Schedule

WS 2014/2015

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.

SS 2013/2014

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.,

WS 2013/2014

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.

SS 2012/2013

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.

WS 2012/2013

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.
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.
Thanks to Red Bull for sponsoring.

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.

SS 2011/2012

Friday June 22 2012, 12:00 @ seminar room 2nd floor :
general discussion
Visualizing heavy fermions emerging in a quantum critical Kondo lattice ,
Pegor Aynajia, Eduardo H. da Silva Neto, Andras Gyenis, Ryan E Baumbach, J.D. Thompson, Zachery Fisk, ERic D. Bauer and Ali Yazdani
Nature 486, 201-206 (2012)

Abstract
In solids containing elements with f orbitals, the interaction between f-electron spins and those of itinerant electrons leads to the development of low-energy fermionic excitations with a heavy effective mass. These excitations are fundamental to the appearance of unconventional superconductivity and non-Fermi-liquid behaviour observed in actinide- and lanthanide-based compounds. Here we use spectroscopic mapping with the scanning tunnelling microscope to detect the emergence of heavy excitations with lowering of temperature in a prototypical family of cerium-based heavy-fermion compounds. We demonstrate the sensitivity of the tunnelling process to the composite nature of these heavy quasiparticles, which arises from quantum entanglement of itinerant conduction and f electrons. Scattering and interference of the composite quasiparticles is used to resolve their energy-momentum structure and to extract their mass enhancement, which develops with decreasing temperature. The lifetime of the emergent heavy quasiparticles reveals signatures of enhanced scattering and their spectral lineshape shows evidence of energy-temperature scaling. These findings demonstrate that proximity to a quantum critical point results in critical damping of the emergent heavy excitation of our Kondo lattice system.
Thanks to Red Bull for sponsoring.

Friday June 15 2012, 12:00 @ seminar room 2nd floor :
Benjamin Kollmitzer on
Structure and Elasticity of Liquid Ordered Lipid Domains ,
Michael Rappolt and Georg Pabst

Abstract
Biological membranes regulate diverse processes on the cellular level, including material transport, signal transduction and various chemical reactions. An about 4 nm thin layer of phospholipids and proteins is the central element of these membranes and there is ample evidence that lipids and proteins mutually regulate their function. Of particular interest is the organization of lipids and proteins into functional platforms, also known as rafts, to perform specific signaling or transport tasks. Sphingomyelin and cholesterol are considered to be the major lipid components of these rafts. Clearly, basic physicochemical understanding of raft properties is of need for ground-breaking developments in human health care. Biophysical studies on complex mixtures of "raft lipids" can contribute significantly to such insights. These mixtures show phase separation into coexisting liquid ordered ($L_o$) and liquid disordered ($L_\alpha$) domains. $L_o$ domains show several features of membrane rafts, including enrichment in cholesterol and capability of glycophosphatidylinositol (GPI)- anchored proteins to insert into these lipid environments. However, the recently discovered exclusion of raft-associating transmembrane proteins (TMP) from $L_o$ domains questions their applicability as models for membrane rafts. The aim of the project is to derive lipid-only models for membrane rafts that recover the partitioning of raft-associating TMP, thus creating improved and experimentally stringently controlled lipid-only models of membrane rafts. The goal will be achieved by deriving those structural and elastic properties of $L_o$ and $L_\alpha$ domains that couple to partitioning of TMP using a broad selection of biophysical techniques. In particular, we will determine lateral packing density (area per lipid), membrane thickness, spontaneous curvature, bending rigidity and Gaussian curvature modulus for several lipid mixtures, from which we will calculate the preferential insertion of protein models into a given lipid environment. This way we will understand why TMP proteins do not partition into current lipid-only raft models and how to tune $L_o$ phase properties such that TMP partition into these environments. Our results will the basis for several follow-up applications and will thus be an important contribution to close the gap between membrane biophysics and cell biology.

