- Monday June 26 2017, 16:00 @ seminar room 3rd floor:
Matthias Gruber on
"Dirac quantum time mirror" ,
P. Reck, C. Gorini, A. Goussev, V. Krueckl, M. Fink, and K. Richter
Phys. Rev. B 95, 165421 (2017)
Abstract
Both metaphysical and practical considerations related to time inversion have intrigued scientists for generations.
Physicists have strived to devise and implement time-inversion protocols, in particular different forms of ''time mirrors''
for classical waves. Here we propose an instantaneous time mirror for quantum systems, i.e., a controlled time discontinuity
generating wave function echoes with high fidelities. This concept exploits coherent particle-hole oscillations in a Dirac spectrum
in order to achieve population reversal, and can be implemented in systems such as (real or artificial) graphene.
- Monday June 19 2017, 16:00 @ seminar room 3rd floor:
Robert Triebl on
"Measurement of Impulsive Thrust from a Closed Radio-Frequency
Cavity in Vacuum" ,
H. White, P. March, J. Lawrence, J. Vera, A. Sylvester, D. Brady, and P. Bailey
Journal of Propulsion and Power
, doi: 10.2514/1.B36120
Abstract
A vacuum test campaign evaluating the impulsive thrust performance of a tapered radio-frequency test article excited in the
transverse magnitude 212 mode at 1937 MHz has been completed. The test campaign consisted of a forward thrust phase and reverse
thrust phase at less than 8 x 10^-6 torr vacuum with power scans at 40, 60, and 80 W. The test campaign included a null thrust test
effort to identify any mundane sources of impulsive thrust; however, none were identified. Thrust data from forward, reverse, and
null suggested that the system was consistently performing with a thrust-to-power ratio of 1.2 +- 0.1 mN/kW.
- Monday June 12 2017, 16:00 @ seminar room 3rd floor:
Max Sorantin on
"On the origin of gravity and the laws of Newton" ,
E. Verlinde
J. High Energ. Phys., 29 (2011)
Abstract
Starting from first principles and general assumptions we present a heuristic argument that shows that Newton's law of gravitation
naturally arises in a theory in which space emerges through a holographic scenario. Gravity is identified with an entropic force caused
by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments
directly leads to the Einstein equations. When space is emergent even Newtons law of inertia needs to be explained. The equivalence principle
auggests that it is actually the law of inertia whose origin is entropic.
- Monday May 29 2017, 16:00 @ seminar room 3rd floor:
Irakli Titvinidze on
"Experimentally accessible invariants encoded in interparticle correlations of harmonically trapped ultra-cold few-fermion mixtures" ,
D. Pecak, M. Gajda, T. Sowinski
arXiv:1703.08116 (2017)
Abstract
System of a two-flavor mixture of ultra-cold fermions confined in a one-dimensional harmonic trap is studied in the frame of the center of mass.
We present a numerical method of obtaining energetic spectra in this frame for an arbitrary mass ratio of fermionic species. We identify a specific
invariant encoded in many-body correlations which enable one to determine an eigenstate of the Hamiltonian and to label excitations of the center of mass.
The tool presented may be particularly useful in experimental analysis of the interparticle interactions which do not affect the center of mass excitations
in a harmonic potential.
- Monday May 22 2017, 16:00 @ seminar room 3rd floor:
Andriy Smolyanyuk on
"Dynamics of Panic Pedestrians in Evacuation" ,
S. Dongmei, Z. Wenyao, W. Binghong
arXiv:1701.01236 (2017)
Abstract
A modified lattice gas model is proposed to study pedestrian evacuation from a single room. The payoff matrix in this model
represents the complicated interactions between selfish individuals, and the mean force imposed on an individual is given by
considering the impacts of neighborhood payoff, walls, and defector herding. Each passer-by moves to his selected location according
to the Fermi function, and the average velocity of pedestrian flow is defined as a function of the motion rule. Two pedestrian types
are included: cooperators, who adhere to the evacuation instructions; and defectors, who ignore the rules and act individually. It is
observed that the escape time increases with the panic level, and the system remains smooth for a low panic level, but exhibits three
stages for a high panic level. We prove that the panic level determines the dynamics of this system, and the initial density of cooperators
has a negligible impact. The system experiences three phases, a single phase of cooperator, a mixed two-phase pedestrian, and a single phase
of defector sequentially as the panic level upgrades. The phase transition has been proven basically robust to the changes of empty site
contribution, wall's pressure, and noise amplitude in the motion rule. It is further shown that pedestrians derive the greatest benefit
from overall cooperation, but are trapped in the worst situation if they are all defectors.
- Monday May 15 2017, 16:00 @ seminar room 3rd floor:
Michael Rumetshofer on
"Destructive quantum interference in electron transport: A reconciliation of the molecular orbital and the atomic orbital perspective" ,
X. Zhaoa, V. Geskina, R. Stadlera
The Journal of Chemical Physics 146, 092308 (2017)
Abstract
Destructive quantum interference (DQI) in single molecule electronics is a purely quantum mechanical effect and is entirely
defined by the inherent properties of the molecule in the junction such as its structure and symmetry. This definition of DQI
by molecular properties alone suggests its relation to other more general concepts in chemistry as well as the possibility of
deriving simple models for its understanding and molecular device design. Recently, two such models have gained a wide spread
attention, where one was a graphical scheme based on visually inspecting the connectivity of the carbon sites in conjugated π
systems in an atomic orbital (AO) basis and the other one puts the emphasis on the amplitudes and signs of the frontier molecular
orbitals (MOs). There have been discussions on the range of applicability for these schemes, but ultimately conclusions from topological
molecular Hamiltonians should not depend on whether they are drawn from an AO or a MO representation, as long as all the orbitals are
taken into account. In this article, we clarify the relation between both models in terms of the zeroth order Green's function and compare
their predictions for a variety of systems. From this comparison, we conclude that for a correct description of DQI from a MO perspective,
it is necessary to include the contributions from all MOs rather than just those from the frontier orbitals. The cases where DQI effects
can be successfully predicted within a frontier orbital approximation we show them to be limited to alternant even-membered hydrocarbons,
as a direct consequence of the Coulson-Rushbrooke pairing theorem in quantum chemistry.
- Monday May 08 2017, 16:00 @ seminar room 3rd floor:
Daniel Bauernfeind on
"Thermalization and its mechanism for generic isolated quantum systems" ,
M. Rigol, V. Dunjko1, M. Olshanii
Nature 452, 854-858 (2008)
Abstract
An understanding of the temporal evolution of isolated many-body quantum systems has long been elusive.
Recently, meaningful experimental studies1, of the problem have become possible, stimulating theoretical
interest. In generic isolated systems, non-equilibrium dynamics is expected to result in thermalization:
a relaxation to states in which the values of macroscopic quantities are stationary, universal with respect
to widely differing initial conditions, and predictable using statistical mechanics. However, it is not
obvious what feature of many-body quantum mechanics makes quantum thermalization possible in a sense
analogous to that in which dynamical chaos makes classical thermalization possible. For example,
dynamical chaos itself cannot occur in an isolated quantum system, in which the time evolution is linear
and the spectrum is discrete. Some recent studies even suggest that statistical mechanics may give
incorrect predictions for the outcomes of relaxation in such systems. Here we demonstrate that a generic
isolated quantum many-body system does relax to a state well described by the standard statistical-mechanical
prescription. Moreover, we show that time evolution itself plays a merely auxiliary role in relaxation, and
that thermalization instead happens at the level of individual eigenstates, as first proposed by Deutsch and
Srednicki. A striking consequence of this eigenstate-thermalization scenario, confirmed for our system, is
that knowledge of a single many-body eigenstate is sufficient to compute thermal averages-any eigenstate in
the microcanonical energy window will do, because they all give the same result.
- Monday April 24 2017, 16:00 @ seminar room 3rd floor:
Viktor Eisler on
"Disorder-free localization" ,
A. Smith, J. Knolle, D. L. Kovrizhin, R. Moessner
arXiv:1701.04748 (2017)
Abstract
The study of localization phenomena - pioneered in Anderson's seminal work on the absence of diffusion in certain
random lattices - is receiving redoubled attention in the context of the physics of interacting systems showing many-body localization.
While in these systems the presence of quenched disorder plays a central role, suggestions for interaction-induced localization in
disorder-free systems appeared early in the context of solid Helium. However, all of these are limited to settings having inhomogeneous
initial states. Whether quenched disorder is a general precondition for localization has remained an open question. Here, we provide an
explicit example to demonstrate that a disorder-free system can generate its own randomness dynamically, which leads to localization in
one of its subsystems. Our model is exactly soluble, thanks to an extensive number of conserved quantities, which we identify, allowing
access to the physics of the long-time limit. The model can be extended while preserving its solubility, in particular towards investigations
of disorder-free localization in higher dimensions.
- Monday April 03 2017, 16:00 @ seminar room 3rd floor:
Gernot Kraberger on
"Could a Neuroscientist Understand a Microprocessor?" ,
E. Jonas, K. P. Kording
PLoS Comput Biol 13(1):e1005268 (2016)
Abstract
There is a popular belief in neuroscience that we are primarily data limited, and that producing large, multimodal, and complex
datasets will, with the help of advanced data analysis algorithms, lead to fundamental insights into the way the brain processes
information. These datasets do not yet exist, and if they did we would have no way of evaluating whether or not the algorithmically-generated
insights were sufficient or even correct. To address this, here we take a classical microprocessor as a model organism, and use our ability to
perform arbitrary experiments on it to see if popular data analysis methods from neuroscience can elucidate the way it processes information.
Microprocessors are among those artificial information processing systems that are both complex and that we understand at all levels, from the
overall logical flow, via logical gates, to the dynamics of transistors. We show that the approaches reveal interesting structure in the data
but do not meaningfully describe the hierarchy of information processing in the microprocessor. This suggests current analytic approaches in
neuroscience may fall short of producing meaningful understanding of neural systems, regardless of the amount of data. Additionally, we argue
for scientists using complex non-linear dynamical systems with known ground truth, such as the microprocessor as a validation platform for
time-series and structure discovery methods.
- Monday March 27 2017, 16:00 @ seminar room 3rd floor:
Florian Maislinger on
"Entropic Dynamics" ,
A. Caticha
Entropy 17, 6110-6128 (2015)
Abstract
Entropic Dynamics is a framework in which dynamical laws are derived as an application of
entropic methods of inference. No underlying action principle is postulated. Instead, the dynamics
is driven by entropy subject to the constraints appropriate to the problem at hand. In this paper we
review three examples of entropic dynamics. First we tackle the simpler case of a standard diffusion
process which allows us to address the central issue of the nature of time. Then we show that imposing
the additional constraint that the dynamics be non-dissipative leads to Hamiltonian dynamics. Finally,
considerations from information geometry naturally lead to the type of Hamiltonian that describes
quantum theory.
- Monday January 30 2017, 16:00 @ seminar room 3rd floor:
Irakli Titvinidze on
"Nonequilibrium Dynamical Mean-Field Theory for the Charge-Density-Wave Phase of the Falicov-Kimball Model" ,
O. P. Matveev, A. M. Shvaika, T. P. Devereaux, J. K. Freericks
Journal of Superconductivity and Novel Magnetism, Volume 29, Issue 3, pp 581-585 (2016)
Abstract
Nonequilibrium dynamical mean-field theory (DMFT) is developed for the case of the charge-density-wave ordered phase. We consider the spinless
Falicov-Kimball model which can be solved exactly. This strongly correlated system is then placed in an uniform external dc electric field. We
present a complete derivation for nonequilibrium dynamical mean-field theory Green's functions defined on the Keldysh-Schwinger time contour.
We also discuss numerical issues involved in solving the coupled equations.
- Monday January 23 2017, 16:00 @ seminar room 3rd floor:
Johann Pototschnig on
"The quantum physics of chronology protection" ,
M. Visser
arXiv:gr-qc/0204022 (2002)
Abstract
This is a brief survey of the current status of Stephen Hawking's ''chronology protection conjecture''. That is: ''Why does nature abhor a time machine?''
I'll discuss a few examples of spacetimes containing ''time machines'' (closed causal curves), the sorts of peculiarities that arise, and the reactions
of the physics community. While pointing out other possibilities, this article concentrates on the possibility of ''chronology protection''. As Stephen puts it:
''It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.''
- Monday January 16 2017, 16:00 @ seminar room 3rd floor:
Michael Scherbela on
"Big Data of Materials Science: Critical Role of the Descriptor" ,
L. M. Ghiringhelli, J. Vybiral, S. V. Levchenko, C. Draxl, and M. Scheffler
Phys. Rev. Lett. 114, 105503 (2015)
Abstract
Statistical learning of materials properties or functions so far starts with a largely silent, nonchallenged step: the choice of the set
of descriptive parameters (termed descriptor). However, when the scientific connection between the descriptor and the actuating mechanisms
is unclear, the causality of the learned descriptor-property relation is uncertain. Thus, a trustful prediction of new promising materials,
identification of anomalies, and scientific advancement are doubtful. We analyze this issue and define requirements for a suitable descriptor.
For a classic example, the energy difference of zinc blende or wurtzite and rocksalt semiconductors, we demonstrate how a meaningful descriptor
can be found systematically.
- Monday January 09 2017, 16:00 @ seminar room 3rd floor:
Viktor Eisler on
"Minimizing irreversible losses in quantum systems by local counter-diabatic driving" ,
D. Sels, and A. Polkovnikov
arXiv:1607.05687 (2016)
Abstract
Counter-diabatic driving protocols were proposed as a means to do fast changes in the Hamiltonian without exciting transitions.
Such driving in principle allows one to realize arbitrarily fast annealing protocols or implement fast dissipationless driving,
circumventing standard adiabatic limitations requiring infinitesimally slow rates. These ideas were tested and used both experimentally
and theoretically in small systems, but in larger chaotic systems it is known that exact counter-diabatic protocols do not exist. In this
work we develop a simple variational approach allowing one to find best possible counter-diabatic protocols given physical constraints
like locality. These protocols are easy to derive and implement both experimentally and numerically. We show that, using these approximate
protocols, one can drastically decrease dissipation and increase fidelity of quantum annealing protocols in complex many-particle systems.
In the fast limit these protocols provide an effective dual description of adiabatic dynamics where the coupling constant plays the role
of time and the counter-diabatic term plays the role of the Hamiltonian.
