Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A2: Frontiers in the Theory of Non-Equilibrium Physics: From Nanosystems to Cold AtomsInvited
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Sponsoring Units: DCMP DAMOP Chair: Marcos Rigol, Pennsylvania State University Room: Ballroom II |
Monday, March 14, 2016 8:00AM - 8:36AM |
A2.00001: Out-of-equilibrium phenomena and Transport in Cold Atoms Invited Speaker: Thierry Giamarchi Transport of particle or charge current between two reservoirs is one of the most studied phenomenon in the context of condensed matter. Despite its apparent simplicity this phenomenon is in fact a case of an out of equilibrium situation requiring in principle new theoretical tools and concepts for its solution. One way to sweep the difficulty under the rug has been usually to tackle this problem in the linear response, where one can come back to the comfortable case of equilibrium. There are however many cases when the linear response is not enough and when a full solution of the non-equilibrium problem is needed. This is in particular the case for quantum point contacts or junctions where the full current-voltage characteristics gives direct information on the physics of the problem. In the recent years, in complement to condensed matter experimental realizations, due to the full control on the parameters of the problem and the fact that they realize isolated quantum systems cold atoms have proven a fantastic laboratory to produce out of equilibrium situations. This ranges from the case of quenches, to more recently via experiments of the ETHZ group to the case of real transport between reservoirs. This experimental activity has in turn thus stimulated strongly theoretical developments in this field. I will discuss in this talk some of the recent advances and realizations both at the experimental and of course the theoretical level. I will in particular focus on a recent study [1] which was able to realize a tunable, ballistic quantum point contact between two fermi reservoirs with a tunable interaction allowing to reach unitarity and to provide a theoretical description of the out-of equilibrium corresponding problem. In such a system the current has been shown to originate from multiple Andreev reflections which leads to a very non-linear current-chemical potential characteristics. The geometry of the contact can be changed showing a competition between superfluidity and thermally activated transport which leads to a conductance minimum and poses several theoretical questions for its theoretical description. [1] ``Connecting strongly correlated superfluids by a quantum point contact'', D. Husmann, S. Uchino, S. Krinner, M. Lebrat, T. Giamarchi, T. Esslinger and J.-P. Brantut, arXiv:1508.00578 (2015). [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 9:12AM |
A2.00002: 'Consistent bosonization-debosonization': A resolution of the non-equilibrium transport puzzle blazes a new path forward Invited Speaker: Nayana Shah In this talk, we will critically reexamine the bosonization-debosonization procedure for systems including certain types of localized features (although more general scenarios are possible). By focusing on the case of a tunneling junction out of equilibrium, I will show that the conventional approach gives results that are not consistent with the exact solution of the problem even at the qualitative level and highlight the inconsistencies that can adversely affect the results of all types of calculations. I will subsequently report on a `Consistent bosonization-debosonization’ procedure that we have developed to resolve the aforementioned non-equilibrium transport puzzle and argue that this framework should be widely applicable [1]. I will touch upon its application for the two-lead Kondo problem [2] that besides being a key theoretical prototype of a strongly correlated system is also of immediate experimental relevance in many ways (see also related talk by Bolech). [1] Nayana Shah,C. J. Bolech, arXiv:1508.03078, [2] C. J. Bolech, Nayana Shah, arXiv:1508.03079 [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:48AM |