Friday June 01 2012, 12:00 @ seminar room 2nd floor :
Markus Aichhorn on
Electron Correlations beyond Coulomb: Strong Magnetism due to Hund's exchange ,
J. Mravlje, M. Aichhorn, and A. Georges
Phys. Rev. Lett. 108, 197202 (2012)

Abstract
In the quest for new materials with novel functionalities materials exhibiting strong electronic correlations play a prominent role. For instance, unconventional superconductors or multiferroic materials are well known, intensively studied, and very promising for potential applications. Although quite some knowledge on different strongly correlated systems has been aquired during the last decades, nature always has something up its sleeve. The key properties of these materials arise due to the correlations between electrons, making the description within single-electron approaches difficult. In order to get a more quantitave, or even predictive description one has to treat the underlying many-body problem properly. A method of choice that is able to deal with strong correlations is the combination of Density Functional Theory with the Dynamical Mean-Field Theory. In this talk, after introducing the basic notions of the method, we will focus on a class of materials of recent interest, perovskites based on technetium. Contrary to what has been assumed for many years, not only 3d electrons can be strongly correlated, but also 4d (or even compounds with open 5d shells) can show effects of electronic correlations. The origin of those correlations, however, can be traced back to different mechanisms, such as the Hund's exchange for 4d, or spin-orbit coupling for 5d elements. We will discuss the unexpectedly and extraordinarily high Neel temperature in technetium compounds, and argue that the reason for the strong magnetism can be found in the vicinity of the compound to a localised/itinerant transition. This location of technetium is special among 4d elements. There, Hund's exchange is very effective and is, thus, the main mechanism determining the electronic properties of this system.

Friday May 25 2012, 12:00 @ seminar room 2nd floor :
Faruk Geles on
Quantum phase transitions from a superfluid to a Mott insulator in a gas of ultracold atoms ,
Markus Greiner, Olaf Mandel, Tilman Esslinger, Theodor W. HÃ€nsch and Immanuel Bloch
Nature 415, 39-44 (2002)

Abstract
For a system at a temperature of absolute zero, all thermal fluctuations are frozen out, while quantum fluctuations prevail. These microscopic quantum fluctuations can induce a macroscopic phase transition in the ground state of a many-body system when the relative strength of two competing energy terms is varied across a critical value. Here we observe such a quantum phase transition in a BoseâEinstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. As the potential depth of the lattice is increased, a transition is observed from a superfluid to a Mott insulator phase. In the superfluid phase, each atom is spread out over the entire lattice, with long-range phase coherence. But in the insulating phase, exact numbers of atoms are localized at individual lattice sites, with no phase coherence across the lattice; this phase is characterized by a gap in the excitation spectrum. We can induce reversible changes between the two ground states of the system.

Friday May 11 2012, 12:00 @ seminar room 2nd floor :
Klaus Lang on
Remote Joule heating by a carbon nanotube ,
Kamal H. Baloch, Norvik Voskanian, Merijntje Bronsgeest and John Cumings
Nature Nanotechnology (April, 2012) @ pdf

Abstract
Minimizing Joule heating remains an important goal in the design of electronic devices. The prevailing model of Joule heating relies on a simple semiclassical picture in which electrons collide with the atoms of a conductor, generating heat locally and only in regions of non-zero current density, and this model has been supported by most experiments. Recently, however, it has been predicted that electric currents in graphene and carbon nanotubes can couple to the vibrational modes of a neighbouring material, heating it remotely. Here, we use in situ electron thermal microscopy to detect the remote Joule heating of a silicon nitride substrate by a single multiwalled carbon nanotube. At least 84% of the electrical power supplied to the nanotube is dissipated directly into the substrate, rather than in the nanotube itself. Although it has different physical origins, this phenomenon is reminiscent of induction heating or microwave dielectric heating. Such an ability to dissipate waste energy remotely could lead to improved thermal management in electronic devices.

Friday May 4 2012, 12:00 @ seminar room 2nd floor :
Martin Ganahl on the relation between integrability and relaxation dynamics using two publications:
Thermalization and its mechanism for generic isolated quantum systems ,
Marcos Rigol, Vanja Dunjko1 and Maxim 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, 2 of the problem have become possible, stimulating theoretical interest3, 4, 5, 6, 7. In generic isolated systems, non-equilibrium dynamics is expected8, 9 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 possible10. For example, dynamical chaos itself cannot occur in an isolated quantum system, in which the time evolution is linear and the spectrum is discrete11. Some recent studies4, 5 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 Deutsch12 and Srednicki13. 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 averagesâany eigenstate in the microcanonical energy window will do, because they all give the same result.