- Monday December 12 2016, 16:00 @ seminar room 3rd floor:
Max Sorantin on
"Approach to typicality in many-body quantum systems" ,
S. Dubey, L. Silvestri, J. Finn, S. Vinjanampathy, and K. Jacobs
Phys. Rev. E 85, 011141 (2012)
Abstract
The recent discovery that for large Hilbert spaces, almost all (that is, typical) Hamiltonians have eigenstates that place small
subsystems in thermal equilibrium, has shed much light on the origins of irreversibility and thermalization. Here we give numerical
evidence that many-body lattice systems generically approach typicality as the number of subsystems is increased, and thus provide
further support for the eigenstate thermalization hypothesis. Our results indicate that the deviation of many-body systems from
typicality decreases exponentially with the number of systems. Further, by averaging over a number of randomly selected nearest-neighbor
interactions, we obtain a powerlaw for the atypicality as a function of the Hilbert space dimension, distinct from the power
law possessed by random Hamiltonians.
- Monday December 05 2016, 16:00 @ seminar room 3rd floor:
Andriy Smolyanyuk on
"Diverging Color Maps for Scientific Visualization" ,
K. Moreland
ISVC 2009: Advances in Visual Computing pp 92-103
Abstract
One of the most fundamental features of scientific visualization is the process of mapping scalar values to colors. This process allows us to view scalar
fields by coloring surfaces and volumes. Unfortunately, the majority of scientific visualization tools still use a color map that is famous for its ineffectiveness:
the rainbow color map. This color map, which naively sweeps through the most saturated colors, is well known for its ability to obscure data, introduce artifacts, and
confuse users. Although many alternate color maps have been proposed, none have achieved widespread adoption by the visualization community for scientific visualization.
This paper explores the use of diverging color maps (sometimes also called ratio, bipolar, or double-ended color maps) for use in scientific visualization, provides a diverging
color map that generally performs well in scientific visualization applications, and presents an algorithm that allows users to easily generate their own customized color maps.
- Monday November 28 2016, 16:00 @ seminar room 3rd floor:
Gernot Kraberger on
"Experimental Time-Optimal Universal Control of Spin Qubits in Solids" ,
J. Geng, Yang Wu, X. Wang, K. Xu, F. Shi, Y. Xie, X. Rong, and J. Du
Phys. Rev. Lett. 117, 170501 (2015)
Abstract
Quantum control of systems plays an important role in modern science and technology. The ultimate goal of quantum control is to achieve
high-fidelity universal control in a time-optimal way. Although high-fidelity universal control has been reported in various quantum systems,
experimental implementation of time-optimal universal control remains elusive. Here, we report the experimental realization of time-optimal
universal control of spin qubits in diamond. By generalizing a recent method for solving quantum brachistochrone equations
[X. Wang et al., Phys. Rev. Lett. 114, 170501 (2015)], we obtained accurate minimum-time protocols for multiple qubits with
fixed qubit interactions and a constrained control field. Single- and two-qubit time-optimal gates are experimentally implemented
with fidelities of 99% obtained via quantum process tomography. Our work provides a time-optimal route to achieve accurate quantum
control and unlocks new capabilities for the emerging field of time-optimal control in general quantum systems.
- Monday November 21 2016, 16:00 @ seminar room 3rd floor:
Michael Rumetshofer on
"Some Statistics concerning the Austrian Presidential Election 2016" ,
E. Neuwirth and W. Schachermayer
arXiv:1609.00506 (2016)
Abstract
The 2016 Austrian presidential runoff election has been repealed by the Austrian constitutional court. The results of the counted
votes had yielded a victory of Alexander van der Bellen by a margin of 30.863 votes as compared to the votes for Norbert Hofer. However,
the constitutional court found that 77.769 votes were "contaminated" as there have been -- at least on a formal level - violations of the
legal procedure when counting those votes. For example, the envelopes were opened prematurely, or not all the members of the electoral board
were present during the counting etc. Hence the court considered the scenario that the irregular counting of these votes might have caused a
reversal of the result as possible. The constitutional court sentenced that this possibility presents a sufficient irregularity in order to order
a repetition of the entire election. While it is, of course, possible that the irregular counting of those 77.769 votes reversed the result, we
shall show that the probability, that this indeed has happened, is ridiculously low.
- Monday November 14 2016, 16:00 @ seminar room 3rd floor:
Florian Maislinger on
"Solving the Quantum Many-Body Problem with Artificial Neural Networks" ,
G. Carleo, M. Troyer
arXiv:1606.02318
Abstract
The challenge posed by the many-body problem in quantum physics originates from the difficulty of describing the non-trivial correlations
encoded in the exponential complexity of the many-body wave function. Here we demonstrate that systematic machine learning of the wave function
can reduce this complexity to a tractable computational form, for some notable cases of physical interest. We introduce a variational representation
of quantum states based on artificial neural networks with variable number of hidden neurons. A reinforcement-learning scheme is then demonstrated,
capable of either finding the ground-state or describing the unitary time evolution of complex interacting quantum systems. We show that this approach
achieves very high accuracy in the description of equilibrium and dynamical properties of prototypical interacting spins models in both one and two
dimensions, thus offering a new powerful tool to solve the quantum many-body problem.
- Monday November 07 2016, 16:00 @ seminar room 3rd floor:
Martin Tazreiter on
"The Investigation of Electron Distribution in Atoms by Electron Diffraction" ,
L. S. Bartell and L. O. Brockway
Phys. Rev. 90, 833 (1953)
Abstract
A procedure is developed for obtaining the radial distribution of electrons in free atoms from electron diffraction data
which makes use of the similarity of the expressions characterizing the scattering of fast electrons and x-rays by atoms.
Requirements are formulated pertaining to accuracy and angular range of data for such studies. A brief comparison is made of
the method with the analogous x-ray method. Electron diffraction data for argon over the range of 0.9 to 44 in units of
4 π sin(θ/2)/ λ are presented and found to give an experimental electron distribution resolving the K, L, and M shells of
argon in satisfactory agreement with the Hartree distribution.
- Monday October 17 2016, 16:00 @ seminar room 3rd floor:
Daniel Bauernfeind on
"More Is Different" ,
P. W. Anderson
Science V177,4047, 393-396 (1972)
Abstract
Broken symmetry and the nature of hierarchical structure of science.
- Monday October 10 2016, 16:00 @ seminar room 3rd floor:
Robert Triebl on
"Observation of quantum Hawking radiation and its entanglement in an analogue black hole" ,
J. Steinhauer
Nature Physics (2016) 10.1038/nphys3863
Abstract
We observe spontaneous Hawking radiation, stimulated by quantum vacuum fluctuations, emanating from an analogue black hole in an atomic Bose–Einstein condensate.
Correlations are observed between the Hawking particles outside the black hole and the partner particles inside. These correlations indicate an approximately
thermal distribution of Hawking radiation. We find that the high-energy pairs are entangled, while the low-energy pairs are not, within the reasonable assumption
that excitations with different frequencies are not correlated. The entanglement verifies the quantum nature of the Hawking radiation. The results are consistent
with a driven oscillation experiment and a numerical simulation.
- Tuesday June 28 2016, 17:00 @ seminar room 3rd floor:
Antonius Dorda on
"Many-body current formula and current conservation for non-equilibrium fully interacting nanojunctions" ,
H. Ness and L. K. Dash
Journal of Physics A: Mathematical and Theoretical, 45, 19
Abstract
We consider the electron transport properties through fully interacting nanoscale junctions beyond the linear-response regime. We calculate
the current flowing through an interacting region connected to two interacting leads, with interaction crossing at the left and right contacts,
by using a non-equilibrium Green function technique. The total current at one interface (the left one for example) is made of several terms which
can be regrouped into two sets. The first set corresponds to a very generalized Landauer-like current formula with physical quantities defined only
in the interacting central region and with renormalized lead self-energies. The second set characterizes inelastic scattering events occurring in the
left lead. We show how this term can be negligible or even vanish due to the pseudo-equilibrium statistical properties of the lead in the thermodynamic
limit. The expressions for the different Green functions needed for practical calculations of the current are also provided. We determine the constraints
imposed by the physical condition of current conservation. The corresponding equation imposed on the different self-energy quantities arising from the current
conservation is derived. We discuss in detail its physical interpretation and its relation with previously derived expressions. Finally several important key
features are discussed in relation to the implementation of our formalism for calculations of quantum transport in realistic systems.
- Monday June 20 2016, 16:00 @ seminar room 3rd floor:
Daniel Bauernfeind on
"Undecidability of the spectral gap",
T. S. Cubitt, D. Perez-Garcia and M. M. Wolf
Nature 528, 207-211, (2015)
The spectral gap-the energy difference between the ground state and first excited state of a system-is central to quantum many-body physics.
Many challenging open problems, such as the Haldane conjecture, the question of the existence of gapped topological spin liquid phases, and
the Yang-Mills gap conjecture, concern spectral gaps. These and other problems are particular cases of the general spectral gap problem: given
the Hamiltonian of a quantum many-body system, is it gapped or gapless? Here we prove that this is an undecidable problem. Specifically, we construct
families of quantum spin systems on a two-dimensional lattice with translationally invariant, nearest-neighbour interactions, for which the spectral gap
problem is undecidable. This result extends to undecidability of other low-energy properties, such as the existence of algebraically decaying ground-state
correlations. The proof combines Hamiltonian complexity techniques with aperiodic tilings, to construct a Hamiltonian whose ground state encodes the
evolution of a quantum phase-estimation algorithm followed by a universal Turing machine. The spectral gap depends on the outcome of the corresponding
"halting problem". Our result implies that there exists no algorithm to determine whether an arbitrary model is gapped or gapless, and that there exist
models for which the presence or absence of a spectral gap is independent of the axioms of mathematics.
- Monday June 13 2016, 16:00 @ seminar room 3rd floor:
Florian Maislinger on
"Classic Nintendo Games are (Computationally) Hard" ,
G. Aloupis, E. D. Demaine, A. Guo, G. Viglietta
arXiv:1203.1895 (2012)
Abstract
We prove NP-hardness results for five of Nintendo's largest video game franchises: Mario, Donkey Kong, Legend of Zelda, Metroid,
and Pokemon. Our results apply to generalized versions of Super Mario Bros. 1-3, The Lost Levels, and Super Mario World; Donkey
Kong Country 1-3; all Legend of Zelda games; all Metroid games; and all Pokemon role-playing games. In addition, we prove
PSPACE-completeness of the Donkey Kong Country games and several Legend of Zelda games.
- Monday June 6 2016, 16:00 @ seminar room 3rd floor:
Michael Rumetshofer on
"Quantum Thermal Transistor" ,
K. Joulain, J. Drevillon, Y. Ezzahri, and J. Ordonez-Miranda
Phys. Rev. Lett. 116, 200601 (2016)
Abstract
We demonstrate that a thermal transistor can be made up with a quantum system of three interacting subsystems, coupled to a thermal reservoir each.
This thermal transistor is analogous to an electronic bipolar one with the ability to control the thermal currents at the collector and at the emitter
with the imposed thermal current at the base. This is achieved by determining the heat fluxes by means of the strong-coupling formalism. For the case of
three interacting spins, in which one of them is coupled to the other two, that are not directly coupled, it is shown that high amplification can be obtained
in a wide range of energy parameters and temperatures. The proposed quantum transistor could, in principle, be used to develop devices such as a thermal
modulator and a thermal amplifier in nanosystems.
- Monday May 23 2016, 16:00 @ seminar room 3rd floor:
Johann Pototschnig on
"Lattice simulations of the thermodynamics of strongly interacting elementary particles and the exploration of new phases of matter in relativistic heavy ion collisions" ,
F. Karsch
Journal of Physics: Conference Series, 2006, 46, 122
Abstract
At high temperatures or densities matter formed by strongly interacting elementary particles (hadronic matter) is expected to undergo a transition to a new form of
matter - the quark gluon plasma - in which elementary particles (quarks and gluons) are no longer confined inside hadrons but are free to propagate in a thermal
medium much larger in extent than the typical size of a hadron. The transition to this new form of matter as well as properties of the plasma phase are studied
in large scale numerical calculations based on the theory of strong interactions - Quantum Chromo Dynamics (QCD). Experimentally properties of hot and dense elementary
particle matter are studied in relativistic heavy ion collisions such as those currently performed at the relativistic heavy ion collider (RHIC) at BNL. We review here
recent results from studies of thermodynamic properties of strongly interacting elementary particle matter performed on Teraflops-Computer. We present results on the QCD
equation of state and discuss the status of studies of the phase diagram at non-vanishing baryon number density.
- Monday May 09 2016, 16:15 @ seminar room 3rd floor:
Gerhard Dorn on
"True Randomness: Indeterministic quantum mechanics vs hidden variable theories (de Brolie-Bohm Theory)" ,
Abstract
This journal club will provide a general discussion of the terminus "randomness", meant is real randomness addressing
the question of determinism in quantum mechanics. Properties of QM (Heisenberg's uncertainty principle, EPR-paradoxon) have led to intensive discussion about "spooky actions at a distance", about reality and determinism itself.
Beside the Copenhager interpretation there have been also different approaches to address this fundamental question.
I will present some ideas of the following sources to give an overview of available theories and leave some space for discussion.
- Monday May 02 2016, 16:00 @ seminar room 3rd floor:
Max Sorantin on
"Symmetry Breaking and the Geometry of Reduced Density Matrices" ,
V. Zauner, D. Draxler, L. Vanderstraeten, J. Haegeman, F. Verstraete
arXiv:1412.7642 (2014)
Abstract
The concept of symmetry breaking and the emergence of corresponding local order parameters constitute the pillars of modern day many body physics.
The theory of quantum entanglement is currently leading to a paradigm shift in understanding quantum correlations in many body systems and in
this work we show how symmetry breaking can be understood from this wavefunction centered point of view. We demonstrate that the existence of
symmetry breaking is a consequence of the geometric structure of the convex set of reduced density matrices of all possible many body wavefunctions.
The surfaces of those convex bodies exhibit non-analytic behavior in the form of ruled surfaces, which turn out to be the defining signatures for the
emergence of symmetry breaking and of an associated order parameter. We illustrate this by plotting the convex sets arising in the context of three
paradigmatic examples of many body systems exhibiting symmetry breaking: the quantum Ising model in transverse magnetic field, exhibiting a second
order quantum phase transition; the classical Ising model at finite temperature in two dimensions, which orders below a critical temperature Tc; and
a system of free bosons at finite temperature in three dimensions, exhibiting the phenomenon of Bose-Einstein condensation together with an associated
order parameter <ψ>. Remarkably, these convex sets look all very much alike. We believe that this wavefunction based way of looking at phase transitions
demystifies the emergence of order parameters and provides a unique novel tool for studying exotic quantum phenomena.
