Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G01: Topological States in AMO Systems IFocus

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Sponsoring Units: DAMOP DCMP Chair: Zhexuan Gong Room: 103 
Tuesday, March 3, 2020 11:15AM  11:51AM 
G01.00001: Observation of the Topological Anderson Insulator in Disordered Atomic Wires Invited Speaker: Bryce Gadway Disorder and topology have many deep connections, and can have a rich combined influence on the transport properties of quantum particles. Some of the most useful and intriguing properties of topological systems relate to the robustness of their edge states with respect to weak disorder. Rich phenomena are expected to occur as strong disorder is added to such systems, relating to the global unwinding of the nonlocal topological order. Surprisingly, it has even been predicted that the addition of disorder can induce topological order in otherwise topologically trivial materials. Despite these deep and rich connections, the difficulty of creating controlled disorder in real materials has prevented the exploration of disorderdriven changes in topology in experiments. Here we describe how a type of synthetic lattice can be engineered for dilute atomic gases at ultracold temperatures, allowing us to mimic the physics of disordered topological materials. We describe two types of topological transitions driven by the addition of disorder to onedimensional atomic wires. The first is the breakdown of a nontrivial band topology by the addition of strong disorder, relating to a randomsinglet topological transition. The second is a counterintuitive ``order by disorder'' transition in which nontrivial topology arises due to disorder, relating to the observation of the topological Anderson insulator phase predicted nearly a decade ago. We discuss extensions to this result, including prospects for exploring disordered topological materials in higher dimensions. 
Tuesday, March 3, 2020 11:51AM  12:03PM 
G01.00002: Topological Phases of Interacting Fermions in Optical Lattices Vito Scarola, Chuanchang Zeng, Tudor Dan Stanescu, Chuanwei Zhang, Sumanta Tewari Recent experiments have placed cold atoms into optical lattices in the presence of synthetic fields. This talk will review studies of HubbardHofstadter models in regimes revealing topological phases of ultracold fermions arising from the interplay of interparticle interactions and synthetic fields in kagome and square optical lattices. Focusing on one regime in particular, attractive interactions in a square optical lattice, we find that attractive swave interactions lead to a higherorder topological superfluid [1]. Higherorder topological superconductors hosting MajoranaKramers pairs as corner modes have recently been proposed in solids. Here, we show that such MajoranaKramers pairs can be realized using a conventional swave superfluid in an optical lattice but with a soliton. The MajoranaKramers pairs emerge at the “corners” defined by the intersections of line solitons and the onedimensional edges of the system. Our scheme sets the stage for observing possible higherorder topological superfluidity with conventional swave superfluids of cold atoms. 
Tuesday, March 3, 2020 12:03PM  12:15PM 
G01.00003: Topological Phase Transitions in Finitesize Systems Across Boundary Conditions Yang Ge, Marcos Rigol Systems driven unitarily across different topological phases seem to exhibit contrary behaviors depending on their boundary conditions [1] and the commensurability of system sizes [2]. In particular, a “nogo” theorem forbidding the change of the Chern number exists for periodic boundary systems. Here we first demonstrate the scaling of dynamical phase transition points in driven periodic boundary systems for different turnon speeds and incommensurate system sizes, which consist of a LandauZener governed regime and an adiabatic following regime. Similar regimes are also identified in the case of open boundary systems, proving that these boundary conditions agree in the thermodynamic limit. Finally, we show that with slow turnons the dc Hall response of a driven fermi sea starting from a trivial state does acquire a nontrivial value in a finitesize system, even when the “nogo” theorem applies. 