Relaxation in a Completely Integrable Many-Body Quantum System: An Ab Initio Study of the Dynamics of the Highly Excited States of 1D Lattice Hard-Core Bosons ,
Marcos Rigol, Vanja Dunjko, Vladimir Yurovsky and Maxim Olshanii
Phys. Rev. Lett. 98, 050405 (2007)

Abstract
In this Letter we pose the question of whether a many-body quantum system with a full set of conserved quantities can relax to an equilibrium state, and, if it can, what the properties of such a state are. We confirm the relaxation hypothesis through an ab initio numerical investigation of the dynamics of hard-core bosons on a one-dimensional lattice. Further, a natural extension of the Gibbs ensemble to integrable systems results in a theory that is able to predict the mean values of physical observables after relaxation. Finally, we show that our generalized equilibrium carries more memory of the initial conditions than the usual thermodynamic one. This effect may have many experimental consequences, some of which have already been observed in the recent experiment on the nonequilibrium dynamics of one-dimensional hard-core bosons in a harmonic potential [T. Kinoshita et al., Nature (London) 440 900 (2006)].

Friday Apr 7 2012, 12:00 @ seminar room 2nd floor :
general discussion about
Probing the relaxation towards equilibrium in an isolated strongly correlated one-dimensional Bose gas ,
S. Trotzky, Y-A. Chen, A. Flesch, I. P. McCulloch, U. Schollwoeck, J. Eisert and I. Bloch
Nature Physics 8, 325-330 (2012)

Abstract
The problem of how complex quantum systems eventually come to rest lies at the heart of statistical mechanics. The maximum-entropy principle describes which quantum states can be expected in equilibrium, but not how closed quantum many-body systems dynamically equilibrate. Here, we report the experimental observation of the non-equilibrium dynamics of a density wave of ultracold bosonic atoms in an optical lattice in the regime of strong correlations. Using an optical superlattice, we follow its dynamics in terms of quasi-local densities, currents and coherencesâall showing a fast relaxation towards equilibrium values. Numerical calculations based on matrix-product states are in an excellent quantitative agreement with the experimental data. The system fulfills the promise of being a dynamical quantum simulator, in that the controlled dynamics runs for longer times than present classical algorithms can keep track of.

Friday Apr 20 2012, 12:00 @ seminar room 2nd floor :
Johann Pototschnig on
Resonant quantum transitions in trapped antihydrogen atoms ,
C. Amole, M. D. Ashkezari, M. Baquero-Ruiz, W. Bertsche, P. D. Bowe, E. Butler, A. Capra, C. L. Cesar, M. Charlton, A. Deller, P. H. Donnan, S. Eriksson, J. Fajans, T. Friesen, M. C. Fujiwara, D. R. Gill, A. Gutierrez, J. S. Hangst, W. N. Hardy, M. E. Hayden, A. J. Humphries, C. A. Isaac, S. Jonsell, L. Kurchaninov, A. Little, N. Madsen, J. T. K. McKenna, S. Menary, S. C. Napoli, P. Nolan, K. Olchanski, A. Olin, P. Pusa, C. O. Rasmussen, F. Robicheaux, E. Sarid, C. R. Shields, D. M. Silveira, S. Stracka, C. So, R. I. Thompson, D. P. van der Werf and J. S. Wurtele
Nature 483, 439-443 (2012)

Abstract
The hydrogen atom is one of the most important and influential model systems in modern physics. Attempts to understand its spectrum are inextricably linked to the early history and development of quantum mechanics. The hydrogen atomâs stature lies in its simplicity and in the accuracy with which its spectrum can be measured and compared to theory. Today its spectrum remains a valuable tool for determining the values of fundamental constants and for challenging the limits of modern physics, including the validity of quantum electrodynamics andâby comparison with measurements on its antimatter counterpart, antihydrogenâthe validity of CPT (charge conjugation, parity and time reversal) symmetry. Here we report spectroscopy of a pure antimatter atom, demonstrating resonant quantum transitions in antihydrogen. We have manipulated the internal spin state of antihydrogen atoms so as to induce magnetic resonance transitions between hyperfine levels of the positronic ground state. We used resonant microwave radiation to flip the spin of the positron in antihydrogen atoms that were magnetically trapped in the ALPHA apparatus. The spin flip causes trapped anti-atoms to be ejected from the trap. We look for evidence of resonant interaction by comparing the survival rate of trapped atoms irradiated with microwaves on-resonance to that of atoms subjected to microwaves that are off-resonance. In one variant of the experiment, we detect 23 atoms that survive in 110 trapping attempts with microwaves off-resonance (0.21 per attempt), and only two atoms that survive in 103 attempts with microwaves on-resonance (0.02 per attempt). We also describe the direct detection of the annihilation of antihydrogen atoms ejected by the microwaves.