- Monday April 25 2016, 16:00 @ seminar room 3rd floor:
Elias Assmann on
"Infinite variance in fermion quantum Monte Carlo calculations" ,
H. Shi, and S. Zhang
Phys. Rev. E 93, 033303, (2016)
Abstract
For important classes of many-fermion problems, quantum Monte Carlo (QMC) methods allow exact calculations of ground-state and finite-temperature
properties without the sign problem. The list spans condensed matter, nuclear physics, and high-energy physics, including the half-filled repulsive
Hubbard model, the spin-balanced atomic Fermi gas, and lattice quantum chromodynamics calculations at zero density with Wilson Fermions, and is growing
rapidly as a number of problems have been discovered recently to be free of the sign problem. In these situations, QMC calculations are relied on to provide
definitive answers. Their results are instrumental to our ability to understand and compute properties in fundamental models important to multiple subareas in
quantum physics. It is shown, however, that the most commonly employed algorithms in such situations have an infinite variance problem. A diverging variance
causes the estimated Monte Carlo statistical error bar to be incorrect, which can render the results of the calculation unreliable or meaningless. We discuss
how to identify the infinite variance problem. An approach is then proposed to solve the problem. The solution does not require major modifications to standard
algorithms, adding a "bridge link" to the imaginary-time path integral. The general idea is applicable to a variety of situations where the infinite variance
problem may be present. Illustrative results are presented for the ground state of the Hubbard model at half-filling.
- Monday April 18 2016, 16:00 @ seminar room 3rd floor:
Irakli Titvinidze on
"Renormalization-group approach to the Anderson model of dilute magnetic alloys. I. Static properties for the symmetric case"
,
H. R. Krishna-murthy, J. W. Wilkins, and K. G. Wilson
Phys. Rev. B 21, 1003, (1980)
Abstract
The temperature-dependent impurity susceptibility for the symmetric Anderson model is calculated for all physically relevant values
of its parameters U (the Coulomb correlation energy) and Γ (the impurity-level width). It is shown that, when U>πΓ, for temperatures
T < U/(10*k_B) the symmetric Anderson model exhibits a local moment and that its susceptibility maps neatly onto that of the spin-1/2 Kondo
model with an effective coupling given by ρJeff=-8Γ/(πU). Furthermore, this mapping is shown for remarkably large values of |ρJeff|.
At very low temperatures (much smaller than the Kondo temperature) the local moment is frozen out, just as for the Kondo model,
leading to a strong-coupling regime of constant susceptibility at zero termperature. The results also depict the formation of a
local moment from the free orbital as T drops below U, a feature not present in the Kondo model. Finally, when U<<πΓ there is a direct
transition from free-orbital regime for T>>Γ to the strong-coupling regime for T<<Γ. The calculations were performed using the numerical
renormalization group originally developed by Wilson for the Kondo problem. In addition to the actual numerical calculations, analytic
results are presented. In particular, the effective Hamiltonians, i.e., fixed-point Hamiltonian plus relevant and marginal operators,
are constructed for the free-orbital, local-moment, and strong-coupling regimes. Analytic formulas for the impurity susceptibility and
free energy in all three regimes are developed. The impurity specfic heat in the strong-coupling regime is calculated.
- Monday April 11 2016, 16:00 @ seminar room 3rd floor:
Andriy Smolyanyuk on
"A multi-directional backlight for a wide-angle, glasses-free three-dimensional display"
,
D. Fattal, Z. Peng, T. Tran, S. Vo, M. Fiorentino, J. Brug, R. G. Beausoleil
Nature 495, 348-351 (2013)
Abstract
Multiview three-dimensional (3D) displays can project the correct perspectives of a 3D image in many spatial directions simultaneously.
They provide a 3D stereoscopic experience to many viewers at the same time with full motion parallax and do not require special glasses
or eye tracking. None of the leading multiview 3D solutions is particularly well suited to mobile devices (watches, mobile phones or tablets),
which require the combination of a thin, portable form factor, a high spatial resolution and a wide full-parallax view zone (for short viewing
distance from potentially steep angles). Here we introduce a multi-directional diffractive backlight technology that permits the rendering of
high-resolution, full-parallax 3D images in a very wide view zone (up to 180 degrees in principle) at an observation distance of up to a metre.
The key to our design is a guided-wave illumination technique based on light-emitting diodes that produces wide-angle multiview images in colour
from a thin planar transparent lightguide. Pixels associated with different views or colours are spatially multiplexed and can be independently
addressed and modulated at video rate using an external shutter plane. To illustrate the capabilities of this technology, we use simple ink masks
or a high-resolution commercial liquid-crystal display unit to demonstrate passive and active (30 frames per second) modulation of a 64-view backlight,
producing 3D images with a spatial resolution of 88 pixels per inch and full-motion parallax in an unprecedented view zone of 90 degrees. We also
present several transparent hand-held prototypes showing animated sequences of up to six different 200-view images at a resolution of 127 pixels
per inch.
- Monday March 21 2016, 16:00 @ seminar room 3rd floor:
Gernot Kraberger on
"Evidence of an odd-parity hidden order in a spin-orbit coupled correlated iridate"
,
L. Zhao, D. H. Torchinsky, H. Chu, V. Ivanov, R. Lifshitz, R. Flint, T. Qi, G. Cao, D. Hsieh
Nature Physics 12, 32-36 (2016)
Abstract
A rare combination of strong spin-orbit coupling and electron-electron correlations makes the iridate Mott insulator Sr2IrO4 a promising
host for novel electronic phases of matter. The resemblance of its crystallographic, magnetic and electronic structures
to La2CuO4, as well as the emergence on doping of a pseudogap region and a low-temperature d-wave gap, has particularly strengthened
analogies to cuprate high-Tc superconductors. However, unlike the cuprate phase diagram, which features a plethora of broken symmetry phases
in a pseudogap region that includes charge density wave, stripe, nematic and possibly intra-unit-cell loop-current orders, no broken symmetry
phases proximate to the parent antiferromagnetic Mott insulating phase in Sr2IrO4 have been observed so far, making the comparison of iridate
to cuprate phenomenology incomplete. Using optical second-harmonic generation, we report evidence of a hidden non-dipolar magnetic order in
Sr2IrO4 that breaks both the spatial inversion and rotational symmetries of the underlying tetragonal lattice. Four distinct domain types
corresponding to discrete 90°-rotated orientations of a pseudovector order parameter are identified using nonlinear optical microscopy,
which is expected from an electronic phase that possesses the symmetries of a magneto-electric loop-current order. The
onset temperature of this phase is monotonically suppressed with bulk hole doping, albeit much more weakly than the Néel temperature,
revealing an extended region of the phase diagram with purely hidden order. Driving this hidden phase to its quantum critical point
may be a path to realizing superconductivity in Sr2IrO4.
- Monday March 14 2016, 16:00 @ seminar room 3rd floor:
Robert Triebl on
"Observation of Gravitational Waves from a Binary Black Hole Merger"
,
B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)
Phys. Rev. Lett. 116, 061102
Abstract
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed
a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0x10-21.
It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single
black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per
203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410+160-180 Mpc corresponding to a
redshift z=0.09+0.03-0.04. In the source frame, the initial black hole masses are 36+5-4 M☉ and 29+4-4 M☉,
and the final black hole mass is 62+4-4 M☉, with 3.0+0.5-0.5M☉c2 radiated in gravitational waves. All uncertainties define 90% credible intervals.
These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation
of a binary black hole merger.
- Monday February 29 2016, 16:00 @ seminar room 3rd floor:
Manuel Zingl on
"Rare-earth vs. heavy metal pigments and their colors from first principles"
,
J. M. Tomczak, L. V. Pourovskii, L. Vaugier, A. Georges, S. Biermann
PNAS, Vol. 110 No. 3, 904-907
Abstract
Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest
for nontoxic alternatives based on rare-earth elements. However, the computation of colors from first principles is a challenge
to electronic structure methods, especially for materials with localized f-orbitals. Here, starting from atomic positions only,
we compute the colors of the red pigment cerium fluorosulfide as well as mercury sulfide (classic vermilion). Our methodology uses
many-body theories to compute the optical absorption combined with an intermediate length-scale modelization to assess how coloration
depends on film thickness, pigment concentration, and granularity. We introduce a quantitative criterion for the performance of a
pigment. While for mercury sulfide, this criterion is satisfied because of large transition matrix elements between wide bands,
cerium fluorosulfide presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi-2D
and the localized nature of Graphic states. Our work shows the power of modern computational methods, with implications for the
theoretical design of materials with specific optical properties.
- Friday June 19 2015, 14:00 c.t. @ seminar room C208, Steyrergasse 30/II :
Univ.-Prof. Ulisse Stefanelli from University of Vienna on
"Geometry optimization in carbon"
Abstract
Carbon nanostructures such as graphene, nanotubes, and fullerenes are
locally planar: each atom forms three covalents bonds which (ideally)
create bonding-angles of 2π/3. In distinguished regimes, this
phenomenology can be modeled by minimizing specific atomic-interaction
potentials including three-body interaction terms [T].
I intend to review some of the existing crystllization results for this
kind of potentials [E,M,M2]. In particular, I will focus on the
possibility of characterizing the geometry of energy minimizers,
especially in three-dimensions. The stability, fine geometry, and partly
the nanomechanics of classes of nanotubes and fullerenes will be discussed.
This is joint work with M. Friedrich, E. Mainini, H. Murakawa, and P. Piovano.
[E] W. E, D. Li. On the crystallization of 2D hexagonal lattices. Comm.Math. Phys. 286 (2009) 1099-1140.
[M] E. Mainini, U. Stefanelli. Crystallization in carbon nanostructures. Comm. Math. Phys. 328 (2014) 545-571.
[M2] E. Mainini, P.Piovano, U. Stefanelli. Finite crystallization in the square lattice. Nonlinearity, 27 (2014) 717-737.
[T] J. Tersoff. New empirical approach for the structure and energy of covalent systems. Phys. Rev. B 37 (1988) 6991-7000.
Thanks to Basisgruppe Physik and ITPcp for support.
- Wednesday June 17 2015, 13:30 @ seminar room 3rd floor:
Johann Pototschnig on
"The permanent electric dipole moments of Rb-alkaline earth diatomic molecules"
Abstract
In a recent series of combined experimental and theoretical studies we investigated
the ground state and several excited states of the Rb-alkaline earth molecules RbSr and RbCa[1,2,3].
The group of alkali-alkaline earth (AK-AKE) molecules has drawn attention in the last years for
applications in ultracold molecular physics[4] and the measurement of fundamental constants[5] due
to their large permanent electric and magnetic dipole moment in the ground state. These properties
should allow for an easy manipulation of the molecules and simulations of spin models in optical lattices.
In our studies we found that the permanent electric dipole moment points in the opposite direction for
certain electronically excited states, and changes the sign in some cases as a function of the binding length.
We summarize our results, give possible causes for the measured trends in terms of molecular orbital theory
and extrapolate the tendencies to other combinations of AK and AKE-elements.
[1] F. Lackner, Phys. Rev. Lett., 2014, 113, 153001
[2] G. Krois, Phys. Chem. Chem. Phys., 2014, 16, 22373
[3] J. Pototschnig, J. Mol. Spectrosc., 2015, 310, 126-134
[4] A. Micheli, Nature Physics, 2006, 2, 341-347
[5] M. Kajita, J. Mol. Spectrosc., 2014, 300, 99-107
Thanks to Basisgruppe Physik and ITPcp for support.
- Wednesday June 10 2015, 13:30 @ seminar room 3rd floor :
Gernot Kraberger on
Evolutionary games of condensates in coupled birth-death processes
,
J. Knebel, M.F. Weber, T. Krueger, E. Frey
Nature Communications 6 (2015), 6977
Abstract
Condensation phenomena arise through a collective behaviour of particles. They are observed
in both classical and quantum systems, ranging from the formation of traffic jams in mass transport
models to the macroscopic occupation of the energetic ground state in ultra-cold bosonic gases
(Bose-Einstein condensation). Recently, it has been shown that a driven and dissipative system of bosons
may form multiple condensates. Which states become the condensates has, however, remained elusive
thus far. The dynamics of this condensation are described by coupled birth-death processes, which also
occur in evolutionary game theory. Here we apply concepts from evolutionary game theory to explain
the formation of multiple condensates in such driven-dissipative bosonic systems. We show that the
vanishing of relative entropy production determines their selection. The condensation proceeds exponentially
fast, but the system never comes to rest. Instead, the occupation numbers of condensates may oscillate,
as we demonstrate for a rock-paper-scissors game of condensates.
Thanks to Basisgruppe Physik and ITPcp for support.
- Wednesday June 03 2015, 13:30 @ seminar room 3rd floor :
Jakob Neumayer on
GOLLUM: a next-generation simulation tool for electron, thermal and spin transport
,
J. Ferrer, C.J. Lambert, V.M. García-Suárez, D.Zs. Manrique, D. Visontai, L. Oroszlany,
R. Rodríquez-Ferradás, I. Grace, S.W.D. Bailey, K. Gillemot, Hatef Sdeghi and L. A. Algharagholy
New Journal of Physics 16 (2014) 093029
Abstract
We have developed an efficient simulation tool 'GOLLUM' for the computation of electrical, spin and thermal transport characteristics
of complex nanostructures. The new multi-scale, multi-terminal tool addresses a number of new challenges and functionalities that
have emerged in nanoscale-scale transport over the past few years. To illustrate the flexibility and functionality of GOLLUM, we
present a range of demonstrator calculations encompassing charge, spin and thermal transport, corrections to density functional theory
such as local density approximation +U (LDA+U) and spectral adjustments, transport in the presence of non-collinear magnetism, the
quantum Hall effect, Kondo and Coulomb blockade effects, finite-voltage transport, multi-terminal transport, quantum pumps,
superconducting nanostructures, environmental effects, and pulling curves and conductance histograms for mechanically-controlled
break-junction experiments.
Thanks to Basisgruppe Physik and ITPcp for support.
- Tuesday May 19 2015, 15:00 @ seminar room 3rd floor :
Marco Gandolfi from Universita Cattolica del Sacro Cuore (Italy) on
"QueDynamics of the Broken Symmetry Phases And Temperature Super-Lattice."
Abstract
The diffusion equation regulates the incoherent propagation of a local perturbation in many
fundamental physical problems, such as the propagation of a temperature gradient or the relaxation
of the order parameter in a broken-symmetry phase. The possibility of experimentally accessing the
ultrafast timescale recently paved the way to the study of the non-equilibrium dynamics in the coherent
regime, i.e. before a local thermalization is achieved. In this perspective, I analyze two prototypical
cases in which a non-trivial wave-like behaviour emerges:
(i) The generalized Fourier equation, which describes the propagation of temperature gradients upon non-adiabatic excitations.
(ii) The coherent motion equation of the order parameter in a symmetry-broken phase, such as charge-density wave or superconductivity. The dynamics reduces to
that described by the conventional time-dependent Ginzburg-Landau equation for the incoherent relaxation.
