Feb 2018

Feb 1- Feb 3 Jiansong Pan, Feb 4-Feb 8 Ahmet Keles, Feb 9-Feb 13 Max Aarzamazovs, Feb 14-Feb 18 Xuguang Yue, Feb 19-Feb 23 Huang Biao, Feb 24-Feb 28 Haiyuan Zou

Feb 28

arXiv:1802.10018 [pdf, other]
Title: Spin Transport in a Mott Insulator of Ultracold Fermions
Authors: Matthew A. Nichols, Lawrence W. Cheuk, Melih Okan, Thomas R. Hartke, Enrique Mendez, T. Senthil, Ehsan Khatami, Hao Zhang, Martin W. Zwierlein
Comments: 14 pages, 9 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
Strongly correlated electronic materials such as the high-Tc cuprates are expected to feature unconventional transport properties, where charge, spin and heat conduction are potentially independent probes of the dynamics. However, the measurement of spin transport in such materials is - in contrast to charge transport - highly challenging. Here we observe spin diffusion in a Mott insulator of ultracold fermionic atoms with single-atom resolution. The system realizes the Fermi-Hubbard model, believed to capture the essence of the cuprate phenomenology. We find that for strong interactions, spin diffusion is driven by super-exchange and strongly violates the quantum limit of charge diffusion. The technique developed in this work can be extended to finite doping, which can shed light on the complex interplay between spin and charge in the Hubbard model. 

arXiv:1802.10061 [pdf, other]
Title: Dynamical classification of topological quantum phases
Authors: Lin Zhang, Long Zhang, Sen Niu, Xiong-Jun Liu
Comments: 7+pages, 4 figures, plus Supplementary Material
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
Topological phase of matter has been a mainstream of research in condensed matter physics, of which the classification, synthesis, and detection of topological states have brought great excitements over the recent decade while remain incomplete with ongoing challenges in both theory and experiment. A topological phase is usually classified by bulk topological number defined in static regime, and hosts protected boundary modes via bulk-boundary correspondence. Here we propose to establish a universal dynamical characterization of the topological quantum phases classified by integers, with the framework of this study consisting of fundamental theorems. First, we uncover that classifying a generic d -dimensional (d D) gapped topological phase can reduce to a (d−1 )D invariant defined on the so-called band inversion surfaces (BISs), rendering a fundamental {\it bulk-surface} duality. Further, we show in quenching across phase boundary the unitary (pseudo)spin dynamics to exhibit unique topological patterns on BISs, which attribute to the bulk topology and manifest a dynamical {\it bulk-surface} correspondence. The topological phase is finally classified by a dynamical topological invariant measured from the dynamical spin-texture field across the BISs. Applications to quenching experiments on feasible models are proposed and studied. This work opens a new direction to classify and detect topological phases by quantum dynamics.

Feb 27
arXiv:1802.08702 [pdf, other]
Title: Quantum spin dynamics of individual neutral impurities coupled to a Bose-Einstein condensate
Authors: Felix Schmidt, Daniel Mayer, Quentin Bouton, Daniel Adam, Tobias Lausch, Nicolas Spethmann, Artur Widera
Comments: 4 pages with 4 figures, 5 pages of supplementary material
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
We report on the controlled immersion of individual localized neutral Cesium atoms of total angular momentum, i.e. quasi-spin, F=3 into a BEC of Rubidium in F=1 . We study spin-exchange dynamics of the impurities interacting with the BEC with both position and time resolution. We observe a clear signature of the BEC's internal mF state in the rate of spin-exchange and demonstrate the interaction of impurities with the condensate. Moreover, by preparing the impurities in a quantum superposition of internal hyperfine states, we study the coherent dynamics within the quantum bath. We find that coherence is maintained in the presence of elastic collisions with BEC atoms. Our results pave the way for future local and coherent single-atom probing of a quantum many-body system, highly relevant for, e.g., insights into thermalization of non-equilibrium systems, local state manipulation or cooling of quantum information carriers by superfluid immersion.
Feb 26
arXiv:1802.08393 [pdf, other]
Title: Boltzmann transport theory for many body localization
Authors: Jae-Ho Han, Ki-Seok Kim
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Strongly Correlated Electrons (cond-mat.str-el)
We investigate a many-body localization transition based on a Boltzmann transport theory. Introducing weak localization corrections into a Boltzmann equation and taking into account self-consistency for a diffusion coefficient, the Wolfle-Vollhardt self-consistent equation for the diffusion coefficient has been derived, which describes an Anderson metal-insulator transition in a continuous fashion [Phys. Rev. B {\bf 34}, 2147 (1986)]. We generalize this Boltzmann equation framework, introducing electron-electron interactions into the Hershfield-Ambegaokar Boltzmann transport theory based on the study of Zala-Narozhny-Aleiner [Phys. Rev. B {\bf 64}, 214204 (2001)], where not only Altshuler-Aronov corrections but also dephasing effects are taken into account. As a result, we obtain a self-consistent equation for the diffusion coefficient in terms of the disorder strength and temperature, which extends the Wolfle-Vollhardt self-consistent equation in the presence of electron correlations. Solving our self-consistent equation numerically, we find a many-body localization insulator-metal transition, where a metallic phase appears from dephasing effects dominantly instead of renormalization effects at high temperatures. Although the mechanism for the many-body localization transition is consistent with that of recent seminal papers [Ann. Phys. (N. Y). {\bf 321}, 1126 (2006); Phys. Rev. Lett. {\bf 95}, 206603 (2005)], we find that nature of this three-dimensional metal-insulator transition differs from all of the previous studies in one dimension. We discuss the nature of our many-body localization transition carefully.