Friday Mar 30 2012, 12:00 @ seminar room 2nd floor :
Max Sorantin on
Demonstration of temporal cloaking ,
Moti Fridman, Alessandro Farsi, Yoshitomo Okawachi, Alexander L. Gaeta
Nature 481, 62-65 (2012)
There is some introduction available at
Nature 481, 35-36 (2012)

Abstract
Recent research has uncovered a remarkable ability to manipulate and control electromagnetic fields to produce effects such as perfect imaging and spatial cloaking. To achieve spatial cloaking, the index of refraction is manipulated to flow light from a probe around an object in such a way that a âholeâ in space is created, and the object remains hidden. Alternatively, it may be desirable to cloak the occurrence of an event over a finite time period, and the idea of temporal cloaking has been proposed in which the dispersion of the material is manipulated in time, producing a âtime holeâ in the probe beam to hide the occurrence of the event from the observer. This approach is based on accelerating the front part of a probe light beam and slowing down its rear part to create a well controlled temporal gapâinside which an event occursâsuch that the probe beam is not modified in any way by the event. The probe beam is then restored to its original form by the reverse manipulation of the dispersion. Here we present an experimental demonstration of temporal cloaking in an optical fibre-based system by applying concepts from the spaceâtime duality between diffraction and dispersive broadening. We characterize the performance of our temporal cloak by detecting the spectral modification of a probe beam due to an optical interaction and show that the amplitude of the event (at the picosecond timescale) is reduced by more than an order of magnitude when the cloak is turned on. These results are a significant step towards the development of full spatio-temporal cloaking.

Friday Mar 23 2012, 12:00 @ seminar room 2nd floor :
general discussion about
Sculpting oscillators with light within a nonlinear quantum fluid ,
G. Tosi, G. Christmann, N. G. Berloff, P. Tsotsis, T. Gao, Z. Hatzopoulos, P. G. Savvidis and J. J. Baumberg
Nature Physics 8, 190-194 (2012)

Abstract
Seeing macroscopic quantum states directly remains an elusive goal. Particles with boson symmetry can condense into quantum fluids, producing rich physical phenomena as well as proven potential for interferometric devices1. However, direct imaging of such quantum states is only fleetingly possible in high-vacuum ultracold atomic condensates, and not in superconductors. Recent condensation of solid-state polariton quasiparticles, built from mixing semiconductor excitons with microcavity photons, offers monolithic devices capable of supporting room-temperature quantum states that exhibit superfluid behaviour. Here we use microcavities on a semiconductor chip supporting two-dimensional polariton condensates to directly visualize the formation of a spontaneously oscillating quantum fluid. This system is created on the fly by injecting polaritons at two or more spatially separated pump spots. Although oscillating at tunable THz frequencies, a simple optical microscope can be used to directly image their stable archetypal quantum oscillator wavefunctions in real space. The self-repulsion of polaritons provides a solid-state quasiparticle that is so nonlinear as to modify its own potential. Interference in time and space reveals the condensate wavepackets arise from non-equilibrium solitons. Control of such polariton-condensate wavepackets demonstrates great potential for integrated semiconductor-based condensate devices.

Friday Mar 16 2012, 12:00 @ seminar room 2nd floor:
Martin Nuss on
Phonon-mediated superconductivity in graphene by lithium deposition ,
Gianni Profeta, Matteo Calandra and Francesco Mauri
Nature Physics 8, 131-134 (2012)

Abstract
Graphene is the physical realization of many fundamental concepts and phenomena in solid-state physics. However, in the list of grapheneâs many remarkable properties superconductivity is notably absent. If it were possible to find a way to induce superconductivity, it could improve the performance and enable more efficient integration of a variety of promising device concepts including nanoscale superconducting quantum interference devices, single-electron superconductorâquantum dot devices nanometre-scale superconducting transistors and cryogenic solid-state coolers. To this end, we explore the possibility of inducing superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms, in a manner analogous to which superconductivity is induced in graphite intercalated compounds (GICs). As for GICs, we find that the electrical characteristics of graphene are sensitive to the species of adatom used. However, contrary to what happens in GICs, Li-covered graphene is superconducting at a much higher temperature with respect to Ca-covered graphene.