Using a finite-element analysis, I investigate the coherent and incoherent dynamics in relation with time-resolved experiments.
Furthermore, I will adress the possibility of creating a superlattice for the temperature and order-parameter waves.
Thanks to Basisgruppe Physik and ITPcp for support.
- Wednesday May 06 2015, 13:30 @ seminar room 3rd floor :
Zhian Asadzadeh on
"Quench dynamics in transverse field Ising model: non-equilibrium cluster perturbation theory (NE-CPT) approach"
Abstract
The recent generalization of cluster perturbation theory (CPT) [1] to out of equilibrium [2], so called non-equilibrium cluster perturbation theory (NE-CPT),
is applied to study magnetic quench dynamics in transverse field Ising model. We mainly focus on the one dimensional case where exact solution is possible
to demonstrate the strengths and the shortcomings of the approach.
First at equilibrium we show that CPT could describe the system in quantum disordered phase but fails in ordered phase. By introducing source and drain
like (broken U(1) symmetry ) variational parameter in the cluster Hamiltonian we produce almost exact results for the ordered phase as well.
In the presence of U(1) symmetry breaking terms we must use the recent extension of CPT which has been presented for strongly correlated bosons in superfluid phase [3].
After well established equilibrium results we apply NE-CPT to study real time dynamics after magnetic quenches in the system. Instead of using general
approach of non-equilibrium self energy functional theory [4] we approximate the time dependent variational parameters at mean-field level. We show that NE-CPT
for finite systems produces results (for example 'z' component of spin operator) quite close to exact one if the quench does not cross the critical point (the point where
separates ordered from disordered one at equilibrium). We show finally the dynamics for infinite system and compare them with exact one. We will discuss about some
numerical details, efficient implementation and specially time propagation of the Kadanoff-Baym equations in the reformulated NE-CPT equation after using Langreth theorem.
[1] David Sénéchal, http://arxiv.org/abs/0806.2690
[2] Matthias Balzer and Michael Potthoff, Phys. Rev. B 83, 195132 - (2011)
[3] Michael Knap, Enrico Arigoni, and Wolfgang von der Linden, Phys. Rev. B 83, 134507 (2011)
[4] Felix Hofmann, Martin Eckstein, Enrico Arrigoni, and Michael Potthoff, Phys. Rev. B 88, 165124 ( 2013)
Thanks to Basisgruppe Physik and ITPcp for support.
- Wednesday April 29 2015, 13:30 @ seminar room 3rd floor :
Irakli Titvinidze on
"Many body physics with ultracold atoms"
Abstract
In my talk I will make short introduction in the ultracold atomic physics.
The topics what I plan to touch are the following (according to [1]):
(i) Cooling atoms (Section 4 (mainly introduction))
(ii) Feshbach resonance (5.4.2 Elastic scattering and Feshbach resonances)
(iv) Optical lattices (14.1 Generation of optical lattices)
(v) Many body effects in Optical lattices (Section 14)
[1] C. J. Pethick and H. Smith, Bose-Einstein Condensation in Dilute Gases.
2nd edition, Cambridge University Press, 2008
Thanks to Basisgruppe Physik and ITPcp for support.
- Friday April 24 2015, 11:00 @ seminar room 2nd floor :
Almas Hilman Muhtadi from KU Leuven (Belgium) on
"Investigation of structure, chemical stability, electronic and magnetic properties
of di- and tri-atomic Ln-based species (Dy, Tb, Ho, Er) and prediction of magnetic
blocking in some model compounds"
Abstract
Currently, there are large numbers of Ln-based molecular magnets reported in the
literature. Most of the well-performing molecular magnets are obtained for the Ln3+
ions, which have a well-defined 4fN ground electronic configuration. There is also a wellestablished
computational methodology based on multiconfigurational ab initio
calculations, enabling their theoretical investigation. The best performing molecular
magnet predicts to date is the simple Dy-O+ cation, with a blocking barrier of some
3000 K. However, this prediction did not take into account the effect of other electronic
shells of the Dy ion (5d, 6s, and 6p). These shells are known to be important for the
species, which containing Ln2+and Ln1+. In this project, we investigated theoretically the
structure of diatomic [Ln-X] and triatomic species[X-Ln-X] and [Ln-X2] (X= F, O, N, C).
Optimization of the structure of several oxidation states of these species has been done
with DFT B3LYP functional in Gaussian09 software. Further, the optimized structures
has been proceded to high-level multiconfigurational ab initio wave function
calculations such as RASSCF/RASPT2/ RASSI/SINGLE_ANISO using MOLCAS
program package in order to determine the ground and low-lying excited spin-orbit
states, magnetic anisotropy of low-lying states and magnetism of the investigated
species. Within this study, the importance of the lanthanides 5d, 6s and 6p shells in the
ground and excited states of the studied species has been clarified. Finally, the influence
of these shells towards the magnetization blocking of these species are revealed.
Thanks to Basisgruppe Physik and ITPcp for support.
- Friday April 17 2015, 11:00 @ seminar room 3rd floor :
Andriy Smolyanuyk from Uzhorod National University (Ukraine) on
"Magnetic properties of single molecular magnets. Classical spin dynamics theory"
Abstract
The talk presents short introduction into the field of molecular magnetism and classical spin dynamics theory.
The systems of interest are Fe4 star-shaped molecules. The aim of this work is to obtain coefficients of exchange interaction and magnetic anisotropy of this systems.
The investigation is handled by using ab initio DFT approach with included consideration of non-collinear magnetism and spin-orbital interaction.
Description of classical spin dynamics theory is given in Landau-Lifshitz-Gilbert ansatz and gives its implementation and application for investigation of small systems.
Thanks to Basisgruppe Physik and ITPcp for support.
- Wednesday March 25 2015, 13:30 @ seminar room 3rd floor :
Robert Triebl on
"Experimental realization of the topological Haldane model with ultracold fermions"
,
Gregor Jotzu, Michael Messer, Remi Desbuquois, Martin Lebrat, Thomas Uehlinger, Daniel Greif, Tilman Esslinger
Nature 515 237-240 (2014)
Abstract
The Haldane model on a honeycomb lattice is a paradigmatic example of a Hamiltonian featuring topologically distinct phases of matter.
It describes a mechanism through which a quantum Hall effect can appear as an intrinsic property of a band structure, rather than being caused by an external magnetic field.
Although physical implementation has been considered unlikely, the Haldane model has provided the conceptual basis for theoretical and experimental research exploring topological
insulators and superconductors. Here we report the experimental realization of the Haldane model and the characterization of its topological band structure, using ultracold fermionic
atoms in a periodically modulated optical honeycomb lattice. The Haldane model is based on breaking both time-reversal symmetry and inversion symmetry. To break time-reversal symmetry,
we introduce complex next-nearest-neighbour tunnelling terms, which we induce through circular modulation of the lattice position. To break inversion symmetry, we create an energy offset
between neighbouring sites. Breaking either of these symmetries opens a gap in the band structure, which we probe using momentum-resolved interband transitions. We explore the resulting Berry
curvatures, which characterize the topology of the lowest band, by applying a constant force to the atoms and find orthogonal drifts analogous to a Hall current. The competition between the two
broken symmetries gives rise to a transition between topologically distinct regimes. By identifying the vanishing gap at a single Dirac point, we map out this transition line experimentally
and quantitatively compare it to calculations using Floquet theory without free parameters. We verify that our approach, which allows us to tune the topological properties dynamically, is suitable
even for interacting fermionic systems. Furthermore, we propose a direct extension to realize spin-dependent topological Hamiltonians.
Thanks to Basisgruppe Physik and ITPcp for support.
- Tuesday December 2 2014, 14:00 @ seminar room 3rd floor :
Jakob Neumayer on
"Pictures around the world! Part 1:Geological wonders of the USA - Arches, Canyons, Waves and Hoodoos"
,
Abstract
Follow me into the sandstone desert of Utah, Arizona and Nevada, where we find relicts of the landscape on
our earth millions of years ago. Our journey will start in the capital city of the Mor mon Culture Salt Lake City
and will take us to the most scenic National Parks in the western USA. We will find beautiful natural stone
arches in the Arches National park, deeply carved canyons in the Canyonlands National park, the amazing stone
pillars („Hoodoos“) of the Bryce Canyon, the unbeatable beauty of one of the most protected landmarks of the
USA - „The Wave“ and loads of other stunning natural beauties like „Giant Sequoias“, „The Nose“ and „The Half
Dome", „Devil’s Golf Course“, ...
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Tuesday November 25 2014, 14:00 @ seminar room 3rd floor :
Antonius Dorda on
"Solving nonequilibrium dynamical mean-field theory using matrix product states"
,
F. Alexander Wolf, Ian P. McCulloch and Ulrich Schollwöck
arXiv:1410.3342 (2014)
Abstract
We solve the nonequilibrium dynamical mean-field theory (DMFT) using matrix product states (MPS).
This allows us to treat much larger bath sizes and by that reach substantially longer times (factor ∼ 2
-- 3) than with exact diagonalization. We show that the star geometry of the underlying impurity
problem can have substantially better entanglement properties than the previously favoured chain
geometry. This has immense consequences for the efficiency of an MPS-based description of general
impurity problems: in the case of equilibrium DMFT, it leads to an orders-of-magnitude speedup. We
introduce an approximation for the two-time hybridization function that uses time-translational
invariance, which can be observed after a certain relaxation time after a quench to a time-
independent Hamiltonian.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Tuesday November 18 2014, 14:00 @ seminar room 3rd floor :
Johann Potoschnig on
"Magnetotransport near a quantum critical point in a simple metal"
,
Ya. B. Bazaliy, R. Ramazashvili, Q. Si and M. R. Norman
Phys. Rev. B 69, 144423 (2004)
Abstract
We use geometric considerations to study transport properties, such as the conductivity and Hall
coefficient, near the onset of a nesting-driven spin-density wave in a simple metal. In particular,
motivated by recent experiments on vanadium-doped chromium, we study the variation of transport
coefficients with the onset of magnetism within a mean-field treatment of a model that contains
nearly nested electron and hole Fermi surfaces. We show that most transport coefficients display a
leading dependence that is linear in the energy gap. The coefficient of the linear term, though, can be small. In particular, we find that the Hall conductivity σxy is essentially unchanged, due to electron-
hole compensation, as the system goes through the quantum critical point. This conclusion extends a similar observation we made earlier for the case of completely flat Fermi surfaces to the immediate
vicinity of the quantum critical point where nesting is present but not perfect.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Tuesday November 11 2014, 14:00 @ seminar room 3rd floor :
Manuel Zingl on
"Thermopower of the Correlated Narrow Gap Semiconductor FeSi and
Comparison to RuSi"
,
by Jan M. Tomczak, K. Haule and G. Kotliar
arXiv:1210.3379 (2012)
Abstract
Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have received a lot of attention because they exhibit a large thermopower, as well as striking similarities to heavy fermion Kondo insulators. Many proposals have been advanced, however, lacking quantitative methodologies
applied to this problem, a consensus remained elusive to date. Here, we employ realistic many-body
calculations to elucidate the impact of electronic correlation effects on FeSi. Our methodology
accounts for all substantial anomalies observed in FeSi: the metallization, the lack of conservation of spectral weight in optical spectroscopy, and the Curie susceptibility. In particular we find a very good agreement for the anomalous thermoelectric power. Validated by this congruence with experiment, we further discuss a new physical picture of the microscopic nature of the insulator-to-metal
crossover. Indeed, we find the suppression of the Seebeck coefficient to be driven by correlation
induced incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic homologue
RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will exhibit an increased thermopower at
intermediate temperatures.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Tuesday November 4 2014, 14:00 @ seminar room 3rd floor :
Irakli Titvinidze on
"Inverse indirect magnetic exchange in one and higher dimensions "
,
by Andrej Schwabe, Irakli Titvinidze, and Michael Potthoff
Phys. Rev. B 88, 121107(R) (2013)
Abstract
The magnetic ground-state properties of the periodic Anderson and Kondo-lattice model with a regular depletion of the correlated sites are analyzed by means of different theoretical and numerical approaches. We consider the model on the one-dimensional chain and on the two-dimensional square lattice with hopping between nearest neighbors. At half-filling and with correlated impurities present at every second site, the depleted periodic Anderson and Kondo lattice models are the most simple systems where the indirect magnetic coupling mediated by the conduction electrons is ferromagnetic. We discuss the underlying electronic structure and the possible mechanisms that result in ferromagnetic long-range order. To this end, different numerical and analytical concepts are applied to the depleted Anderson and also to the related depleted Kondo lattice and are contrasted with each other. This includes numerical approaches, i.e. Hartree-Fock theory, density-matrix renormalization and dynamical mean-field theory, as
well as analytical concepts, namely a variant of the Lieb-Mattis theorem, the concept of flat-band ferromagnetism, and perturbative approaches, i.e. the effective RKKY exchange in the limit of weak and the “inverse indirect magnetic exchange” in the limit of strong coupling between the conduction band and the impurities. Finally, we also investigate robustness of the ferromagnetic state for finite temperatures and away of the half-filling.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Tuesday October 28 2014, 14:00 @ seminar room 3rd floor :
Priyanka Seth on
"Frequency-dependent local interactions and low-energy effective models
from electronic structure calculations"
,
by F. Aryasetiawan, M. Imada, A. Georges, G. Kotliar, S. Biermann, and A. I. Lichtenstein
Phys. Rev. B 70, 195104 (2004)
Abstract
We propose a systematic procedure for constructing effective models of strongly correlated
materials. The parameters, in particular the on-site screened Coulomb interaction U, are calculated
from first principles, using the random-phase approximation. We derive an expression for the
frequency-dependent U(w) and show, for the case of nickel, that its high-frequency part has
significant influence on the spectral functions. We propose a scheme for taking into account the
energy dependence of U(w), so that a model with an energy-independent local interaction can still
be used for low-energy properties.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Tuesday October 21 2014, 14:00 @ seminar room 2nd floor :
Christopher Albert on
"Space... The final frontier...
These are the voyages of the Starship Voyager 1.
Its continuing mission:
To explore strange new worlds...
To seek out new life; new civilisations...
To boldly go where no one has gone before!"