Feb. 20

Measuring Electromagnetic and Gravitational Responses of Photonic Landau Levels

Nathan Schine, Michelle Chalupnik, Tankut Can, Andrey Gromov, Jonathan Simon

(Submitted on 13 Feb 2018)

The topology of an object describes global properties that are insensitive to local perturbations. Classic examples include string knots and the genus (number of handles) of a surface: no manipulation of a closed string short of cutting it changes its "knottedness"; and no deformation of a closed surface, short of puncturing it, changes how many handles it has. Topology has recently become an intense focus of condensed matter physics, where it arises in the context of the quantum Hall effect [1] and topological insulators [2]. In each case, topology is defined through invariants of the material's bulk [3-5], but experimentally measured through chiral/helical properties of the material's edges. In this work we measure topological invariants of a quantum Hall material through local response of the bulk: treating the material as a many-port circulator enables direct measurement of the Chern number as the spatial winding of the circulator phase; excess density accumulation near spatial curvature quantifies the curvature-analog of charge known as mean orbital spin, while the moment of inertia of this excess density reflects the chiral central charge. We observe that the topological invariants converge to their global values when probed over a few magnetic lengths lB, consistent with intuition that the bulk/edge distinction exists only for samples larger than a few lB. By performing these experiments in photonic Landau levels of a twisted resonator [6], we apply quantum-optics tools to topological matter. Combined with developments in Rydberg-mediated interactions between resonator photons [7], this work augurs an era of precision characterization of topological matter in strongly correlated fluids of light.



arXiv:1802.06704 [pdf, other]
Quantum simulation of lattice gauge theories using Wilson fermions
T. V. Zache, F. Hebenstreit, F. Jendrzejewski, M. K. Oberthaler, J. Berges, P. Hauke
Comments: 19 pages, 11 figures
Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Lattice (hep-lat); High Energy Physics - Phenomenology (hep-ph); Quantum Physics (quant-ph)
Quantum simulators have the exciting prospect of giving access to real-time dynamics of lattice gauge theories, in particular in regimes that are difficult to compute on classical computers. Future progress towards scalable quantum simulation of lattice gauge theories, however, hinges crucially on the efficient use of experimental resources. As we argue in this work, due to the fundamental non-uniqueness of discretizing the relativistic Dirac Hamiltonian, the lattice representation of gauge theories allows for an optimization that up to now has been left unexplored. We exemplify our discussion with lattice quantum electrodynamics in two-dimensional space-time, where we show that the formulation through Wilson fermions provides several advantages over the previously considered staggered fermions. Notably, it enables a strongly simplified optical lattice setup and it reduces the number of degrees of freedom required to simulate dynamical gauge fields. Exploiting the optimal representation, we propose an experiment based on a mixture of ultracold atoms trapped in a tilted optical lattice. Using numerical benchmark simulations, we demonstrate that a state-of-the-art quantum simulator may access the Schwinger mechanism and map out its non-perturbative onset.



Strongly-correlated bosons on a dynamical lattice

Daniel González-Cuadra, Przemysław R. Grzybowski, Alexandre Dauphin, Maciej Lewenstein

(Submitted on 15 Feb 2018)

We study a one-dimensional system of strongly-correlated bosons interacting with a dynamical lattice. A minimal model describing the latter is provided by extending the standard Bose-Hubbard Hamiltonian to include extra degrees of freedom on the bonds of the lattice. We show that this model is capable of reproducing phenomena similar to those present in usual fermion-phonon models. In particular, we discover a bosonic analog of the Peierls transition, where the translational symmetry of the underlying lattice is spontaneously broken. The latter provides a dynamical mechanism to obtain a topological insulator in the presence of interactions, analogous to the Su-Schrieffer-Heeger (SSH) model for electrons. We numerically characterize the phase diagram of the model, which includes different types of bond order waves and topological solitons. Finally, we study the possibility of implementing the model experimentally using atomic systems.