Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V27: Non-Equilibrium Physics in AMO Systems III |
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Sponsoring Units: DAMOP DCMP Chair: Simon Weidinger, Technical University of Munich Room: LACC 404B |
Thursday, March 8, 2018 2:30PM - 2:42PM |
V27.00001: Observation of a Dynamical Phase Transition in a Quantum Simulation of the Collective Heisenberg Model Ben Olsen , Scott Smale , Peiru He , Haille Sharum , Kenneth Jackson , Andrew Koller , Jamir Marino , Ana Maria Rey , Joseph Thywissen We engineer a quantum simulator of the collective Heisenberg model with a local longitudinal field. The model is implemented using the two lowest hyperfine states of a Fermi-degenerate gas of Potassium (40K) atoms, in a weakly interacting regime with motion frozen in single-particle eigenstates. We initialize a coherent superposition with maximal transverse magnetization, and measure magnetization dynamics using a Ramsey sequence. We observe a dynamical phase transition between two steady states: an ordered ferromagnetic state, where the transverse magnetization is stabilized by a large energy gap, and a demagnetized state. We explore the dynamical phase diagram of the model by tuning both interaction strength (with a Feshbach resonance) and inhomogeneity variance (with vector light shifts). We also validate experimentally the spin model description of the dynamics. We find excellent agreement with theoretical calculations based on a mean-field treatment of the Heisenberg model. The observed stabilization of many-body coherence over long times opens a new window for the generation of correlated quantum states in fermions, with applications to enhanced metrology and advanced materials. |
Thursday, March 8, 2018 2:42PM - 2:54PM |
V27.00002: Out-of-time-ordered Correlators in Integrable Quantum Ising Model Cheng-Ju Lin , Olexei Motrunich Out-of-time-ordered correlators (OTOC) have been proposed to characterize quantum chaos in generic systems. However, they can also show interesting behavior in integrable models resembling the OTOC in chaotic systems in some aspects. In this work, we study the OTOC in the 1d quantum Ising model. We find that the OTOC for spin operators that are local in terms of the Jordan-Wigner fermions has a ``shell-like'' structure: after the wavefront passes, the OTOC approaches its starting value in the long-time limit in a $t^{-1}$ fashion, showing no sign of scrambling. On the other hand, the OTOC for spin operators that are non-local in the Jordan-Wigner fermions has a ``ball-like'' structure, with its value reaching zero in the long-time limit, looking like a signature of scrambling; the approach to zero, however, is described by a slow power-law $t^{-1/4}$. These long-time power-law behaviors in the lattice model are not captured by conformal field theory calculations. The mixed OTOC with both local and non-local operators in the Jordan-Wigner fermions also has a ``ball-like'' structure, but the limiting values and the decay behavior appear to be non-universal. In all cases, we are not able to define a parametrically large window around the wavefront to extract the Lyapunov exponent. |
Thursday, March 8, 2018 2:54PM - 3:06PM |
V27.00003: Nonequilibrium melting of spin and orbital-order in the two-band Hubbard model Jiajun Li , Martin Eckstein We study the dynamics of spin and orbital order after strong non-equilibrium excitation in a two-band Hubbard model, using non-equilibrium dynamical mean field theory. The model features an A-type antiferromagnetic phase with antiferro-orbital ordering in equilibrium. Various charge excitations are created during the strong electric pulse, causing both spin and orbital order to melt dynamically. However, due to strong anisotropy in the hopping of electrons in the ordered phase, the orbital-order defects are easier to create and move than the spin-order defects. Therefore, the antiferromagnetic order typically melts slower than the antiferro-orbital order. In addition, varying Hund's coupling modifies the energy spectrum of the two-electron sector in the local Hilbert space. We demonstrate that this can be utilized to control the relative populations of non-equilibrium excitations and therefore the dynamical melting of order parameters. Our finding reveals the possibility of preparing non-thermal spin and orbital-ordered phases and may lead to a way of simultaneously controlling the order parameters with non-equilibrium techniques. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V27.00004: Scaling of the light-cone in the time evolution of long-range interacting quantum spin-chains Lorenzo Cevolani , Julien Despres , Giuseppe Carleo , Luca Tagliacozzo , Laurent Sanchez-Palencia We provide a universal picture of correlation spreading in spin systems with long-range spin-exchange term. Using the quasi-particle picture, we demonstrate that the space-time correlation map exhibits a two-fold structure. While the outer structure determines the correlation edge, the inner structure determines the propagation of local extrema. The outer structure depends on the spectrum and on the observable and it is sub-ballistic for observable without a strong infrared divergence. This means that the spreading is slower than the one predicted by the Lieb-Robinson bounds for these models. On the other side, the inner structure depends just on the energy spectrum and it is ballistic for gapped Hamiltonians and faster-than-ballistic for gapless Hamiltonians. Our results shed new light on correlation spreading in quantum systems and provides a unified picture to interpret previous results present in the literature. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V27.00005: Symmetry Protected Dynamical Symmetry in the Generalized Hubbard Models Jinlong Yu , Ning Sun , Hui Zhai In this talk we present a theorem on the dynamics of the generalized Hubbard models. This theorem shows that the symmetry of the single particle Hamiltonian can protect a kind of dynamical symmetry driven by the interactions. Here the dynamical symmetry refers to that the time evolution of certain observables are symmetric between the repulsive and attractive Hubbard models. We demonstrate our theorem with three different exam- ples in which the symmetry involves bipartite lattice symmetry, reflection symmetry and translation symmetry, respectively. Each of these examples relates to one recent cold atom experiment on the dynamics in the optical lattices where such a dynamical symmetry is manifested. These experiments include expansion dynamics of cold atoms, chirality of atomic motion within a synthetic magnetic field and melting of charge-density-wave order. Therefore, our theorem provides a unified view of these seemingly disparate phenomena. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V27.00006: Emergent CDW order in a 1D correlated electron system with underlying magnetic microstructure Thomas Koehler , Sebastian Paeckel , Salvatore Manmana We address the question of charge density wave order (CDW) emerging in a nonequilibrium situation in the course of a photo excitation. Using time-dependent matrix product states (tMPS), we model a pump excitation using Peierls substitution in a Hubbard chain at quarter filling, which is in the presence of an alternating magnetic background field. This magnetic microstructure is obtained for a 1D toy-manganate system, in which the $t_{2g}$ electrons form a frozen lattice of Zener polarons, but do not participate in the charge transport. We investigate the interplay of the magnetic microstructure and the Hubbard $U$ term for the dynamics of the system and discuss possible realizations with ultracold atoms on optical lattices. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V27.00007: Generation of single-particle excitations by time dependent impurity potential Jhih-Shih You , Richard Schmidt , Michael Knap , Dmitri A. Ivanov , Eugene Demler Generation of single-particle excitations (levitons) can occur in systems of heavy quantum impurities coupled to a bath of fermions. To induce such minimal excitations on top of the Fermi energy of a one-component Fermi gas, one can use a designed time dependent impurity-bath interaction. For impurities coupled to a two-component Fermi gas with such designed interaction, a proper operation of spin rotation which mixes two-component states leads to an optimal transport from one component to the other: The counting statistics of the transport is of single particle character. In particular we find that optimization of the transport is easily achieved in the regime of narrow Feshbach resonance and identify that the leviton physics manifests even in small systems. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V27.00008: Solvable Hydrodynamics of Quantum Integrable Systems Vir Bulchandani , Romain Vasseur , Christoph Karrasch , Xiangyu Cao , Joel Moore Quantum integrable systems, such as the interacting Bose gas in one dimension and the XXZ quantum spin chain, have an extensive number of local conserved quantities that endow them with exotic thermalization and transport properties. We discuss recently introduced hydrodynamic techniques to describe far-from-equilibrium dynamics in such integrable systems. Explicit comparison with density matrix renormalization group time evolution of a thermal expansion in the XXZ model shows that hydrodynamical predictions from smooth initial conditions can be remarkably accurate, even for small system sizes. Solutions are also obtained in the Lieb-Liniger model for free expansion into vacuum and collisions between clouds of particles, which model experiments on ultracold one-dimensional Bose gases. Finally, we discuss possible applications of hydrodynamics to scenarios where integrability is broken, for example, by a trapping potential. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V27.00009: Non-Equilibrium DMFT with Inchworm Quantum Monte Carlo Joseph Kleinhenz , Igor Krivenko , Guy Cohen , Emanuel Gull We present a real-time, numerically exact, inchworm quantum Monte Carlo solver for non-equilibrium impurity problems with fully time dependent parameters. Our solver works on the forward-backward Keldysh contour so correlations develop dynamically from an initially uncorrelated tensor product density matrix instead of being generated by an imaginary time evolution. We benchmark and compare this approach with the recently developed inchworm quantum Monte Carlo solver on the full Keldysh contour including equilibrium branch and show applications to quenches of the single-band Hubbard model on the infinite coordination number Bethe lattice within the framework of non-equilibrium dynamical mean field theory. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V27.00010: Absorbing state phase transition with competing quantum and classical fluctuations Michael Buchhold , Sebastian Diehl Stochastic processes with absorbing states feature remarkable examples of non-equilibrium universal phenomena. While a broad understanding has been progressively established in the classical regime, relatively little is known about the behavior of these non-equilibrium systems in the presence of quantum fluctuations. Here we theoretically address such a scenario in an open quantum spin model which in its classical limit undergoes a directed percolation phase transition. By mapping the problem to a non-equilibrium field theory, we show that the introduction of quantum fluctuations stemming from coherent, rather than statistical, spin-flips alters the nature of the transition such that it becomes first-order. In the intermediate regime, where classical and quantum dynamics compete on equal terms, we highlight the presence of a bicritical point with universal features different from the directed percolation class in low dimension. We finally propose how this physics could be explored within gases of interacting atoms excited to Rydberg states. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V27.00011: Quantum Integrability in Generic Time Dependent Hamiltonians Nikolai Sinitsyn , Emil Yuzbashyan , Vladimir Chernyak , Aniket Patra , Chen Sun We formulate a set of conditions under which the scattering problem for a time-dependent quantum Hamiltonian is integrable. The main requirement is the existence of a nonabelian gauge field with zero curvature in the space of system parameters. Known solvable multistate Landau-Zener models satisfy these conditions. Our method produces a strategy to incorporate time-dependence into various models that are convensionally known as integrable, so that the resulting non-stationary Schrodinger equation is explicitly solvable. We also validate some prior conjectures, including the solution of the driven Tavis-Cummings model. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V27.00012: Integrability in multistate Landau-Zener and Landau-Zener-Coulomb problems Nikolai Sinitsyn , Emil Yuzbashyan , Vladimir Chernyak , Aniket Patra , Chen Sun The multistate Landau-Zener (MLZ) problem is to find scattering amplitudes in models with linearly time-dependent evolution and Hamiltonians of the form H(t)=A+Bt, where A and B are arbitrary Hermitian matrices and t is time. This problem has long history. It originates from the work of Majorana in 1932 and finds numerous applications in modern atomic and mesocsopic physics. Recently, many new solvable MLZ classes have been found after the discovery of empirical integrability conditions. We use our theory of MLZ integrability in order to explain previous findings that so far have had the status of conjectures. We extend already known solutions to broader solvable classes, such as the Landau-Zener-Coulomb systems with H(t)=A+Bt+C/t, and show how to obtain solutions of highly nontrivial and physically interesting solvable many-body models, such as the BCS model with decaying as 1/t coupling. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V27.00013: A large class of solvable multistate Landau-Zener models and quantum integrability Chen Sun , Nikolai Sinitsyn We identify a new class of exactly solvable multistate Landau-Zener (MLZ) models. Such models can have an arbitrary number N of interacting states and quickly growing with N numbers of exact adiabatic energy crossing points, which appear at different values of time. At each N, transition probabilities in these systems can be found analytically and exactly but complexity and variety of solutions in this class also grow with N quickly. By exploring several low-dimensional sectors, we find features that shed light on the common properties of these solutions and, generally, on quantum integrability. We also show that the previously known bowtie model can be entirely derived as a special limit of our solvable class. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V27.00014: Thermalization after an ionic potential quench in the Ionic Hubbard Mode Soumen Bag , Hulikal Krishnamurthy Here we study the consequences of an ionic potential quench in the Ionic Hubbard Model on a Bethe lattice using non-equilibrium Dynamical Mean Field Theory. The system is quenched from a staggered charge density order(ionic potential Δ≠0) to a uniform state(for Δ = 0) for different values of the interaction U. We find that the staggered occupancy(δn) decreases rather fast from its initial value(for Δ ≠0) to the final δn =0(which is equilibrium δn when Δ = 0) , but in an oscillatory fashion. At large times(t > 3.0 $\hbar$/J, where J is the nearest neighbor hopping amplitude) these oscillations can be well fitted by a function of the form [Acos(ωt+φ)\exp{-Γ t}]/t^{α}. As one increases U the frequency of the oscillation(ω) is almost constant(≈4*J/$\hbar$) but the rate of decay of the oscillations, Γ, increases with increasing U, where as α deceases. When U =0 α ≈ 1.5 and Γ ≈ 0.0. We also find that none of the three fitting parameters (ω,α,Γ) depend on the initial staggered potential. It remains a challenging question to understand the microscopic origins of these parameters. We have also studied the evolution of the double occupancy under these quenches, and while it also thermalizes, its time dependence seems more complicated. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V27.00015: Supersolitons in a 2D anisotropic semimetal Seth Davis , Matthew S. Foster We study the effects of strong correlations on far- from-equilibrium quantum transport. We consider a |
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