,
by S. M. Krimigis, R. B. Decker, E. C. Roelof, M. E. Hill, T. P. Armstrong, G. Gloeckler, D. C. Hamilton and
L. J. Lanzerotti
Science 34 6142, 144-147 (2013)
by L. F. Burlaga, N. F. Ness and E. C. Stone
Science 34 6142, 147-150 (2013)
by E. C. Stone, A. C. Cummings, F. B. McDonald, B. C. Heikkila, N. Lal and W. R. Webber
Science 34 6142, 150-153 (2013)
Abstract
We report measurements of energetic (>40 kiloelectron volts) charged particles on Voyager 1 from the
interface region between the heliosheath, dominated by heated solar plasma, and the local interstellar
medium, which is expected to contain cold nonsolar plasma and the galactic magnetic field. Particles of solar
origin at Voyager 1, located at 18.5 billion kilometers (123 astronomical units) from the Sun, decreased by a
3factor of >10 on 25 August 2012, while those of galactic origin (cosmic rays) increased by 9.3% at the same
time. Intensity changes appeared first for particles moving in the azimuthal direction and were followed by
those moving in the radial and antiradial directions with respect to the solar radius vector. This unexpected
heliospheric “depletion region” may form part of the interface between solar plasma and the galaxy
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday June 26 2014, 13:00 @ seminar room 2nd floor :
Robert Triebl on
BICEP2 I: Detection Of B-mode Polarization at Degree Angular Scales,
by BICEP2 Collaboration
arXiv:1403.3985v2 (2014)
Abstract
We report r esults from the BICEP2 experiment, a Cosmic Microwa ve Background (CMB) polarimeter specifically
designed to search for the signal of inflationary gravitational waves in the B-mode power spectrum around
l=80. The telescope comprised a 26 cm aperture all -cold refracting optical system equipped with a focal plane
of 512 antenna coupled transition edge sensor (TES) 150 GHz bolometers each with temperature sensitivity of
approx. 300 uk.sqrt(s). BICEP2 observed from the South Pole for three seasons from 2010 to 2012. A low -
foreground region of sky with an effecti ve area of 380 square degrees was observed to a depth of 87 nK-
degrees in Stokes Q and U. In this paper we describe the observations, data reduction, maps, simulations and
results. We find an excess of B-mode power over the base lensed-LCDM expectation in the range 305σ. Through jackknife tests and simulations based on
detailed calibration measurements we show that systematic contamination is much smaller than the observed
excess. We also estimate potential foreground signals and find that available models predict these to be
considerably smaller than the observed signal. These foreground models possess no significant cross -
correlation with our maps. Additionally, cross -correlating BICEP2 against 100 GHz maps from the BICEP1
experiment, the excess signal is confirmed with 3σ significance and its spectral index is found to be consistent
with that of the CMB, disfavoring synchrotron or dust at 2.3σ and 2.2σ, respectively. The observed B-mode
power spectrum is well -fit by a lensed-LCDM + tensor theoretical model with tensor/scalar ratio
r=0.20+0.07−0.05, with r=0 disfavored at 7.0σ. Subtracting the best available estimate for foreground dust
modifies the likelihood slightly so that r=0 is disfavored at 5.9σ.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday June 13 2014, 10:30 @ seminar room 2nd floor :
Aleksey Kolmogorov on
Prediction and discovery of exotic materials with an evolutionary search ,
[1] A.N. Kolmogorov, http://maise-guide.org (2009)
[2] A.G. Van Der Geest and A.N. Kolmogorov, CALPHAD, 46 184 (2014)
[3] H. Gou, N. Dubrovinskaia, E. Bykova, A. A. Tsirlin, D. Kasinathan, A. Ric hter, M. Merlini, M. Hanfland, A. M.
Abakumov, D. Batuk, G. Van Tendeloo, Y. Nakajima, A. N. Kolmogorov, L. Dubrovinsky, PRL 111, 157002 (2013)
[4] A. N. Kolmogorov, S. Shah, E. R. Margine, A. K. Kleppe, and A. P. Jephcoat, PRL 109, 075501 (2012)
[5] A.N. Kolmogorov, S. Shah, E.R. Margine, A.F. Bialon, T. Hammerschmidt, R. Drautz PRL 105, 217003 (2010)
Abstract
In the search for new phonon-mediated superconductors, we have recently completed the most extensive ab
initio analysis of metal boride materials. A combination of high-throughput screening, targeted evolutionary
search [1], and rational design has been employed to examine over 12,000
relevant M-B candidate materials [2]. Dozens of synthesizable brand-new
structures or compounds have been identified under ambient and elevated
pressures. Some of the predictions have already been confirmed in joint
experiments [3-5]. They discovered materials include an unexpected Fe-based
phonon-mediated superconductor and a surprisingly complex high-pressure
polymorph of CaB6 .
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday May 22 2014, 13:00 @ seminar room 2nd floor :
Gerhard Unger on
Numerical methods for eigenvalue problems using contour integration ,
T. Sakurai, J. Futamura, H. Tadano: Efficient parameter estimation and implementation of a contour integral -
based eigensolver, J. Algorithms Comput. Technol. 7 (2013), no. 3, 249--269.
E. Polizzi: A density matrix-based algorithm for solving eigenvalue problems , Phys. Rev. B 79, 115112 (2009).
Abstract
Numerical methods for eigenvalue problems using contour integration have attracted recently much
attention. These methods are suitable for the approximation of all eigenvalues (and all related
eigenvectors) of an eigenvalue problem inside of a given contour in the complex plane. The basic
principle of these methods is that the invariant subspace (span of eigenvectors), which is related to
the eigenvalues inside of a given contour, can be represented by using the integral over the contour
of the resolvent. An efficient approximation of such contour integrals is possible by standard
numerical quadrature schemes, e. g., by the composite trapezoidal rule.
One important variant of these methods can be considered as a Rayleigh-Ritz type method where the
eigenvalue problem is projected to the approximated invariant subspace which is computed by the
contour integration of the resolvent. One advantage of this variant compared with a Krylov-type
method is the better and easier scalability on parallel computer architectures.
In my talk the basic principles of numerical methods for eigenvalue problems which are based on
contour integration are introduced and the Rayleigh-Ritz type variant is described.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday May 22 2014, 13:00 @ seminar room 2nd floor :
Martin Ganahl on
Characterizing Topological Order by Studying the Ground States on an Infinite Cylinder,
by L. Cincio and G. Vidal
PRL 110, 067208 (2013)
Abstract
Given a microscopic lattice Hamiltonian for a topologically ordered phase, we propose a numerical
approach to characterize its emergent anyon model and, in a chiral phase, also its gapless edge
theory. First, a tensor network representation of a complete, orthonormal set of ground states on a
cylinder of infinite length and finite width is obtained through numerical optimization. Each of these
ground states is argued to have a different anyonic flux threading through the cylinder. Then a
quasiorthogonal basis on the torus is produced by chopping off and reconnecting the tensor network
representation on the cylinder. From these two bases, and by using a number of previous results,
most notably the recent proposal of Y. Zhang et al. [Phys. Rev. B 85, 235151 (2012)] to extract the
modular U and S matrices, we obtain (i) a complete list of anyon types i, together with (ii) their
quantum dimensions di and total quantum dimension D, (iii) their fusion rules Nijk, (iv) their mutual
statistics, as encoded in the off-diagonal entries Sij of S, (v) their self-statistics or topological spins θi,
(vi) the topological central charge c of the anyon model, and, in a chiral phase (vii) the low energy
spectrum of each sector of the boundary conformal field theory. As a concrete application, we study
the hard-core boson Haldane model by using the two-dimensional density matrix renormalization
group. A thorough characterization of its universal bulk and edge properties unambiguously shows
that it realizes a ν=1/2 bosonic fractional quantum Hall state.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday May 15 2014, 13:00 @ seminar room 2nd floor :
Johann Pototschnig on
Subjective probability and quantum certainty,
by Carlton M. Caves, Christopher A. Fuchs and Ruediger Schack
arXiv:quant-ph/0608190
Abstract
In the Bayesian approach to quantum mechanics, probabilities--and thus quantum states--represent
an agent's degrees of belief, rather than corresponding to objective properties of physical systems. In
this paper we investigate the concept of certainty in quantum mechanics. Particularly, we show how
the probability-1 predictions derived from pure quantum states highlight a fundamental difference
between our Bayesian approach, on the one hand, and Copenhagen and similar interpretations on
the other. We first review the main arguments for the general claim that probabilities always
represent degrees of belief. We then argue that a quantum state prepared by some physical device
always depends on an agent's prior beliefs, implying that the probability-1 predictions derived from
that state also depend on the agent's prior beliefs. Quantum certainty is therefore always some
agent's certainty. Conversely, if facts about an experimental setup could imply agent-independent
certainty for a measurement outcome, as in many Copenhagen-like interpretations, that outcome
would effectively correspond to a preexisting system property. The idea that measurement
outcomes occurring with certainty correspond to preexisting system properties is, however, in
conflict with locality. We emphasize this by giving a version of an argument of Stairs [A. Stairs, Phil.
Sci. 50, 578 (1983)], which applies the Kochen-Specker theorem to an entangled bipartite system.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday May 8 2014, 13:00 @ seminar room 2nd floor :
Georg Huhs on
SIESTA-PEXSI: Massively parallel method for
efficient and accurate ab initio materials
simulation without matrix diagonalization
,
by Georg Huhs
Abstract
We describe a scheme for efficient large-scale electronic-structure calculations based on the
combination of the pole expansion and selected inversion (PEXSI) technique with the SIESTA method,
which uses numerical atomic orbitals within the Kohn-Sham density functional theory (KSDFT)
framework. The PEXSI technique can efficiently utilize the sparsity pattern of the Hamiltonian and
overlap matrices generated in SIESTA, and for large systems has a much lower computational
complexity than that associated with the matrix diagonalization procedure. The PEXSI technique can
evaluate the electron density, free energy, atomic forces, density of states and local density of states
without computing any eigenvalue or eigenvector of the Kohn-Sham Hamiltonian, and with accuracy
fully comparable to that obtained from the matrix diagonalization procedure for general systems,
including metallic systems at low temperature. The PEXSI method is also highly scalable, with the
recently developed massively parallel PEXSI technique making efficient usage of more than 10, 000
processors on high performance machines. We demonstrate the performance and accuracy of the
SIESTA-PEXSI method using several examples of large scale electronic structure calculations, including
1D, 2D and bulk problems with insulating, semi-metallic, and metallic character.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday April 10 2014, 13:00 @ seminar room 2nd floor :
Manuel Zingl on
Spin Density Waves in the Hubbard model - A DMFT approach,
by Robert Peters, Norio Kawakami
arXiv:1403.4315
Abstract
We analyze spin density waves (SDWs) in the Hubbard model on a square lattice within the framework of inhomogeneous dynamical mean field theory (iDMFT). Doping the half-filled Hubbard model results in a change of the antiferromagnetic N\'eel state, which exists exactly at half filling, to a phase of incommensurate SDWs. Previous studies of this phase mainly rely on static mean field calculations. In this paper, we will use large-scale iDMFT calculations to study properties of SDWs in the Hubbard model. A great advantage of iDMFT over static mean field approaches is the inclusion of local screening effects and the easy access to dynamical correlation functions. Furthermore, this technique is not restricted to the Hubbard model, but can be easily used to study incommensurate phases in various strongly correlated materials.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday March 27 2014, 13:00 @ seminar room 2nd floor :
Markus Aichhorn on
Solar-energy conversion and light emission in an atomic monolayer p–n diode,
by Andreas Pospischil, Marco M. Furchi and Thomas Mueller
Nature Nanotechnology (2014)
Abstract
The limitations of the bulk semiconductors currently used in electronic devices—rigidity, heavy weight and high costs—have recently shifted the research efforts to two-dimensional atomic crystals1 such as graphene2 and atomically thin transition-metal dichalcogenides3, 4. These materials have the potential to be produced at low cost and in large areas, while maintaining high material quality. These properties, as well as their flexibility, make two-dimensional atomic crystals attractive for applications such as solar cells or display panels. The basic building blocks of optoelectronic devices are p–n junction diodes, but they have not yet been demonstrated in a two-dimensional material. Here, we report a p–n junction diode based on an electrostatically doped5 tungsten diselenide (WSe2) monolayer. We present applications as a photovoltaic solar cell, a photodiode and a light-emitting diode, and obtain light–power conversion and electroluminescence efficiencies of ~0.5% and ~0.1%, respectively. Given recent
advances in the large-scale production of two-dimensional crystals6, 7, we expect them to profoundly impact future developments in solar, lighting and display technologies.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Thursday March 20 2014, 13:00 @ seminar room 2nd floor :
Georg Winkler on
Defining and detecting quantum speedup,
by Troels F. Ronnow, Zhihui Wang, Joshua Job, Sergio Boixo, Sergei V. Isakov, David Wecker, John M. Martinis, Daniel A. Lidar and Matthias Troyer
arXiv:1401.2910 (2010)
see also
Evidence for quantum annealing with more than one hundred qubits
Abstract
The development of small-scale digital and analog quantum devices raises the question of how to fairly assess and compare the computational power of classical and quantum devices, and of how to detect quantum speedup. Here we show how to define and measure quantum speedup in various scenarios, and how to avoid pitfalls that might mask or fake quantum speedup. We illustrate our discussion with data from a randomized benchmark test on a D-Wave Two device with up to 503 qubits. Comparing the performance of the device on random spin glass instances with limited precision to simulated classical and quantum annealers, we find no evidence of quantum speedup when the entire data set is considered, and obtain inconclusive results when comparing subsets of instances on an instance-by-instance basis. Our results for one particular benchmark do not rule out the possibility of speedup for other classes of problems and illustrate that quantum speedup is elusive and can depend on the question posed.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Wednesday March 12 2014, 12:00 @ seminar room 2nd floor :
general discussion on
We will fix this year schedule and dates.,
- Friday January 31 2014, 14:00 @ seminar room 2nd floor :
Markus Aichhorn on
Asking Photons Where They Have Been,
by A. Danan, D. Farfurnik, S. Bar-Ad and L. Vaidman
Phys. Rev. Lett. 111, 240402 (2013)
viewpoint
physicsworld comment
Abstract
We present surprising experimental evidence regarding the past of photons passing through an interferometer. The information about the positions through which the photons pass in the interferometer is retrieved from modulations of the detected signal at the vibration frequencies of mirrors the photons bounce off. From the analysis we conclude that the past of the photons is not represented by continuous trajectories, although a common sense analysis adopted in various welcher weg measurements, delayed-choice which-path experiments, and counterfactual communication demonstrations yields a single trajectory. The experimental results have a simple explanation in the framework of the two-state vector formalism of quantum theory.Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday January 24 2014, 12:00 @ seminar room 2nd floor :
Robert Triebl on
Wild swarms of midges linger at the edge of an ordering phase transition,
by A. Attanasi, A. Cavagna, L. Del Castello, I. Giardina, S. Melillo, L. Parisi, O. Pohl, B. Rossaro, E. Shen, E. Silvestri and M. Viale
arXiv:1307.5631
Abstract
The most notable hallmark of collective behaviour in biological systems is the emergence of order: individuals polarize their state, giving the stunning impression that the group behaves as one. Mating swarms of mosquitoes and midges, however, do not display global order and it is therefore unclear whether swarms are a true instance of collective behaviour or a mere epiphenomenon of the independent response of each insect to an environmental stimulus. Here, we experimentally study wild swarms of midges by measuring their susceptibility, namely the capability to collectively respond to an external perturbation. The susceptibility is way larger than that of a noninteracting system, indicating the presence of strong coordination, and it increases sharply with the swarm density, a distinctive mark of an incipient ordering phase transition. We find that swarms live at the near-critical edge of this transition, suggesting that their size and density are tuned to maximize collective response.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday January 17 2014, 14:00 @ seminar room 2nd floor :