WS 2011/2012

Friday Jan 27 2012, 12:30 @ seminar room 2nd floor:
Christoph Heil on
Electrons surfing on a sound wave as a platform for quantum optics with flying electrons ,
Sylvain Hermelin, Shintaro Takada, Michihisa Yamamoto, Seigo Tarucha, Andreas D. Wieck, Laurent Saminadayar, Christopher Bï¿œuerle and Tristan Meunier
Nature 477, 435-438 (2011)

Abstract
Electrons in a metal are indistinguishable particles that interact strongly with other electrons and their environment. Isolating and detecting a single flying electron after propagation, in a similar manner to quantum optics experiments with single photons is therefore a challenging task. So far only a few experiments have been performed in a high-mobility two-dimensional electron gas in which the electron propagates almost ballistically. In these previous works, flying electrons were detected by means of the current generated by an ensemble of electrons, and electron correlations were encrypted in the current noise. Here we demonstrate the experimental realization of high-efficiency single-electron source and detector for a single electron propagating isolated from the other electrons through a one-dimensional channel. The moving potential is excited by a surface acoustic wave, which carries the single electron along the one-demensional channel at a speed of 3mum ns^-1. When this quantum channel is placed between two quantum dots several micrometres apart, a single electron can be transported from one quantum dot to the other with quantum efficiencies of emission and detection of 96% and 92%, respectively. Furthermore, the transfer of the electron can be triggered on a timescale shorter than the coherence time T2* of GaAs spin qubits. Our work opens new avenues with which to study the teleportation of a single electron spin and the distant interaction between spatially separated qubits in a condensed-matter system.

Friday Jan 13 2012, 12:00 @ seminar room 2nd floor:
general discussion about
Tail-assisted pitch control in lizards, robots and dinosaurs ,
Thomas Libby, Talia Y. Moore, Evan Chang-Siu, Deborah Li, Daniel J. Cohen, Ardian Jusufi and Robert J. Full
Nature 481, 181-184 (2012)

Abstract
In 1969, a palaeontologist proposed that theropod dinosaurs used their tails as dynamic stabilizers during rapid or irregular movements, contributing to their depiction as active and agile predators. Since then the inertia of swinging appendages has been implicated in stabilizing human walking aiding acrobatic manoeuvres by primates and rodents, and enabling cats to balance on branches. Recent studies on geckos suggest that active tail stabilization occurs during climbing, righting and gliding. By contrast, studies on the effect of lizard tail loss show evidence of a decrease, an increase or no change in performance. Application of a control-theoretic framework could advance our general understanding of inertial appendage use in locomotion. Here we report that lizards control the swing of their tails in a measured manner to redirect angular momentum from their bodies to their tails, stabilizing body attitude in the sagittal plane. We video-recorded Red-Headed Agama lizards (Agama agama) leaping towards a vertical surface by first vaulting onto an obstacle with variable traction to induce a range of perturbations in body angular momentum. To examine a known controlled tail response, we built a lizard-sized robot with an active tail that used sensory feedback to stabilize pitch as it drove off a ramp. Our dynamics model revealed that a body swinging its tail experienced less rotation than a body with a rigid tail, a passively compliant tail or no tail. To compare a range of tails, we calculated tail effectiveness as the amount of tailless body rotation a tail could stabilize. A model Velociraptor mongoliensis supported the initial tail stabilization hypothesis1, showing as it did a greater tail effectiveness than the Agama lizards. Leaping lizards show that inertial control of body attitude can advance our understanding of appendage evolution and provide biological inspiration for the next generation of manoeuvrable search-and-rescue robots.

Friday Dec 16 2011, 12:30 @ seminar room 2nd floor:
Klaus Lang on
Tie knots, random walks and topology ,
Thomas M. A. Fink and Yong Mao
Physica A 276, 1-2, 109-121 (2000)

Abstract
Necktie knots are inherently topological structures; what makes them tractable is the particular manner in which they are constructed. This observation motivates a map between tie knots and persistent walks on a triangular lattice. The topological structure embedded in a tie knot may be determined by appropriately manipulating its projection; we derive corresponding rules for tie knot sequences. We classify knots according to their size and shape and quantify the number of knots in a class. Aesthetic knots are characterised by the conditions of symmetry and balance. Of the 85 knots which may be tied with conventional tie, we recover the four traditional knots and introduce nine new aesthetic ones. For large (though impractical) half-winding number, we present some asymptotic results.

