Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H27: Topological Physics in AMO Systems II |
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Sponsoring Units: DAMOP DCMP Chair: Iacopo Carusotto, Universita' di Trento Room: LACC 404B |
Tuesday, March 6, 2018 2:30PM - 2:42PM |
H27.00001: Quantum Spin Dynamics in Spin-orbit Coupled Bose Gas Wai Ho Tang, Shizhong Zhang Spin-orbit coupling (SOC) has given rise to many novel features of physical systems, from single atom to solid materials. Since its recent experimental realisation in neutral atoms, many theoretical and experimental studies have revealed new phases and properties of Bose condensate with SOC. On the other hand, little attention has been paid to the effects of spin-orbit interaction in thermal Bose vapour with SOC. In this study, we derive transport equations governing the spin dynamics of a two-component Bose gas with SOC. The coupled dynamics of density and spin results in new features of spin dynamics. Analytic solution of spin helix in quasi-one-dimensional situation is obtained. In the adiabatic regime, persistent spin helix (PSH) protected by SU(2) symmetry is found. In the vicinity of PSH, the decay of transverse spin proceeds either parabolically or exponentially, depending on initial polarisation. By contrast, far away from adiabatic limit, transverse spin density and current oscillate in a way similar to the charge-current oscillation in LC circuit. Lastly, the leading correction in short duration of Rabi coupling on PSH will be discussed. |
Tuesday, March 6, 2018 2:42PM - 2:54PM |
H27.00002: Floquet engineering of an optical solenoid to probe Chern insulators Nur Unal, Botao Wang, Andre Eckardt The idea of inserting a local magnetic flux, representing the field of a thin solenoid, plays an important role in various condensed matter models, especially in the understanding of topological systems. One example is the creation and manipulation of quasiparticle or hole excitations in these systems, which are essential for fault-tolerant quantum information processing. Implementing such local fluxes in cold atom experiments promises great potential. Here, we propose an experimental scheme to realize a local flux in a cold atom setting which takes advantage of the recent developments in synthetic gauge fields and quantum microscopes. To demonstrate the feasibility of our method, we consider quantum-Hall-type lattice systems and study the dynamical creation of topological excitations. We analyze the adiabatic charge pumping by tuning the strength of the local flux. |
Tuesday, March 6, 2018 2:54PM - 3:06PM |
H27.00003: Topological states with molecular systems Joel Yuen-Zhou In this talk, I'll briefly explain our recent efforts to design topologically protected states using molecular degrees of freedom in organic materials. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H27.00004: Edge State Dynamics in Bosonic Fractional Chern Insulators Xiaoyu Dong, Adolfo Grushin, Johannes Motruk, Frank Pollmann The experimental realization of the Harper-Hofstadter model in ultra-cold atomic gases has placed fractional states of matter in these systems within reach---a fractional Chern insulator state (FCI) is expected to emerge for sufficiently strong interactions when half-filling the lowest band. The experimental setups naturally allow to probe the dynamics of this topological state, yet little is known about its out-of-equilibrium properties. We explore, using density matrix renormalization group (DMRG) simulations, the response of the FCI state to spatially localized perturbations. After confirming the static properties of the phase we show that the characteristic, gapless features are clearly visible in the edge dynamics. We find that a local edge perturbation in this model propagates chirally independent of the perturbation strength. This contrasts the behavior of single particle models with counter-propagating edge states, such as the non-interacting Harper-Hofstadter model, where the chirality is manifest only for weak perturbations. Additionally, our simulations show that there is inevitable density leakage into the bulk, preventing a naive chiral Luttinger theory interpretation of the dynamics. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H27.00005: Measurement of topologically protected corner states in a microwave metamaterial quadrupole insulator Christopher Peterson, Wladimir Benalcazar, Taylor Hughes, Gaurav Bahl The electric polarization of a crystalline insulator can be expressed in terms of the Berry phase of its electronic ground state. Recently, this concept was extended to higher electric multipole moments and a new class of previously unobserved topological phases was predicted. Using a GHz frequency reconfigurable microwave metamaterial circuit, we implemented the first member of this class -- a topological insulator with a quantized bulk quadrupole moment. We experimentally confirm the existence of the new topological phase through spectroscopic measurements and identify corner states manifested by the bulk topology. By deforming the edge of our metamaterial, we observe that these corner states are robustly protected by the bulk topology. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H27.00006: Magneto-Mechanical Topological Lego Inbar Grinberg, Mao Lin, Wladimir Benalcazar, Taylor Hughes, Gaurav Bahl There is a rising interest in producing topological insulating metamaterials, due to their robustness in face of defects, which may lead to producing disorder resilient waveguides and communication lines. In the last decade many such synthetic topological insulators have been produced in electronic, optical, and acoustic systems. However, these metamaterials are not easily reconfigurable, they are designed based on a specific set of parameters, and therefore limited in their ability to explore different phases of matter. We present a Lego like tool kit, which can be utilized to produce different acoustic metamaterials with tailored topological properties. Our system consists of discrete coupled magnet-loaded resonators, where the coupling rates are controlled by the distance over which the magnet interaction takes place. We demonstrate experimentally, that by changing the coupling rates arrangement in an array, a variety of different topological phases in 1d and 2d can be achieved, using the same Lego like elements. This toolkit offer flexibility in design, and can be used to produce easily reconfigurable acoustic metamaterials. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H27.00007: Berry Electrodynamics - Anomalous Drift and Pump from Electric Field Analog Swati Chaudhary, Gil Refael, Manuel Endres Berry curvature can significantly affect the dynamics of a wave packet in a 2D Bloch band. It gives rise to a Hall drift and thus can be treated as an analog of magnetic field in quasi-momentum space. Indeed, we can even define an Electric field analog from the time derivative of the Berry connection. This analog field appears as an anomalous velocity and integrates into a transverse drift in the semi-classical equation of motion for a wave packet undergoing adiabatic evolution in a Bloch band. Time-dependent Berry connection can lead to interesting results in non-adiabatic processes as well. An anomalous drift proportional to a shift vector is observed during band switching. This shift vector depends not only on the topological properties of the involved bands but also on the nature of drive. In some special cases, band population can be flipped without a significant anomalous drift. We simulate these effects and show that a combination of adiabatic and non-adiabatic steps in a cycle can amplify the transverse drift due to the Electric field analog, and can be combined to provide a persistent pumping effect. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H27.00008: Topological Protection of Coherence in a Dissipative Environment Zhengzhi Ma, Lorenzo Campos Venuti, Stephan Haas, Hubert Saleur One dimensional topological insulators are characterized by edge states with exponentially small energies. According to one generalization of topological phases to non-Hermitian systems, a finite system in a non-trivial topological phase displays surface states with exponentially long life times. In this work we explore the possibility of exploiting such non-Hermitian topological phases to enhance the quantum coherence of a fiducial qubit embedded in a dissipative environment. We first show that a network of qubits interacting with lossy cavities can be represented, in a suitable super-one-particle sector, by a non-Hermitian "Hamiltonian" of the desired form. We then study, both analytically and numerically, one-dimensional geometries with up to three sites per unit cell, and up to a topological winding number W=2. For finite-size systems the number of edge modes is a complicated function of W and the system size N. However we find that there are precisely W modes localized at one end of the chain. In such topological phases the quibt's coherence lifetime is exponentially large in the system size. We verify that, for W>1, at large times, the Lindbladian evolution is approximately a non-trivial unitary. For W=2 this results in oscillations of the qubit's coherence measure. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H27.00009: Topological Phases in the Non-Hermitian Su-Schrieffer-Heeger Model Simon Lieu We address the conditions required for a Z topological classification in the most general form of the non-Hermitian Su-Schrieffer-Heeger (SSH) model. While chiral symmetry ensures a topological transition, we show that it also results in a “conjugated-pseudo-Hermiticity” which is responsible for a quantized “complex” Berry phase. We comment on the PT-symmetric problem, where previous studies have demonstrated that pseudo-anti-Hermiticity ensures a topological transition. We provide the first example where the complex Berry phase of a band is used as a quantized invariant to predict the existence of gapless edge modes in a non-Hermitian model. An intuitive picture is provided by examining eigenvector evolution on the Bloch sphere. We verify our claims numerically and discuss relevant experimental set-ups. Our work sheds light on the general problem of extending topological classification to non-Hermitian models. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H27.00010: Edge modes and topological phases in a generalized Su-Schrieffer-Heeger ladder system Karmela Padavic, Suraj Hegde, Wade DeGottardi, Smitha Vishveshwara Motivated by the realization of topological edge states in Su-Schrieffer-Heeger (SSH) chains in cold atomic systems, we study a ladder model consisting of two coupled SSH chains. Such a system can serve as a testbed for study and detection of edge modes analogous to the bound Majorana states in the Kitaev wire. The SSH ladder Hamiltonian belongs to the BDI symmetry class and is characterized by a Z-valued topological invariant. When the two chains have identical hopping but are off-set by one lattice site, the topological phase of the SSH ladder is characterized by the existence of Dirac zero energy modes at the edge. These modes are similar to the Majorana modes of the Kitaev chain with respect to the spatial profile of associated wavefunctions. For a more general model of coupled SSH chains having four distinct hopping amplitudes, we show that a more complex phase diagram is obtained as the system can support another kind of edge states. Through numerical and analytical methods, we study the distinct topological phases hosting these modes and their robustness with respect to the strength of the inter-chain coupling. Relevant for experimental systems, we additionally consider the effects of finite size on the edge-mode structure and the phase diagram. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H27.00011: Floquet Hopf Insulator in Dipolar Spin Systems Thomas Schuster, Felix Flicker, Snir Gazit, Jun Ye, Joel Moore, Norman Yao A tremendous amount of recent work has focused on non-trivial band structures that fall outside the "periodic table" of topological insulators. These include topological crystalline insulators, higher order topological insulators, as well as unstable topological insulators, whose topology is nontrivial only in low-band models. In three dimensions, the prime example of this latter group is the so-called "Hopf" insulator. Here, we provide the first experimental blueprint for realizing a Hopf insulator in a dipolar spin system as well as explicit evidence for the role of a recently proposed generalized particle-hole symmetry. Second, we extend the Hopf insulator’s classification to Floquet systems and find the existence of a new phase, the "anomalous Floquet Hopf insulator". Contrary to previously classified Floquet-Bloch systems, the change in Hopf invariant (a Z invariant) at topological defects of the Floquet evolution is only conserved modulo two, leading to a Z x Z2 classification for the Floquet phase. We provide an explicit homotopy demonstrating this reduced classification, and highlight its relation to the Hopf insulator’s instability. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H27.00012: Stability of the anomalous Floquet insulator Frederik Nathan, Mark Rudner, Dmitry Abanin, Netanel Lindner, Erez Berg Many-body localization stabilizes periodically driven quantum systems, leading to the possibility of distinct non-equilibrium Floquet phases of matter with no equilibrium counterparts. Here we demonstrate the existence of an anomalous Floquet insulator (AFI) phase in two dimensions. This phase is characterized by a nonzero, quantized magnetization in the bulk, and delocalized chiral states at the edge that support a quantized net current. The phase is stabilized by disorder, and we argue that the bulk remains many-body localized in the presence of interactions, while the chiral edge states display protected thermalization. We numerically investigate the dynamics of AFI in an open geometry. We find that the non-uniform particle density profiles remain stable in the bulk, out to the longest timescales that we can access, while the propagating edge states persist and thermalize, despite being coupled to the localized bulk. This opens up the possibility of observing quantized edge transport at high temperature, in the presence of interactions. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H27.00013: Quantized large-bias current in the anomalous Floquet-Anderson insulator Netanel Lindner, Arijit Kundu, Mark Rudner, Erez Berg We study two-terminal transport through two-dimensional periodically driven systems in which all bulk Floquet eigenstates are localized by disorder. We focus on the Anomalous Floquet-Anderson Insulator (AFAI) phase, a topologically-nontrivial phase within this class, which hosts topologically protected chiral edge modes coexisting with its fully localized bulk. We show that the unique properties of the AFAI yield remarkable far-from-equilibrium transport signatures: for a large bias between leads, a quantized amount of charge is transported through the system each driving period. Upon increasing the bias, the chiral Floquet edge mode connecting source to drain becomes fully occupied and the current rapidly approaches its quantized value. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H27.00014: Detecting Topological Invariants via Losses Tibor Rakovszky, Janos Asboth, Andrea Alberti We show that the bulk winding number characterizing one-dimensional topological insulators with chiral symmetry can be detected from the displacement of a single particle, observed via losses. Losses represent the effect of repeated weak measurements on one sublattice only, which interrupt the dynamics periodically. When these do not detect the particle, they realize negative measurements. Our repeated measurement scheme covers both time-independent and periodically driven (Floquet) topological insulators, with or without spatial disorder. In the limit of rapidly repeated, vanishingly weak measurements, our scheme describes non-Hermitian Hamiltonians, such as the lossy Su-Schrieffer-Heeger model. We find, contrary to intuition, that the time needed to detect the winding number can be made shorter by decreasing the efficiency of the measurement. We illustrate our results on a discrete-time quantum walk, and propose ways of testing them experimentally. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H27.00015: Topological Triply-Degenerate Points Induced by Spin-Tensor-Momentum Couplings Haiping Hu, Junpeng Hou, Fan Zhang, Chuanwei Zhang The recent discovery of triply-degenerate points (TDPs) in topological materials has opened a new perspective toward the realization of novel quasiparticles without counterparts in quantum field theory. The emergence of such protected nodes is often attributed to spin-vector-momentum coupling (SVMC). Here we show that the interplay between spin-tensor-momentum coupling (STMC) and SVMC can induce three types of TDPs, classified by their different Chern numbers (C = ±2, ±1, 0). Under a Zeeman field, type-I (C = ±2) and type-II (C = ±1) TDPs can be lifted into two Weyl points carrying the same and opposite monopole charges, respectively, whereas a type-III (C = 0) TDP is broken into two pairs of Weyl points of opposite charges. We find that different TDPs of the same type are connected by intriguing Fermi arcs on surfaces, and that transitions between different types are accompanied by level crossings along high-symmetry lines. We further propose an experimental scheme for realizing such TDPs in cold-atom optical lattices. Our results provide a framework for studying STMC-induced TDPs and other exotic quasiparticles. |
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