Gerhard Dorn on
Presentation + demonstration of the very popular microcontroller Arduino and microcomputer Raspberry Pi,
introductory book for raspberry pi (what is it and how to use it)
educational book for raspberry pi (what can be done with raspberry pi in class)
introduction book for arduino
some cool raspberry pi projects
Abstract
What can you do with the Raspberry Pi, a $35 computer the size of a credit card? All sorts of things! If you're learning how to program, or looking to build new electronic projects, this hands-on guide will show you just how valuable this flexible little platform can be.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday January 10 2014, 14:00 @ seminar room 2nd floor :
Christopher Albert on
Sustainable Energy without the hot air – Energy Storage,
by David MacKay
http://www.withouthotair.com/
Abstract
The public discussion of energy options tends to be intensely emotional, polarized, mistrustful, and destructive. Every option is strongly opposed: the public seem to be anti-wind, anti-coal, anti-waste-to-energy, anti-tidal-barrage, anti-fuel-duty, and anti-nuclear. We can't be anti-everything! We need an energy plan that adds up. But there's a lack of numeracy in the public discussion of energy. Where people do use numbers, they select them to sound big, to make an impression, and to score points in arguments, rather than to aid thoughtful discussion. My motivation in writing "Sustainable Energy - without the hot air" (available both on paper, and for free in electronic form [withouthotair.com]) is to promote constructive conversations about energy, instead of the perpetual Punch and Judy show. I've tried to write an honest, educational and fun book.[2] I hope the book will help build a cross-party consensus in favour of urgently making an energy plan that adds up. "Sustainable Energy - without the
hot air" presents the numbers that are needed to answer these questions: How huge are Britain's renewable resources, compared with our current energy consumption? How big do renewable energy facilities have to be, to make a significant contribution? How big would our energy consumption be if we adopted strong efficiency measures? Which efficiency measures offer big savings, and which offer only 5 or 10%? Do new much-hyped technologies such as hydrogen or electric cars reduce energy consumption, or do they actually make our energy problem worse? Wherever possible, I answer these questions from first principles. To make the numbers comparable and comprehensible, I express all energies and powers in a single set of units: energies are measured in kilowatt-hours (the same units that you see on your electricity bills and gas bills, costing 10p a pop), and powers are measured in kilowatt-hours per day, per person. Everyday choices involve small numbers of kWh per day. If I have a hot bath, I use
5 kWh of energy. If I were to drive from Cambridge to London and back in an average car, I would use 130 kWh. Let me give you three examples of what we learn when we work out the numbers. First, switching off the phone charger. I think I first heard this idea from the BBC, the idea that one of the top ten things you should do to make a difference to your energy consumption is to switch off the phone charger when you are not using it. The truth is that leaving the phone charger switched on uses about 0.01 kWh per day. This means that switching the phone charger off for a whole day saves the same energy as is used in driving an average car for one second. Switching off phone chargers is like bailing the Titanic with a teaspoon. Second, hydrogen for transport: all hydrogen-powered transport prototypes _increase_ energy consumption compared to ordinary fossil-cars; whereas electric vehicles are significantly more energy efficient than fossil-cars. So hydrogen vehicles make our energy problem worse, and
electric vehicles make it better. Third, here are the numbers for wave power. We often hear that Britain has a "huge" wave resource. But how huge is the technical potential of wave power compared with our huge consumption? If 1000 km of Atlantic coastline were completely filled with Pelamis wave machines, the average power delivered would be 2.4 kWh per day per person. That is indeed a huge amount of power: but today's British total energy consumption is on average 125 kWh per day per person. (That's for all forms of energy: electrical, transport, heating - not just electricity.) So a country-sized wave farm would deliver an average power equal to 2% of our current power consumption. I'm not saying we should not invest in wave power. But we need to know the truth about the scale of renewables required. This message applies, sadly, to almost all renewables in Britain (wind, tide, photovoltaics, hydroelectricity, biofuels, for example): to make a substantial contribution, renewable facilities have to be
country-sized. And this is perhaps the most important message: the scale of action required to put in place a sustainable energy solution. Even if we imagine strong efficiency measures and smart technology-switches that halved our energy consumption [from 125 kWh per day per person to 60 kWh per day] (which would be lower than the per-capita consumption of any developed country today), we should not kid ourselves about the challenge of supplying 60 kWh per day without fossil fuels. Among the low-carbon energy supply options, the three with the biggest potential are wind power, nuclear power, and concentrating solar power in other peoples' deserts. And here is the scale that is required if (for simplicity) we wanted to get one third from each of these sources: we would have to build wind farms with an area equal to the area of Wales; we would have to build 50 Sizewells of nuclear power; and we would need solar power stations in deserts covering an area twice the size of Greater London. Of course I'm not
recommending this particular mix of options; there are many mixes that add up; and a more detailed story would discuss other technologies such as 'clean coal' with carbon capture and storage (as yet, unproven); and energy storage systems to cope with fluctuations of supply and demand. Whatever mix you choose, if it adds up, we have a very large building task. The simple wind/nuclear/solar mix I just mentioned would involve roughly a hundred-fold increase in wind power over 2006 [3], and a five-fold increase in nuclear power [4]; the solar power in deserts would require new long-distance cables connecting the Sahara to Surrey, with a capacity 25 times greater than the existing England-France interconnector. It's not going to be easy to make a energy plan that adds up; but it is possible. We need to get building.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday December 20 2013, 14:00 @ seminar room 2nd floor :
Johann Pototschnig on
Prevented Mortality and Greenhouse Gas Emissions from Historical and Projected Nuclear Power,
by Pushker A. Kharecha and James E. Hansen
Environ. Sci. Technol., 2013, 47 (9), pp 4889-4895
the speaker will also discuss
Recommendations for a restart of molten salt reactor development,
by R.W. Moir
Energy Conversion an dManagement, Pergamon-elsevier Science Ltd, 2008, 49, 1849-1858
Abstract
In the aftermath of the March 2011 accident at Japan’s Fukushima Daiichi nuclear power plant, the future contribution of
nuclear power to the global energy supply has become somewhat uncertain. Because nuclear power is an abundant, low-
carbon source of base-load power, it could make a large contribution to mitigation of global climate change and air
pollution. Using historical production data, we calculate that global nuclear power has prevented an average of 1.84 million
air pollution-related deaths and 64 gigatonnes of CO2-equivalent (GtCO2-eq) greenhouse gas (GHG) emissions that would
have resulted from fossil fuel burning. On the basis of global projection data that take into account the effects of the
Fukushima accident, we find that nuclear power could additionally prevent an average of 420 000–7.04 million deaths and
80–240 GtCO2-eq emissions due to fossil fuels by midcentury, depending on which fuel it replaces. By contrast, we assess
that large-scale expansion of unconstrained natural gas use would not mitigate the climate problem and would cause far
more deaths than expansion of nuclear power.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday December 20 2013, 13:15 :
13:15 PART I @ seminar room 2nd floor: Andreas Martitsch on
Introductory discussion about Positron physics (material research related),
Basic tutorial.
14:00 PART II @ lecture hall P2: Prof. Dr. Peter Mascher on
Insight into Nanostructured Materials with Positrons
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday December 06 2013, 14:00 @ seminar room 2nd floor :
Christoph Heil on
Superconductivity III:,
Unconventionsl Superconductivty,
by M.R.Norman
arXiv:1302.3176, (2013)
Abstract
A brief review of unconventional superconductivity is given, stretching from the halcyon days of
helium-3 to the modern world of Majorana fermions. Along the way, we will encounter such strange
beasts as heavy fermion superconductors, cuprates, and their iron-based cousins. Emphasis will be
put on the fact that in almost all cases, an accepted microscopic theory has yet to emerge. This is
attributed to the difficulty of constructing a theory of superconductivity outside the Migdal-
Eliashberg framework.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday November 29 2013, 14:00 @ seminar room 2nd floor :
Zhian Asadzadeh on
Superconductivity II: from BCS to high Tc
,
materials:
1) lecture notes: Carsten Timm, “Theory of Superconductivity”, TU Dresden, chapter 10 and following
available at
2) book: Michael Tinkham, “Introduction to superconductivity”
Abstract
This is the second part of our three week session on superconductivity.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday November 22 2013, 14:00 @ seminar room 2nd floor :
Anonius Dorda on
Superconductivity I: BCS theory
,
materials:
1) lecture notes: Carsten Timm, “Theory of Superconductivity”, TU Dresden, chapter 10 BCS theory
available at
2) book: Michael Tinkham, “Introduction to superconductivity”, chapter 3 The BCS Theory
Abstract
This is the first part of our three week session on superconductivity.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday November 15 2013, 14:00 @ seminar room 2nd floor :
Manuel Zingl on
From Few to Many: Observing the Formation of a Fermi Sea One Atom at a Time,
by A. N. Wenz, G. Zürn, S. Murmann, I. Brouzos, T. Lompe and S. Jochim
Science 324, 6157, 457-460 (2013)
or arXiv:1307.3443
Abstract
Knowing when a physical system has reached sufficient size for its macroscopic properties to be well described
by many-body theory is difficult. We investigated the crossover from few- to many-body physics by studying
quasi?one-dimensional systems of ultracold atoms consisting of a single impurity interacting with an increasing
number of identical fermions. We measured the interaction energy of such a system as a function of the
number of majority atoms for different strengths of the interparticle interaction. As we increased the number
of majority atoms one by one, we observed fast convergence of the normalized interaction energy toward a
many-body limit calculated for a single impurity immersed in a Fermi sea of majority particles.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday November 8 2013, 14:00 @ seminar room 2nd floor :
Martin Nuss on
Band-dependent Quasiparticle Dynamics in Single Crystals of the Ba0:6K0:4Fe2As2 Superconductor Revealed by Pump-Probe Spectroscopy,
by Darius H. Torchinsky, G. F. Chen, J. L. Luo, N. L. Wang, and Nuh Gedik
PRL 105, 027005 (2010)
or arXiv:0905.0678v2
Abstract
We report on band-dependent quasiparticle dynamics in Ba0:6K0:4Fe2As2 (TcΠ37 K) measured
using ultrafast pump-probe spectroscopy. In the superconducting state, we observe two distinct
relaxation processes: a fast component whose decay rate increases linearly with excitation density
and a slow component with an excitation density independent decay rate. We argue that these two
components reflect the recombination of quasiparticles in the two hole bands through intraband and
interband processes. We also find that the thermal recombination rate of quasiparticles increases
quadratically with temperature. The temperature and excitation density dependence of the decays
indicates fully gapped hole bands and nodal or very anisotropic electron bands.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday October 25 2013, 14:00 @ seminar room 2nd floor :
Max Sorantin on
Unveiling the Higgs mechanism to students to students,
by Giovanni Organtini
available at
Eur. J. Phys. 33 1397 (2012)
or arXiv:1207.2146
Abstract
In this paper we give the outline of a lecture given to undergraduate students aiming at
understanding why physicists are so much interested in the Higgs boson. The lecture has been
conceived for students not yet familiar with advanced physics and is suitable for several disciplines,
other than physics. The Higgs mechanism is introduced by semi-classical arguments mimicking the
basic field theory concepts, assuming the validity of a symmetry principle in the expression of the
energy of particles in a classical field. The lecture is divided in two parts: the first, suitable even to
high--school students, shows how the mass of a particle results as a dynamical effect due to the
interaction between a massless particle and a field (as in the Higgs mechanism). The audience of the
second part, much more technical, consists mainly of teachers and university students of disciplines
other than physics.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday October 18 2013, 14:00 @ seminar room 2nd floor :
Martin Ganahl on
Bosonisation for beginners â refermionisation for experts,
by Jan von Delft and Herbert Schoeller
Ann. Phys. (Leipzig) 7, 4, 225-306 (1998)
or arXiv:condMat:9805275
Abstract
This tutorial review gives an elementary and self-contained derivation of the standard identities (psi eta(x) ⌠Feta e, etc.) for abelian bosonization in 1 dimension in a system of finite size L, following and simplifying Haldane's constructive
approach. As a non-trivial application, we rigorously resolve (following Furusaki) a recent controversy regarding the tunneling density of states, dos(omega), at the site of an impurity in a Tomonaga-Luttinger liquid: we use finite-size
refermionization to show exactly that for g = 1/2 its asymptotic low-energy behavior is dos(omega) ⌠omega. This agrees with the results of Fabrizio and Gogolin and of Furusaki, but not with those of Oreg and Finkel'stein (probably because we capture
effects not included in their mean-field treatment of the Coulomb gas that they obtained by an exact mapping; their treatment of anti-commutation relations in this mapping is correct, however, contrary to recent suggestions in the
literature). â The tutorial is addressed to readers with little or no prior knowledge of bosonization, who are interested in seeing âall the detailsâ explicitly; it is written at the level of beginning graduate students, requiring only knowledge of
second quantization, but not of field theory (which is not needed here). At the same time, we hope that experts too might find useful our explicit treatment of certain subtleties that can often be swept under the rug, but are crucial for some
applications, such as the calculation of dos(omega) â these include the proper treatment of the so-called Klein factors that act as fermion-number ladder operators (and also ensure the anti-commutation of different species of fermion fields), the
âi phi(x)retention of terms of order 1/L, and a novel, rigorous formulation of finite-size refermionization of both F e and the boson field phi(x) itself.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday October 11 2013, 12:00 @ seminar room 2nd floor :
Journal Club opening about
this semester,
by Martin Nuss
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday June 28 2013, 11:30 @ lecture hall P3, 2nd floor :
Professor Strinati on
Superfluid gap of a fermionic system from the BCS to the Bose-Einstein condensate limit:approach based on a coarse-graining of the Bogoliubov-de Gennes equations.,
Abstract
Inhomogeneous superconductors can, in principle, be described in terms of the Bogoliubov-de Gennes (BdG) equations,
whose difficult implementation, however, limits their use to a few simple cases in practice. Simplifying features occur close
to the critical tempearture, where the BdG equations have been shown long ago by Gor'kov to reduce to the Ginzburg-
Landau differential equation for the gap parameter of highly overlapping Cooper pairs, which is much simpler to solve than
the original BdG equations themelves. In an analogous fashion, more recently it has also been shown that, in the limit of
strong inter-particle interaction where non-overlapping composite bosons form out of the constituent fermions, the BdG
equations reduce to the Gross-Pitaevskii (GP) equation for composite bosons at low temperature, an equation that has
played a major role in the last several years for describing Bose-Einstein condensate of trapped dilute Bose gases. The
question naturally arises whether it would be possible to find a generalized non-liinear differential equaton for the gap
parameter, that would be able to replace the BdG equation in an extended portion of the coupling vs temperature phase
diagram over which the the BCS-BEC crossover from highly overlapping Cooper pairs to dilute composite bosons can occur.