Friday Dec 02 2011, 12:00 @ seminar room 2nd floor:
Johann Pototschnig on
Electronically excited rubidium atom in a helium cluster or film ,
Markku Leino, Alexandra Viel, and Robert E. Zillich
J. Chem. Phys. 129, 184308 (2008)

Abstract
We present theoretical studies of helium droplets and films doped with one electronically excited rubidium atom Rb* (2P). Diffusion and path integral Monte Carlo approaches are used to investigate the energetics and the structure of clusters containing up to 14 helium atoms. The surface of large clusters is approximated by a helium film. The nonpair additive potential energy surface is modeled using a diatomic in molecule scheme. Calculations show that the stable structure of Rb*Hen consists of a seven helium atom ring centered at the rubidium, surrounded by a tirelike second solvation shell. A very different structure is obtained when performing a vertical Monte Carlo transition. In this approach, a path integral Monte Carlo equilibration starts from the stable configuration of a rubidium atom in the electronic ground state adsorbed to the helium surface after switching to the electronically excited surface. In this case, Rb*Hen relaxes to a weakly bound metastable state in which Rb* sits in a shallow dimple. The interpretation of the results is consistent with the recent experimental observations [ G. Aubï¿œck et al., Phys. Rev. Lett. 101, 035301 (2008) ].

Friday Nov 25 2011, 12:00 @ seminar room 2nd floor:
Benjamin Kollmitzer on
Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation ,
Jasper Kirkby, Joachim Curtius, Joao Almeida, Eimear Dunne, Jonathan Duplissy, Sebastian Ehrhart, Alessandro Franchin, Stephanie Gagne, Luisa Ickes, Andreas Kï¿œrten, Agnieszka Kupc,xel Metzger, Francesco Riccobono, Linda Rondo, Siegfried Schobesberger, Georgiossagkogeorgas, Daniela Wimmer, Antonio Amorim, Federico Bianchi, Martinreitenlechner, Andre David, Josef Dommen, Andrew Downard, Mikael Ehn, Richard C.lagan, Stefan Haider, Armin Hansel, Daniel Hauser, Werner Jud, Heikki Junninen, Fabianreissl, Alexander Kvashin, Ari Laaksonen, Katrianne Lehtipalo, Jorge Lima, Edward R.Lovejoy, Vladimir Makhmutov, Serge Mathot, Jyri Mikkilï¿œ, Pierre Minginette, Sandra Mogo,uomo Nieminen, Antti Onnela, Paulo Pereira, Tuukka Petï¿œjï¿œ, Ralf Schnitzhofer, John H. Seinfeld, Mikko Sipilï¿œ, Yuri Stozhkov, Frank Stratmann, Antonio Tome, Joonas Vanhanen, Yrjoiisanen, Aron Vrtala, Paul E. Wagner, Hansueli Walther, Ernest Weingartner, Heike Wex,aul M. Winkler, Kenneth S. Carslaw, Douglas R. Worsnop, Urs Baltensperger and Markku Kulmala
Nature 476, 429-433 (2011)

The speaker will also include information on more elaborate papers:
# http://www.atmos-chem-phys-discuss.net/9/18235/2009/acpd-9-18235-2009.html

Abstract
Atmospheric aerosols exert an important influence on climate through their effects on stratiform cloud albedo and lifetime and the invigoration of convective storms. Model calculations suggest that almost half of the global cloud condensation nuclei in the atmospheric boundary layer may originate from the nucleation of aerosols from trace condensable vapours, although the sensitivity of the number of cloud condensation nuclei to changes of nucleation rate may be small. Despite extensive research, fundamental questions remain about the nucleation rate of sulphuric acid particles and the mechanisms responsible, including the roles of galactic cosmic rays and other chemical species such as ammonia. Here we present the first results from the CLOUD experiment at CERN. We find that atmospherically relevant ammonia mixing ratios of 100 parts per trillion by volume, or less, increase the nucleation rate of sulphuric acid particles more than 100-1000-fold. Time-resolved molecular measurements reveal that nucleation proceeds by a base-stabilization mechanism involving the stepwise accretion of ammonia molecules. Ions increase the nucleation rate by an additional factor of between two and more than ten at ground-level galactic-cosmic-ray intensities, provided that the nucleation rate lies below the limiting ion-pair production rate. We find that ion-induced binary nucleation of H2SO4-H2O can occur in the mid-troposphere but is negligible in the boundary layer. However, even with the large enhancements in rate due to ammonia and ions, atmospheric concentrations of ammonia and sulphuric acid are insufficient to account for observed boundary-layer nucleation.