Tuesday, March 3, 2020 12:15PM  12:27PM 
G01.00004: Tenfold way for quadratic Lindbladians Simon Lieu, Max McGinley, Nigel Cooper We uncover a topological classification applicable to open fermionic systems governed by a general class of Lindblad master equations. These ‘quadratic Lindbladians’ can be captured by a nonHermitian singleparticle matrix which describes internal dynamics as well as systemenvironment coupling. We show that this matrix must belong to one of ten nonHermitian BernardLeClair symmetry classes which reduce to the AltlandZirnbauer classes in the closed limit. The Lindblad spectrum admits a topological classification, which we show results in gapless edge excitations with finite lifetimes. Unlike previous studies of purely Hamiltonian or purely dissipative evolution, these topological edge modes are unconnected to the form of the steady state. We provide onedimensional examples where the addition of dissipators can either preserve or destroy the closed classification of a model, highlighting the sensitivity of topological properties to details of the systemenvironment coupling. 
Tuesday, March 3, 2020 12:27PM  12:39PM 
G01.00005: Lattice gauge theories and string dynamics in Rydberg atom quantum simulators Federica Maria Surace, Paolo Pietro Mazza, Giuliano Giudici, Alessio Lerose, Andrea Gambassi, Marcello Dalmonte Lattice gauge theories are at the basis of our understanding of fundamental interactions and quantum simulations are one of the most promising paths to go beyond the exponential complexity of this quantum many body problem. We show that Rydberg atom setups represent an ideal platform for the investigation of equilibrium and nonequilibrium properties of lattice gauge theories, by proving their equivalence to abelian (and possibly non abelian) gauge theories on the lattice. Building on this correspondence, we show that the recently observed anomalously slow dynamics corresponds to a stringinversion mechanism, reminiscent of the stringbreaking typically observed in gauge theories. 
Tuesday, March 3, 2020 12:39PM  12:51PM 
G01.00006: Exact quantum manybody scar states intrinsic to periodically driven system in the Rydbergblockaded atom chain Sho Sugiura, Tomotaka Kuwahara, Keiji Saito In a periodically driven manybody system, any quantum state usually ends up an infinite temperature state. This is called the Floquet energy eigenstate thermalization hypothesis (Floquet ETH). Here we discuss the violation of the Floquet ETH with realistic Hamiltonians. Our periodically driven model is based on the PXP type interaction, which is demonstrated in a recent experiment in the Rydberg atoms chain by Harvard group. By showing explicit expressions of the wave functions, we exactly prove the existence of the manybody scar states, which show nonthermal behavior, while other states obey the Floquet ETH. In addition, we systematically engineer various periodicallydriven Hamiltonians having Floquet manybody scar states. 
Tuesday, March 3, 2020 12:51PM  1:03PM 
G01.00007: Exact quantum manybody scar states in the twodimensional PXP model and their dynamical signatures ChengJu Lin, Vladimir Calvera, Timothy Hsieh The twodimensional PXP model is an effective model describing the dynamics of a twodimensional Rydberg atom array in the nearestneighbor blockade regime. We discover exponentially many exact quantum manybody scar states in the twodimensional PXP model. These exact scar states have valence bond solid order, therefore breaking the lattice translation symmetry, despite being at effectively infinite temperature. As a manifestation of these scar states, we propose a quench experiment starting from a chargedensitywave initial state, resulting in strong oscillations in the total Rydbergexcitation number. Another signature is longlived valence bond solid order of the scar states after a quench into a weakly perturbed model. 
Tuesday, March 3, 2020 1:03PM  1:15PM 
G01.00008: Fragmented Hilbert Spaces and Slow Dynamics in Quantum Dimer Models Johannes Feldmeier, Frank Pollmann, Michael Knap The presence of dynamical constraints can have severe impacts on the nonequilibrium dynamics of closed quantum systems, leading to slow or even nonergodic behavior at finite energy densities. One mechanism for the occurrence of such features is a fragmentation of the Hilbert space structure in systems of fractons, consisting of particles with reduced mobility [1]. Within this context, we investigate the connectivity of configuration spaces resulting from the hard core constraint in closepacked dimer models. On the square lattice, we uncover the emergence of effectively immobile defects within a staggered background, whose dynamics is impeded up to high orders in perturbation theory. This fractonlike structure of the low energy Hilbert space is shown to give rise to glassy behavior for states at low temperatures [2]. We further discuss extensions to different lattices and dimensions that allow for the explicit construction of large sets of conserved quantities, leading to slow dynamics and localized modes for typical states also at high energies. 