Here, a non-linear (LPDA) differential equation for the gap parameter of a superfluid Fermi system is obtained by
performing a suitable coarse graining of the BdG equations for any coupling throughout the BCS-BEC crossover and from
zero temperature up to the critical temperature, aiming at replacing the time-consuming solution of the original BdG
equations by the simpler solution of this novel equation. We perform a favorable numerical test for the practical validity of
this new LPDA equation over most the temperature-coupling phase diagram, by an explicit comparison with the full
solution of the original BdG equations for an isolated vortex. In addition, the LPDA equation reduces both to the GL
equation for highly-overlapping Cooper pairs in weak coupling close to the critical temperature and to theGP equation for
dilute composite bosons in strong coupling at low temperature.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday June 14 2013, 11:30 @ seminar room 2nd floor :
Wojciech Czart on
Phase diagrams and thermodynamic properties of the Penson-Kolb-Hubbard model: s-wave pairing vs eta-pairing,
by W. R. Czart, S. Robaszkiewicz and B. Tobijaszewska
Phys. Stat. Sol. (b) 244, No. 7, 2327â2330 (2007)
Abstract
The extended Hubbard model with pair-hopping interaction J, i.e. the so-called Penson-Kolb-Hubbard model, is
studied. In the analysis we focus on the properties of the superconducting states with Cooper-pair center-of-
mass momentum q = 0 (S-phase) and q = Q (eta-phase). The evolutions of thermodynamic and electromagnetic
characteristics at T = 0 and the critical temperatures with interaction parameters and particle concentration are
discussed for d-dimensional hypercubic lattices. In the analysis we have used a linear response theory and the
electromagnetic kernel has been evaluated within the HFA-RPA scheme. For d = 2 SQ lattices the effects of
phase fluctuations on the transition temperatures are examined within the Kosterlitz - Thouless (KT) scenario.
The KT critical temperatures Tc are calculated and compared with the ones obtained in BCS-HFA, Tp. The plots
of Tc versus the inverse square penetration depth are determined. Except for weak coupling limit these plots
have a shape similar to the experimental Uemuraâs plots obtained for various classes of short-coherence length
superconductors.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday June 7 2013, 11:30 @ seminar room 2nd floor :
Antonius Dorda on
Thermoelectric transport with electron-phonon coupling and electron-electron interaction in molecular junctions
,
by Jie Ren, Jian-Xin Zhu, James E. Gubernatis, Chen Wang and Baowen Li
Phys. Rev. B 85, 155443 (2012)
Abstract
Within the framework of nonequilibrium Green's functions, we investigate the thermoelectric
transport in a single molecular junction with electron-phonon and electron-electron interactions. By
transforming into a displaced phonon basis, we are able to deal with these interactions non-
perturbatively. Then, by invoking the weak tunneling limit, we are able to calculate the
thermoelectricity. Results show that at low temperatures, resonances of the thermoelectric figure of
merit ZT occur around the sides of resonances of electronic conductance but drops dramatically to
zero at exactly these resonant points. We find ZT can be enhanced by increasing electron-phonon
coupling and Coulomb repulsion, and an optimal enhancement is obtained when these two
interactions are competing. Our results indicate a great potential for single-molecular-junctions as
good thermoelectric devices over a wide range of temperatures.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday May 24 2013, 11:30 @ seminar room 2nd floor :
Christoph Heil on
Charge Frustration in a Tringular Triple Quantum Dot,
by M. Seo, H. K. Choi, S.-Y. Lee1, N. Kim, Y. Chung, H.-S. Sim, V. Umansky, and D. Mahalu
Phys. Rev. Lett. 110, 046803 (2013)
Abstract
We experimentally investigate the charge (isospin) frustration induced by a geometrical symmetry in
a triangular triple quantum dot. We observe the ground-state charge configurations of sixfold
degeneracy, the manifestation of the frustration. The frustration results in omnidirectional charge
transport, and it is accompanied by nearby nontrivial triple degenerate states in the charge stability
diagram. The findings agree with a capacitive interaction model. We also observe unusual transport
by the frustration, which might be related to elastic cotunneling and the interference of trajectories
through the dot. This work demonstrates a unique way of studying geometrical frustration in a
controllable way.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday May 17 2013, 11:30 @ seminar room 2nd floor :
Simon Erker on
Time Crystals,
by Frank Wilczek and Alfred Shapere
Phys. Rev. Lett. 109, 160402 (2012)
Phys. Rev. Lett. 109, 160401 (2012)
Phys. Rev. Lett. 109, 160401 (2012)
Abstract
Some subtleties and apparent difficulties associated with the notion of spontaneous breaking of time-translation symmetry in quantum mechanics are identified and resolved. A model exhibiting that phenomenon is displayed. The possibility and significance of breaking of imaginary time-translation symmetry is discussed.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
- Friday May 3 2013, 11:30 @ seminar room 2nd floor :
Faruk Geles on
Effect of crystal-field splitting and interband hybridization on the metal-insulator transitions of strongly correlated systems
,
by Alexander I. Poteryaev, Michel Ferrero, Antoine Georges, and Olivier Parcollet
Phys. Rev. B 78, 045115 (2008)
Abstract
We investigate a quarter-filled two-band Hubbard model involving a crystal-field splitting, which lifts
the orbital degeneracy as well as an interorbital hopping (interband hybridization). Both terms are
relevant to the realistic description of correlated materials such as transition-metal oxides. The
nature of the Mott metal-insulator transition is clarified and is found to depend on the magnitude of
the crystal-field splitting. At large values of the splitting, a transition from a two-band to a one-band
metal is first found as the on-site repulsion is increased and is followed by a Mott transition for the
remaining band, which follows the single-band (Brinkman-Rice) scenario well documented previously
within dynamical mean-field theory. At small values of the crystal-field splitting, a direct transition
from a two-band metal to a Mott insulator with partial orbital polarization is found, which takes
place simultaneously for both orbitals. This t - Friday March 01 2013, 12:00 @ seminar room 2nd floor :
general discussion about
this semester,
by Martin Nuss
Thanks to Red Bull for sponsoring.
ransition is characterized by a vanishing of the
quasiparticle weight for the majority orbital but has a first-order character for the minority orbital. It
is pointed out that finite-temperature effects may easily turn the metallic regime into a bad metal
close to the orbital polarization transition in the metallic phase.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
Friday April 26 2013, 11:30 @ seminar room 2nd floor :
Markus Aichhorn on
Fluctuation driven topological Hund insulator,
by Jan Carl Budich, Björn Trauzettel and Giorgio Sangiovanni
arXiv:1211.3059 (2012)
Abstract
We investigate in the framework of dynamical mean field theory a two-band Hubbard model based
on the Bernevig-Hughes-Zhang Hamiltonian describing the quantum spin Hall (QSH) effect in HgTe
quantum wells. In the presence of interaction, Friday March 01 2013, 12:00 @ seminar room 2nd floor :
general discussion about
this semester,
by Martin Nuss
Thanks to Red Bull for sponsoring.
we find that a system with topologically trivial non-
interacting parameters can be driven into a QSH phase at finite interaction strength by virtue of local
dynamical fluctuations. For very strong interaction, the system reenters a trivial insulating phase by
going through a Mott transition. We obtain the phase diagram of our model by direct calculation of
the bulk topological invariant of the interacting system in terms of its single particle Green's function.
Thanks to Basisgruppe Physik, Red Bull and ITPcp for support.
Wednesday April 17 2013, 12:45 @ seminar room 2nd floor :
Johann Pototschnig on
The Kerr-de Sitter universe,
by Sarp Akcay and Richard A Matzner
Class. Quantum Grav. 28, 8 085012 (2011)
Abstract
It is now widely accepted that the universe as we understand it is accelerating in expansion and fits the de Sitter model rather well. As
such, a realistic assumption of black holes must place them on a de Sitter background and not Minkowski as is typically done in general
relativity. The most astrophysically relevant black hole is the uncharged, rotating Kerr solution, a member of the more general Kerrâ
Newman metrics. A generalization of the rotating Kerr black hole to a solution of the Einstein's equation with a cosmological constant Î
was discovered by Carter (1973 Les Astres Occlus ed B DeWitt and C M DeWitt (New York: Gordon and Breach)). It is typically referred to as
the Kerrâde Sitter spacetime. Here, we discuss the horizon structure of this spacetime and its dependence on Î. We recall that in a Î > 0
universe, the term 'extremal black hole' refers to a black hole with angular momentum J > M2. We obtain explicit numerical results for the
black hole's maximal spin value and get a distribution of admissible Kerr holes in the (Î, spin) parameter space. We look at the conformal
structure of the extended spacetime and the embedding of the 3-geometry of the spatial hypersurfaces. In analogy with Reissnerâ
Nordströmâde Sitter spacetime, in particular by considering the Kerrâde Sitter causal structure as a distortion of the ReissnerâNordströmâ
de Sitter one, we show that spatial sections of the extended spacetime are 3-spheres containing two-dimensional topologically spherical
sections of the horizons of Kerr holes at the poles. Depending on how a t = constant 3-space is defined, these holes may be seen as black or
white holes (four possible combinations).
Thanks to Red Bull for sponsoring.
Friday April 12 2013, 11:00 @ seminar room 2nd floor :
Martin Ganahl on
Majorana Fermions,
by Jason Alicea
arXiv:1202.1293 (2012)
also using
arXiv:1206.1736 (2012)
Phys.-Usp. 44 131 (2001)
Science 33606084 (2012)
Abstract
The 1937 theoretical discovery of Majorana fermions--whose defining property is that they are their own anti-particles--has since impacted
diverse problems ranging from neutrino physics and dark matter searches to the fractional quantum Hall effect and superconductivity.
Despite this long history the unambiguous observation of Majorana fermions nevertheless remains an outstanding goal. This review article
highlights recent advances in the condensed matter search for Majorana that have led many in the field to believe that this quest may soon
bear fruit. We begin by introducing in some detail exotic `topological' one- and two-dimensional superconductors that support Majorana
fermions at their boundaries and at vortices. We then turn to one of the key insights that arose during the past few years; namely, that it is
possible to `engineer' such exotic superconductors in the laboratory by forming appropriate heterostructures with ordinary s-wave
superconductors. Numerous proposals of this type are discussed, based on diverse materials such as topological insulators, conventional
semiconductors, ferromagnetic metals, and many others. The all-important question of how one experimentally detects Majorana
fermions in these setups is then addressed. We focus on three classes of measurements that provide smoking-gun Majorana signatures:
tunneling, Josephson effects, and interferometry. Finally, we discuss the most remarkable properties of condensed matter Majorana
fermions--the non-Abelian exchange statistics that they generate and their associated potential for quantum computation.
Thanks to Red Bull for sponsoring.
Friday March 22 2013, 11:00 @ seminar room 2nd floor :
Max Sorantin on
What if time really exists?,
by Sean M. Carroll
arXiv:0811.3772 (2008)
Abstract
Despite the obvious utility of the concept, it has often been argued that time does not exist. I take the opposite perspective: let's imagine that time does exist, and the universe is described by a quantum state obeying ordinary time-dependent quantum mechanics. Reconciling this simple picture with the known facts about our universe turns out to be a non-trivial task, but by taking it seriously we can infer deep facts about the fundamental nature of reality. The arrow of time finds a plausible explanation in a Heraclitean universe," described by a quantum state eternally evolving in an infinite-dimensional Hilbert space.
Thanks to Red Bull for sponsoring.
Friday March 08 2013, 12:00 @ seminar room 2nd floor :
Christoph Heil and Martin Nuss on
Shedding light on the pairing mechanism in iron-based superconductors,
by Christoph Heil, Markus Aichhorn, Heinrich Sormann, Ewald Schachinger and Wolfgang von der Linden
see for example arXiv:1210.2593 (2012)
Abstract
Whether superconductivity in iron-pnictides and -chalcogenides stems from local or itinerant effects is a
question still debated on. In order to investigate the influence of Fermi surface nesting on the pairing
mechanism, we calculate from first-principles calculations the static and dynamic susceptibility of various
iron-based compounds. We show that the susceptibility depends sensitively on doping and pressure
application and confront our theoretical results with conclusions drawn from experiments. For instance,
our results give evidence that pairing through Fermi-nesting mechanisms alone is not sufficient to explain
the evolution of the transition temperature with pressure in FeSe.
A variational cluster approach to strongly correlated quantum systems out of equilibrium,
by Martin Nuss, Enrico Arrigoni and Wolfgang von der Linden
see for example Phys. Rev. B 86, 245119 (2012)
Abstract
The theoretical understanding of the non-equilibrium behavior of strongly correlated quantum many-
body systems is a long standing challenge, which has become increasingly relevant with the progress
made in the fields of molecular-and nano- electronics, spintronics, spectroscopy or quantum optics and
simulation. We report on the development of non-equilibrium cluster perturbation theory, and its
variational improvement, the non-equilibrium variational cluster approach for steady-state situations.
Both methods are based on the Keldysh Green's function technique which allows accessing single particle
dynamic quantities. These flexible and versatile techniques can in principle be applied to any fermionic /
bosonic lattice Hamiltonian, including multi-band and multi-impurity systems. We present results for the
steady-state of molecular / nanoscopic devices under bias including the effects of electron-electron
interactions and magnetic fields.
Thanks to Red Bull for sponsoring.
Friday March 01 2013, 12:00 @ seminar room 2nd floor :
general discussion about
this semester,
by Martin Nuss
Thanks to Red Bull for sponsoring.
- Monday January 28 2013, 12:30 @ seminar room 2nd floor :
general discussion about
Unconventional Superconductivity from Local Spin Fluctuations in the Kondo Lattice,
by Oliver Bodensiek, Rok Zitko, Matthias Vojta, Mark Jarrell and Thomas Pruschke
arXiv:1301.5556 (2013)
Abstract
The explanation of heavy-fermion superconductivity is a long-standing challenge to theory. It is commonly thought to be connected to non-local fluctuations of either spin or charge degrees of freedom and therefore of unconventional type. Here we present results for the Kondo-lattice model, a paradigmatic model to describe heavy-fermion compounds, obtained from dynamical mean-field theory which captures local correlation effects only. Unexpectedly, we find robust s-wave superconductivity in the heavy-fermion state. We argue that this novel type of pairing is tightly connected to the formation of heavy quasiparticle bands and the presence of strong local spin fluctuations.
