Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session U6: Focus Session: Transport and Out-of-Equilibrium Dynamics with Ultracold Atoms |
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Chair: Joseph Thywissen, University of Toronto Room: Delaware AB |
Friday, June 12, 2015 10:30AM - 11:00AM |
U6.00001: Scale-Invariant Hydrodynamics and Quantum Viscosity in Fermi Gases Invited Speaker: John Thomas An optically-trapped gas of spin 1/2-up and spin 1/2-down $^{\mathrm{6}}$Li atoms, tuned near a collisional (Feshbach) resonance, provides a unique paradigm for testing predictions that cross interdisciplinary boundaries, from high temperature superconductors to nuclear matter. At resonance, the dilute atomic cloud becomes the most strongly interacting, non-relativistic fluid known: Shock waves are produced when two clouds collide. We observe scale-invariant hydrodynamic expansion of a resonantly interacting gas and determine the quantum shear viscosity $\eta =\alpha {\kern 1pt}\hbar n$, with n the density, as a function of interaction strength and temperature, from nearly the ground state through the superfluid phase transition. We extract the local shear viscosity coefficient $\alpha {\kern 1pt}$ from cloud-averaged data, using iterative methods borrowed from image processing, and observe previously hidden features, which are compared to recent predictions.\\[4pt] In collaboration with Ethan Elliott and James Joseph, Physics Department, North Carolina State University. [Preview Abstract] |
Friday, June 12, 2015 11:00AM - 11:30AM |
U6.00002: Transport in Heisenberg spin chains Invited Speaker: Christian Gross Measurements of spin diffusion in ultracold Fermi gases recently reported astonishingly small values for the diffusivity and a strong dependence on the dimensionality of the system. Furthermore, in Heisenberg quantum magnets even the nature of spin transport -- ballistic or diffusive -- is an open issue. Here we report on transport measurements in Heisenberg spin systems realized with ultracold Bosons in an optical lattice. We study spin transport close to and very far away from equilibrium. For week perturbations, that is, close to equilibrium, we observe ballistic transport of single as well as bound magnons along 1d chains. In contrast, in a far-from-equilibrium situation we find effectively diffusive transport which is microscopically explained by the spectral distribution of the initial far-from equilibrium states. Repeating the latter measurement in 2d, we observe anomalous superdiffusion. [Preview Abstract] |
Friday, June 12, 2015 11:30AM - 11:42AM |
U6.00003: 2D Superexchange-mediated magnetization dynamics in an optical lattice Elizabeth Goldschmidt, Roger Brown, Robert Wyllie, Silvio Koller, Michael Foss-Feig, Trey Porto The interplay of magnetic exchange interactions and tunneling underlies many complex quantum phenomena observed in real materials. We study nonequilibrium magnetization dynamics in an extended 2D system by loading effective spin-1/2 bosons into a spin-dependent optical lattice, and we use the lattice to separately control the resonance conditions for tunneling and superexchange. After preparing a nonequilibrium antiferromagnetically ordered state, we observe relaxation dynamics governed by two well-separated rates, which scale with the underlying Hamiltonian parameters associated with superexchange and tunneling. Remarkably, with tunneling off-resonantly suppressed, we are able to observe superexchange-dominated dynamics over two orders of magnitude in magnetic coupling strength, despite the presence of vacancies. In this regime, the measured timescales are in agreement with simple theoretical estimates, but the detailed dynamics of this 2D, strongly-correlated, and far-from-equilibrium quantum system remain out of reach of current computational techniques. [Preview Abstract] |
Friday, June 12, 2015 11:42AM - 11:54AM |
U6.00004: Negative Differential Conductivity in an Interacting Quantum Gas Bodhaditya Santra, Ralf Labouvie, Simon Heun, Sandro Wimberger, Herwig Ott Negative differential conductivity (NDC) is a widely exploited mechanism in many areas of research dealing with particle and energy transport. We experimentally realize such a many body quantum transport system based on ultracold atoms in a periodic potential. We prepare our system by loading Bose condensed rubidium atoms in a 1D optical lattice with high atom occupancy per lattice site. Subsequently, we remove all the atoms from a central lattice site. While the atoms from neighboring sites tunnel into the empty site, we observe NDC in the resulting current voltage characteristics and investigate the microscopic mechanism behind it [1]. \\[4pt] [1] R. Labouvie, B. Santra, S. Heun, S. Wimberger, H. Ott, arXiv:1411.5632 [Preview Abstract] |
Friday, June 12, 2015 11:54AM - 12:06PM |
U6.00005: Observation of quantized conductance in neutral matter Dominik Husmann, Sebastian Krinner, Martin Lebrat, Charles Grenier, Shuta Nakajima, Samuel H\"ausler, Jean-Philippe Brantut, Tilman Esslinger In transport experiments, the quantum nature of matter becomes directly evident when changes in conductance occur only in discrete steps, with a size determined solely by Planck's constant h. Here we report the observation of quantized conductance in the transport of neutral atoms driven by a chemical potential bias. We use high-resolution lithography to shape light potentials that realize either a quantum point contact or a quantum wire for atoms. These constrictions are imprinted on a quasi-two-dimensional ballistic channel connecting the reservoirs. By varying either a gate potential or the transverse confinement of the constrictions, we observe distinct plateaux in the atom conductance. The conductance in the first plateau is found to be equal to the universal conductance quantum, 1/h. We use Landauer's formula to model our results and find good agreement for low gate potentials, with all parameters determined a priori. We eventually explore the behavior of a strongly interacting Fermi gas in the same configuration, and the consequences of the emergence of superfluidity. [Preview Abstract] |
Friday, June 12, 2015 12:06PM - 12:18PM |
U6.00006: Quantum and mean-field nonequlibrium dynamics of atom transistor Maxim Olshanii, Zhedong Zhang We illustrate that the neutral and ultracold atoms in asymmetric three wells can simulate the behavior of the atomic transistor, analogous to the electronic transistor in semiconductor. Under the mean-field approximation, it is found that the occupation of atoms in the drain can be significantly amplified by manipulating the occupation in the gate, to realize the ``on'' and ``off'' modes of the atomic transistor. We analysis the criterion for switching on the transistor and find the corresponding resonant value of gate occupation, by acquiring the mobility becomes sufficient for the drain occupation to reach its thermal value. To quantify the reliability of our mean-field approach in the semi-classical regime, the comparison between the semi-classical and the full quantum calculations is carried out, which shows a good agreement. [Preview Abstract] |
Friday, June 12, 2015 12:18PM - 12:30PM |
U6.00007: Transport dynamics in quantum lattice models and the discrete truncated Wigner approximation Johannes Schachenmayer, Guido Pupillo, Edoardo Tignone, Claudiu Genes, Alexander Pikovski, Ana Maria Rey Transport of physical quantities such as energy, charge, or information plays a crucial role in a vast variety of scientific fields ranging from materials science/solid-state physics, to photonics/quantum information, to biological systems. The robustness of quantum coherences in the presence of de-coherent sources, and how those affect transport efficiency are important open questions. Addressing them can not only impact our fundamental understanding of quantum science but at the same time can lead to important technological applications. Here, we present a scheme of how to dramatically enhance the energy transport efficiency of a material by coupling it to a cavity mode, an idea with profound implications for organic semi-conductor materials. In addition we report on progress of how to numerically tackle the problem of quantum transport dynamics with a newly developed method, the dTWA, which allows to simulate quantum-dynamics even in large systems and high dimensions. [Preview Abstract] |
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