Tuesday, March 3, 2020 1:15PM  1:27PM 
G01.00009: Thermalization and its Breakdown for a Large Nonlinear Spin Shane Kelly, ShanWen Tsai, Eddy M.E. Timmermans By developing a semiclassical analysis based on the Eigenstate Thermalization Hypothesis, we determine the long time behavior of a large spin evolving with a nonlinear Hamiltonian. Despite integrable classical dynamics, we find the Eigenstate Thermalization Hypothesis is satisfied in the majority of eigenstates and thermalization is generic. The exception is a novel mechanism for the breakdown of thermalization based on an unstable fixed point in the classical dynamics. Using the semiclassical analysis we derive how the equilibrium values of observables encode properties of the initial state. We conclude with a discussion of relevant experiments and the potential generality of this mechanism for the breakdown of thermalization. 
Tuesday, March 3, 2020 1:27PM  1:39PM 
G01.00010: CPT invariant topological phase in nonHermitian spin1/2 quantum systems Tanmoy Das, Ananya Ghatak NonHermitian system has recently found its place in laboratories, raising the hope to obtain unconventional and uncharted physical properties. Over the last two to three years, we have made several contributions to timedependent theory for parity (P), timereversal (T) invariant nonHermitian Hamiltonians; delineating its conservation laws, CPT gauge invariance, and new term in the Berry phase. I will talk about a realizable spinful nonHermitian system exhibiting the nonAbelian Berry phase and supersymmetric edge states. 
Tuesday, March 3, 2020 1:39PM  1:51PM 
G01.00011: Topological insulatorsuperconductor phase transition in a nonHermitian spin chain Yunmei Li, Xiwang Luo, Junpeng Hou, Chuanwei Zhang NonHermitian systems can host nontrivial topological phases with the topological characterization distinct from Hermitian systems. Here we consider a 1D XY spin model with nonHermitian outofplane magnetic field, which is periodically modulated. For isotropic XY interaction, the system corresponds to a topological insulator (TI) with pseudoantiHermiticity, which supports edge states characterized by the quantized Zak phase. The anisotropy of XY interaction gives raise to superconducting paring, and the system experiences a transition from TI to topological superconducting (TSC) as the anisotropy increases. The transition point, where the system undergoes a band touch at E=0, can be tuned by the modulation phases and amplitudes of the nonHermitian field. The TSC phase supports Majorana edge modes, protected by the chiral symmetry. The model can be experimentally simulated with trapped ions. Our work provides a method to investigate the interplay between nonHermiticity and superconducting interaction and paves the way for exploring various topological insulators and superconductors in nonHermitian systems. 
Tuesday, March 3, 2020 1:51PM  2:03PM 
G01.00012: Nonadiabatic topological energy pumps with three incommensurate frequencies Rongchun Ge, Frederik Nathan, Takahiro Morimoto, Snir Gazit, Mark Rudner, Michael Kolodrubetz

Tuesday, March 3, 2020 2:03PM  2:15PM 
G01.00013: Quadrupole Topological Photonic Crystals Li He, Zachariah M Addison, Eugene John Mele, Bo Zhen Quadrupole topological phases, exhibiting protected boundary states that are themselves topological insulators of lower dimensions, have recently been of great interest. Extensions of these ideas from current tight binding models to continuum theories for realistic materials require the identification of quantized invariants describing the bulk quadrupole order. Here we identify the analog of quadrupole order in Maxwell’s equations for a photonic crystal (PhC). Unlike prior studies relying on threaded flux, our quadrupole moment is quantized purely by crystalline symmetries. Furthermore, through the bulkedge correspondence of Wannier bands, we reveal the boundary manifestations of nontrivial quadrupole phases as quantized polarizations at edges and bound states at corners. Finally, we relate the nontrivial corner states to the emergent phenomena of quantized fractional corner charges and a filling anomaly. 
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