Thanks to Red Bull for sponsoring.
- Monday January 21 2013, 12:30 @ seminar room 2nd floor :
Werner Dobrautz on
Computational Complexity and Fundamental Limitations to Fermionic Quantum Monte Carlo Simulations,
by Matthias Troyer and Uwe-Jens Wiese
Phys. Rev. Lett. 94, 170201 (2005)
Abstract
Quantum Monte Carlo simulations, while being efficient for bosons, suffer from the ânegative sign problemâ
when applied to fermionsâcausing an exponential increase of the computing time with the number of
particles. A polynomial time solution to the sign problem is highly desired since it would provide an unbiased
and numerically exact method to simulate correlated quantum systems. Here we show that such a solution is
almost certainly unattainable by proving that the sign problem is nondeterministic polynomial (NP) hard,
implying that a generic solution of the sign problem would also solve all problems in the complexity class NP in
polynomial time.
Thanks to Red Bull for sponsoring.
- Monday January 14 2013, 12:30 @ seminar room 2nd floor :
Faruk Geles on
Janus-faced influence of the Hund's rule coupling in strongly correlated materials
,
by Luca deâ Medici, Jernej Mravlje and Antoine Georges
Phys. Rev. Lett. 107, 256401 (2012)
Abstract
We show that in multiband metals the correlations are strongly affected by Hundâs rule coupling, which
depending on the filling promotes metallic, insulating or bad-metallic behavior. The quasiparticle coherence
and the proximity to a Mott insulator are influenced distinctly and, away from single- and half-filling, in
opposite ways. A strongly correlated bad metal far from a Mott phase is found there. We propose a concise
classification of 3d and 4d transition-metal oxides within which the ubiquitous occurrence of strong
correlations in Ru- and Cr-based oxides, as well as the recently measured high Néel temperatures in Tc-based
perovskites are naturally explained.
For supplementary information see review: âStrong electronic correlations from Hund's couplingâ, by
Antoine Georges, Luca de' Medici, Jernej Mravlje,
available at: arXiv:1207.3033 (2012)
Thanks to Red Bull for sponsoring.
- Monday January 7 2013, 12:30 @ seminar room 2nd floor :
Simon Erker on
Mott criticality and pseudogap in Bose-Fermi mixtures,
by Ehud Altman, Eugene Demler, Achim Rosch
arXiv: 1205.4026
Abstract
We study the Mott transition of a mixed Bose-Fermi system of ultracold atoms in an optical lattice, where the
number of (spinless) fermions and bosons adds up to one atom per lattice, n_F+n_B=1. For weak interactions, a
Fermi surface coexists with a Bose-Einstein condensate while for strong interaction the system is
incompressible but still characterized by a Fermi surface of composite fermions. At the critical point, the
spectral function of the fermions, A(k,w), exhibits a pseudo-gapped behavior, rising as |w| at the Fermi
momentum, while in the Mott phase it is fully gapped. Taking into account the interaction between the critical
modes leads at very low temperatures either to p-wave pairing or the transition is driven weakly first order.
The same mechanism should also be important in antiferromagnetic metals with a small Fermi surface.
Thanks to Red Bull for sponsoring.
- Monday December 17 2012, 12:30 @ seminar room 2nd floor :
Markus Aichhorn on
Quantum Hall Effects, Chapter IV Strong correlations and the Fractional
Quantum Hall Effect (pg 65-89),
by M. O. Goerbig
arXiv:0909.1998
This is part one of our three week session on Quantum Hall effects.
Abstract
These lecture notes yield an introduction to quantum Hall effects both for non-relativistic electrons in
conventional 2D electron gases (such as in semiconductor heterostructures) and relativistic electrons in
graphene. After a brief historical overview in chapter 1, we discuss in detail the kinetic-energy quantisation of
non-relativistic and the relativistic electrons in a strong magnetic field (chapter 2). Chapter 3 is devoted to the
transport characteristics of the integer quantum Hall effect, and the basic aspects of the fractional quantum
Hall effect are described in chapter 4. In chapter 5, we briefly discuss several multicomponent quantum Hall
systems, namely the quantum Hall ferromagnetism, bilayer systems and graphene that may be viewed as a
four-component system.
Thanks to Red Bull for sponsoring.
- Monday December 03 2012, 12:30 @ seminar room 2nd floor :
Christoph Heil on
Topological insulators from the Perspective of first-principles calculations,
by Haijun Zhang and Shou-Cheng Zhang
arXiv:1209.6446
Abstract
Topological insulators are new quantum states with helical gapless edge or surface states inside the bulk band
gap.These topological surface states are robust against the weak time-reversal invariant perturbations, such as
lattice distortions and non-magnetic impurities. Recently a variety of topological insulators have been
predicted by theories, and observed by experiments. First-principles calculations have been widely used to
predict topological insulators with great success. In this review, we summarize the current progress in this field
from the perspective of first-principles calculations. First of all, the basic concepts of topological insulators and the frequently-used techniques within first-principles calculations are briefly introduced. Secondly, we
summarize general methodologies to search for new topological insulators. In the last part, based on the band
inversion picture first introduced in the context of HgTe, we classify topological insulators into three types with s-p, p-p and d-f, and discuss some representative examples for each type.
Thanks to Red Bull for sponsoring.
- Monday November 26 2012, 12:30 @ seminar room 2nd floor :
Martin Nuss on
second part: Quantum Hall Effects, Chapter III Integer Quantum Hall Effect (pg 43-65),
by M. O. Goerbig
arXiv:0909.1998
This is part one of our three week session on Quantum Hall effects.
Abstract
These lecture notes yield an introduction to quantum Hall effects both for non-relativistic electrons in
conventional 2D electron gases (such as in semiconductor heterostructures) and relativistic electrons in
graphene. After a brief historical overview in chapter 1, we discuss in detail the kinetic-energy quantisation of
non-relativistic and the relativistic electrons in a strong magnetic field (chapter 2). Chapter 3 is devoted to the
transport characteristics of the integer quantum Hall effect, and the basic aspects of the fractional quantum
Hall effect are described in chapter 4. In chapter 5, we briefly discuss several multicomponent quantum Hall
systems, namely the quantum Hall ferromagnetism, bilayer systems and graphene that may be viewed as a
four-component system.
Thanks to Red Bull for sponsoring.
- Monday November 19 2012, 12:30 @ seminar room 2nd floor :
Martin Nuss on
first part: Quantum Hall Effects, Chapter III Integer Quantum Hall Effect (pg 43-65),
by M. O. Goerbig
arXiv:0909.1998
This is part one of our three week session on Quantum Hall effects.
Abstract
These lecture notes yield an introduction to quantum Hall effects both for non-relativistic electrons in
conventional 2D electron gases (such as in semiconductor heterostructures) and relativistic electrons in
graphene. After a brief historical overview in chapter 1, we discuss in detail the kinetic-energy quantisation of
non-relativistic and the relativistic electrons in a strong magnetic field (chapter 2). Chapter 3 is devoted to the
transport characteristics of the integer quantum Hall effect, and the basic aspects of the fractional quantum
Hall effect are described in chapter 4. In chapter 5, we briefly discuss several multicomponent quantum Hall
systems, namely the quantum Hall ferromagnetism, bilayer systems and graphene that may be viewed as a
four-component system.
Thanks to Red Bull for sponsoring.
- Monday November 12 2012, 12:30 @ seminar room 2nd floor :
Martin Ganahl on
Quantum Hall Effects, Chapter II Landau Quantisation (pg 21-43),
by M. O. Goerbig
arXiv:0909.1998
This is part one of our three week session on Quantum Hall effects.
Abstract
These lecture notes yield an introduction to quantum Hall effects both for non-relativistic electrons in
conventional 2D electron gases (such as in semiconductor heterostructures) and relativistic electrons in
graphene. After a brief historical overview in chapter 1, we discuss in detail the kinetic-energy quantisation of
non-relativistic and the relativistic electrons in a strong magnetic field (chapter 2). Chapter 3 is devoted to the
transport characteristics of the integer quantum Hall effect, and the basic aspects of the fractional quantum
Hall effect are described in chapter 4. In chapter 5, we briefly discuss several multicomponent quantum Hall
systems, namely the quantum Hall ferromagnetism, bilayer systems and graphene that may be viewed as a
four-component system.
Thanks to Red Bull for sponsoring.
- Monday November 05 2012, 12:30 @ seminar room 2nd floor :
Andreas Martitsch on
Mysteries of Astronomy - What Is Dark Energy? How Hot Is Dark Matter?,
by Robert Coontz and Adrian Cho
Science 336, 6085, 1090 (2012)
The speaker will focus on the first two contributions by Adrian Cho:
What Is Dark Energy?
SHow Hot Is Dark Matter?
Abstract
Endless mysteries lurk in the depths of space. To pare the list down to eight now, there's a challenge. In deciding what to
include in this section, Science's news staff teamed up with Science Associate Editor Maria Cruz and consulted researchers
on the Board of Reviewing Editors and elsewhere. From the outset, the team decided that true mysteries must have staying
power (as opposed to mere questions that researchers might resolve in the near future). Some of the finalists are obvious
shoo-ins; others have received less of the popular limelight. The final selection spans the entire history of the universe on
scales ranging from our sun and its planetary system to the entire cosmos. Each mystery is sure to be solved largely through
astronomical observations if it is solved: In at least one case, experts aren't sure that a seemingly simple question will ever
be answered. (What Is Dark Energy?, How Hot Is Dark Matter?, Where Are the Missing Baryons?, How Do Stars Explode?,
What Reionized the Universe?, What's the Source of the Most Energetic Cosmic Rays?, Why Is the Solar System So Bizarre?,
Why Is the Sun's Corona So Hot?)
Thanks to Red Bull for sponsoring.
- Monday October 22 2012, 12:30 @ seminar room 2nd floor :
Prof. Bernhard Schnizer on
Stark-Zeemann-Effect in the HyperfineStructure of AlkaliAtoms. Level Crossings, Avoided Level Crossings, Geometric Phase: Theoretical investigations on the Stark-Zeeman effect of the 2p 2P3/2-level in 6Li for perpendicularly crossed fields,
by E. Roessl, B. Schnizer, and M. Musso
Eur. Phys. J. D 37, 187-200 (2006)
Abstract
The splitting behaviour of the 2p 2P3/2 hyperfine structure levels is investigated in 6Li for homogeneous crossed
electric and magnetic fields (Stark-Zeeman effect). This is done by diagonalizing the perturbation matrix
comprising the hyperfine interaction, the electronic and nuclear magnetic interaction and the effective electric
interaction obtained by transforming the quadratic Stark effect to a first order perturbation interaction. Symmetries
are used to find analytic formulae for level shifts and crossing points if only one external field is present. A
reflection symmetry unbroken with all three interactions present permits the decomposition of the 12 Ã 12 matrix
into two 6 Ã 6 submatrices. The structure of energy eigenvalue surfaces I_F_MF (B,E) of the two subsystems is
found by numeric diagonalization of the perturbation matrix and is displayed in the ranges |B| < 1 mT, |E| < 300
kV/cm. The total angular momentum F = J + I (J = 3/2), electronic angular momentum, I = 1, nuclear spin) and the
magnetic quantum number MF provide labels for all surfaces. All crossing points of the energy surfaces have been
found. Adiabatic level transfer occurring in atoms traversing a sequence of crossed magnetic and electric fields is explained. Berry phases occur for cycles around some crossing points. Their presence or absence is explained.
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- Monday October 15 2012, 12:30 @ seminar room 2nd floor :
Michael Knap on
Nobel Prize in Physics 2012: MEASURING AND MANIPULATING INDIVIDUAL QUANTUM SYSTEMS,
Serge Haroche and David J. Wineland
www.nobelprize.org (2012) Advanced Press Release
see also popular information:
www.nobelprize.org (2012) Popular Press Release
Abstract
"for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems"
Serge Haroche and David J. Wineland have independently invented and developed methods for measuring and manipulating individual particles while preserving their quantum-mechanical nature, in ways that were previously thought unattainable. The Nobel Laureates have opened the door to a new era of experimentation with quantum physics by demonstrating the direct observation of individual quantum particles without destroying them. For single particles of light or matter the laws of classical physics cease to apply and quantum physics
takes over. But single particles are not easily isolated from their surrounding environment and they lose their mysterious quantum properties as soon as they interact with the outside world. Thus many seemingly bizarre phenomena predicted by quantum physics could not be directly observed, and researchers could only carry out thought experiments that might in principle manifest these bizarre phenomena. Through their ingenious laboratory methods Haroche and Wineland together with their research groups have managed to measure and control very fragile quantum states, which were previously thought inaccessible for direct observation. The new methods
allow them to examine, control and count the particles. Their methods have many things in common. David Wineland traps electrically charged atoms, or ions, controlling and measuring them with light, or photons. Serge Haroche takes the opposite approach: he controls and measures trapped photons, or particles of light, by
sending atoms through a trap. Both Laureates work in the field of quantum optics studying the fundamental interaction between light and matter, a field which has seen considerable progress since the mid-1980s. Their ground-breaking methods have enabled this field of research to take the very first steps towards building a new type of super fast computer based on quantum physics. Perhaps the quantum computer will change our everyday lives in this century in the same radical way as the classical computer did in the last century. The research has also led to the construction of extremely precise clocks that could become the future basis for a new standard of time, with more than hundred-fold greater precision than present-day caesium clocks.
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- Thursday October 11 2012, 12:30 @ seminar room 2nd floor :
Max Sorantin on
On the reality of the quantum state ,
Matthew F. Pusey, Jonathan Barrett and Terry Rudolph
Nat. Phys. 8, 476-479 (2012)
Abstract
Quantum states are the key mathematical objects in quantum theory. It is therefore
surprising that physicists have been unable to agree on what a quantum state truly
represents. One possibility is that a pure quantum state corresponds directly to reality.
However, there is a long history of suggestions that a quantum state (even a pure state)
represents only knowledge or information about some aspect of reality. Here we show that
any model in which a quantum state represents mere information about an underlying
physical state of the system, and in which systems that are prepared independently have
independent physical states, must make predictions that contradict those of quantum
theory.
Thanks to Red Bull for sponsoring.
- Thursday October 4 2012, 12:30 @ seminar room 2nd floor :
Martin Nuss on
Setting up the schedule for WS2012/2013 ,
including free snacks and drinks
Thanks to Red Bull for sponsoring.
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