Friday Nov 18 2011, 12:00 @ seminar room 2nd floor:
Faruk Geles on
Quasipure Bose-Einstein Condensate Immersed in a Fermi Sea ,
F. Schreck, L. Khaykovich, K. L. Corwin, G. Ferrari, T. Bourdel, J. Cubizolles, and C. Salomon
Phys. Rev. Lett. 87, 080403 (2001)

Abstract
We report the observation of coexisting Bose-Einstein condensate (BEC) and Fermi gas in a magnetic trap. With a very small fraction of thermal atoms, the 7Li condensate is quasipure and in thermal contact with a 6Li Fermi gas. The lowest common temperature is 0.28muK=0.2(1)TC = 0.2(1)TF where TC is the BEC critical temperature and TF the Fermi temperature. The 7Li condensate has a one-dimensional character.

Friday Nov 11 2011, 12:00 @ seminar room 2nd floor :
general discussion about
Electrically driven directional motion of a four-wheeled molecule on a metal surface ,
Tibor Kudernac, Nopporn Ruangsupapichat, Manfred Parschau, Beatriz Macia, Nathalie Katsonis, Syuzanna R. Harutyunyan, Karl-Heinz Ernst and Ben L. Feringa
Nature 479, 208-211 (2011)

Abstract
Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature's motor proteins have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems such as DNA walkers and light- and redox-driven molecular motors. But although controlled movement of single molecules along a surface has been reported the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units-our previously reported rotary motors-that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.

Friday Nov 04 2011, 12:30 @ seminar room 2nd floor:
Andreas Martitsch on
Measurement of the neutrino velocity with the OPERA detector in the CNGS beam ,
OPERA
arXiv:1109.4897v1
The speaker will also include information on explainatory papers:
# http://www.nature.com/news/2011/111005/full/news.2011.575.html
# Contaldi, C. R. Preprint at http://xxx.lanl.gov/abs/1109.6160 (2011).
# Henri, G. Preprint at http://arxiv.org/abs/1110.0239 (2011).

Abstract
The OPERA neutrino experiment at the underground Gran Sasso Laboratory has measured the velocity of neutrinos from the CERN CNGS beam over a baseline of about 730 km with much higher accuracy than previous studies conducted with accelerator neutrinos. The measurement is based on high-statistics data taken by OPERA in the years 2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the OPERA detector, as well as a high precision geodesy campaign for the measurement of the neutrino baseline, allowed reaching comparable systematic and statistical accuracies. An early arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (60.7 \pm 6.9 (stat.) \pm 7.4 (sys.)) ns was measured. This anomaly corresponds to a relative difference of the muon neutrino velocity with respect to the speed of light (v-c)/c = (2.48 \pm 0.28 (stat.) \pm 0.30 (sys.)) \times 10-5.

Friday Oct 28 2011, 12:00 @ seminar room 2nd floor:
Martin Nuss on
Experimental control of the transition from Markovian to non-Markovian dynamics of open quantum systems ,
Bi-Heng Liu, Li Li, Yun-Feng Huang, Chuan-Feng Li, Guang-Can Guo, Elsi-Mari Laine, Heinz-Peter Breuer and Jyrki Piilo
Nature Physics September (2011) doi:10.1038/nphys2085

Abstract
Realistic quantum mechanical systems are always exposed to an external environment. This often induces Markovian processes in which the system loses information to its surroundings. However, many quantum systems exhibit non-Markovian behaviour with a flow of information from the environment back to the system. The environment usually consists of large number of degrees of freedom which are difficult to control, but some sophisticated schemes for reservoir engineering have been developed. The control of open systems plays a decisive role, for example, in proposals for entanglement generation and dissipative quantum computation, for the design of quantum memories and in quantum metrology. Here we report an all-optical experiment which allows one to drive the open system from the Markovian to the non-Markovian regime, to control the information flow between the system and the environment, and to determine the degree of non-Markovianity by measurements on the open system.

Martin Nuss | Journal Club

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