A2.00003: \textbf{Non-equilibrium Aspects of Quantum Integrable Systems} Invited Speaker: Natan Andrei The study of non-equilibrium dynamics of interacting many body systems is currently one of the main challenges of modern condensed matter physics, driven by the spectacular progress in the ability to create experimental systems - trapped cold atomic gases are a prime example - that can be isolated from their environment and be highly controlled. Many old and new questions can be addressed: thermalization of isolated systems, nonequilibrium steady states, the interplay between non equilibrium currents and strong correlations, quantum phase transitions in time, universality among others. In this talk I will describe nonequilibrium quench dynamics in integrable quantum systems. I'll discuss the time evolution of the Lieb-Liniger system, a gas of interacting bosons moving on the continuous infinite line and interacting via a short range potential. Considering a finite number of bosons on the line we find that for any value of repulsive coupling the system asymptotes towards a strongly repulsive gas for any initial state, while for an attractive coupling, the system forms a maximal bound state that dominates at longer times. In the thermodynamic limit -with the number of bosons and the system size sent to infinity at a constant density and~the long time limit taken subsequently-~ I'll show that the density and density-density correlation functions for strong but finite positive coupling are described by GGE for translationally invariant initial states with short~ range correlations. As examples I'll discuss quenches from a Mott insulator initial state or a Newton's Cradle. Then I will show that if the initial state is strongly non translational invariant, e.g. a domain wall configuration, the system does not equilibrate but evolves into a nonequilibrium steady state (NESS). A related NESS arises when the quench consists of coupling a quantum dot to two leads held at different chemical potential, leading in the long time limit to a steady state current. Time permitting I will also discuss the quench dynamics of the XXZ Heisenberg chain. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:24AM |
A2.00004: Universal behavior after a quantum quench in interacting field theories Invited Speaker: Aditi Mitra The dynamics of an isolated quantum system represented by a field theory with O(N) symmetry, and in d>2 spatial dimensions, is investigated after a quantum quench from a disordered initial state to the critical point. A perturbative renormalization-group approach involving an expansion around d=4 is employed to study the time-evolution, and is supplemented by an exact solution of the Hartree-Fock equations in the large-N limit. The results show that the dynamics is characterized by a prethermal regime controlled by elastic dephasing where excitations propagate ballistically, and a light cone emerges in correlation functions in real space. The memory of the initial state, together with the absence of time-scales at the critical point, gives rise to universal power-law aging which is characterized by a new non-equilibrium short-time exponent. The dynamics of the entanglement following a quench is also explored, and reveals that while the time evolution of the entanglement entropy itself is not much different between a free bosonic theory and an interacting bosonic theory, the low-energy entanglement spectrum on the other hand shows clear signature of the non-equilibrium short-time exponent related to aging. This work was done in collaboration with Y. Lemonik (NYU), M. Tavora (NYU), A. Chiocchetta (SISSA), A. Maraga (SISSA), and A. Gambassi (SISSA). [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 11:00AM |
A2.00005: Exploring the nonequilibrium dynamics of ultracold quantum gases by using numerical tools Invited Speaker: Fabian Heidrich-Meisner Numerical tools such as exact diagonalization or the density matrix renormalization group method have been vital for the study of the nonequilibrium dynamics of strongly correlated many-body systems. Moreover, they provided unique insight for the interpretation of quantum gas experiments, whenever a direct comparison with theory is possible. By considering the example of the experiment by Ronzheimer et al.~[1], in which both an interaction quench and the release of bosons from a trap into an empty optical lattice (sudden expansion) was realized, I discuss several nonequilibrium effects of strongly interacting quantum gases. These include the thermalization of a closed quantum system and its connection to the eigenstate thermalization hypothesis [2], nonequilibrium mass transport [1], dynamical fermionization [3], and transient phenomena such as quantum distillation or dynamical quasicondensation [4]. I highlight the role of integrability in giving rise to ballistic transport in strongly interacting 1D systems [1] and in determining the asymptotic state after a quantum quench [5]. The talk concludes with a perspective on open questions concerning 2D systems and the numerical simulation of their nonequilibrium dynamics [6].\\ \noindent [1] Ronzheimer et al., Phys. Rev. Lett. 110, 205301 (2013) \newline [2] Sorg et al., Phys. Rev. A 90, 033606 (2014) \newline [3] Vidmar et al., Phys. Rev. B 88, 235117 (2013) \newline [4] Vidmar et al., Phys. Rev. Lett. 115, 175301 (2015) \newline [5] Mei et al., arXiv:1509.00828 \newline [6] Hauschild, Pollmann, FHM, arXiv:1509.00696 [Preview Abstract] |
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