- 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 (TcŒ 37 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) âŒ 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) âŒ 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, BjÃ¶rn 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
NordstrÃ¶mâde Sitter spacetime, in particular by considering the Kerrâde Sitter causal structure as a distortion of the ReissnerâNordstrÃ¶mâ
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 NÃ©el 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.
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- 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?)
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- 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.
Thanks to Red Bull for sponsoring.
- 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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