Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K44: Quantum Phase Transitions: Magnetism and Related Orders |
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Sponsoring Units: DCMP GMAG Chair: Anders Sandvik, Boston University Room: LACC 504 |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K44.00001: Quantum Phase Transition between Plaquette and Neel Phases in a Quantum Magnet on the Shastry-Sutherland Lattice Bowen Zhao , Anders Sandvik A recent experiment [1] on a Shastry-Sutherland (SS) material under pressure provides us with the first evidence of a phase transition between an antiferromagnet and a 2-fold degenerate 4-spin plaquette singlet state in a material. It is also potentially the first experimental realization of the deconfined quantum critical point. However, due to the frustrated couplings in the original SS model, it is challenging to study this model using unbiased numerical approaches, especially because of sign problems in quantum Monte Carlo simulations. Therefore, the nature of the phase transition has not yet been studied in detail. Here we propose a "designer Hamiltonian" describing the same physics as that in the experimental material but without traditional frustration. We instead construct a suitable J-Q model, a Heisenberg model extended by a particular 4-spin interaction on the SS lattice. Sign-free quantum Monte Carlo calculation shows that this model indeed hosts a quantum phase transition from the Neel ground state into a plaquette singlet phases. We will discuss the nature of the transition and a possible emergent higher symmetry. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K44.00002: Phase Transition of Heisenberg Model on Honeycomb Lattice with Mixed Interactions by Quantum Monte Carlo Yizhen Huang , Gang Su We studied the quantum Heisenberg model with J>0 and J'<0 along the zigzag and armchair directions on honeycomb lattice with worm monte carlo method. By size extrapolation of the stripe order, we found that when tuning J' with J fixed the system crossovers from a disordered 1D system to a striped ordered 2D one. An uniform magnetic field h perpendicular to the plane tilts the spins along the field's direction and keeps the transverse components stripe ordered. As long as h ≧2J, all the spins are polarized. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K44.00003: Spin density wave field investigation in quasi-one-dimensional conductors Guoqing Wu , Rongxing Cao , Jun Dong , Yafang Xu , Xiao-Shan Ye , Jinbo Zhang , Xianghua Zeng , Qiuliang Wang , Bing Wu , William Clark Spin Density Wave (SDW) as a type of long range spin ordering exhibits in low dimensional conductors. It has been found to accompany or coexist or even compete with macroscopic coherence like superconductivity. However, its significance is not fully understood. It has been a long mystery regarding the mechanism of the resistivity upturn at ~ 25 K and its association with the superconductivity at lower temperatures exhibiting in the quasi-one-dimensional conductor Li_{0.9}Mo_{6}O_{17. }Here we report a spin density wave field investigation using a simple theoretical model, which helps a much better understanding of the experimental data previously reported and some of the ^{7}Li-NMR data we have as well. Our result suggests that SDW be the mechanism of the resistivity upturn appearing at ~ 25 K and it also be the possible electron quantum ground state of the material. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K44.00004: Numerical Evidence of Emergent Symmetry in Spin-1/2 Heisenberg Chain Pranay Patil , Anders Sandvik , Emanuel Katz The antiferromagnetic Heisenberg chain is expected to have an extended symmetry, SU(2)xSU(2), in the infrared limit, whose physical interpretation is that the spin and dimer order parameters form the components of a common 4-dimensional vector. Here we numerically investigate this emergent symmetry using quantum Monte Carlo simulations of a modified Heisenberg chain (the J-Q model) in which the logarithmic scaling corrections of the conventional Heisenberg chain can be avoided. We show how the two- and three-point spin and dimer correlation functions approach their forms constrained by conformal field theory as the system size increases and characterize the scaling corrections due to violations of the extended symmetry caused by the finite system size. Because of these corrections, it is harder to observe the asymptotic behavior than in simpler conformal field-theory realizations such as the transverse-field Ising chain. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K44.00005: Decoherence from Spin Environments: Loschmidt Echo and Quasiparticle Excitations Henrik Johannesson , Rouhollah Jafari We revisit the problem of decoherence of a qubit centrally coupled to an interacting spin environment, here modeled by a quantum compass chain or an extended XY model in a staggered magnetic field. These two models both support distinct spin liquid phases, adding a new element to the problem. By analyzing their Loschmidt echoes when perturbed by the qubit we find that a fast decoherence of the qubit is conditioned on the presence of propagating quasiparticles which couple to it. Different from expectations based on earlier works on central spin models, our result implies that the closeness of an environment to a quantum phase transition is neither a sufficient nor a necessary condition for an accelerated decoherence rate of a qubit. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K44.00006: Effect of Electric field on Breathing Pyrochlores Pallasena Viswanathan Sriluckshmy , Ipsita Mandal , Subhro Bhattacharjee , Roderich Moessner The coupling between conventional (Maxwell) and emergent electrodynamics in quantum spin ice has been studied by Lantagne-Hurtubise et al. (Phys. Rev. B 96, 125145) where they find that a uniform electric field can be used to tune the properties of both the ground state and excitations of the spin liquid. Extending the study to the case of breathing pyrochlores, we find a sufficiently strong electric field triggers a quantum phase transition into new U (1) quantum spin liquid phases along a direction that did not show a phase transition in the isotropic limit. We also analyse the phase diagram of breathing pyrochlores in the presence of Electric field using gauge mean field theory. Finally, we discuss experimental aspects of our results. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K44.00007: Dynamical structure factor S(ω, k) of longer ranged interacting Ising chain with disorder Jian Wang Dynamical structure factor S(ω, k) is calculated for the longer ranged interacting Ising chain model with disorder. Majorana zeros are observed in their energy spectrums. These disorder induced Majorana zero modes, can be explained as the “boundary” modes of ferromagnetic Griffith rare regions within the paramagnetic system bulk |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K44.00008: Extracting universal features of the 2+1 critical Ising model via entanglement scaling Bohdan Kulchytskyy , Lauren Hayward Sierens , Roger Melko Entanglement entropy has emerged as new a paradigm for studying and characterizing condensed matter systems. The scaling of entropy with the size of the entangled region can reveal universal features of the continuum theory which underlies a lattice model. We perform large-scale Monte-Carlo simulations of a 2+1 Ising model tuned to its critical temperature, belonging to the universality class of the Wilson-Fisher fixed point. We study the universal shape-dependent contribution to the entanglement entropy between two complementary cylindrical regions on finite lattices. In the thin strip limit, we extract a universal constant and relate it to its corresponding value for a free scalar field theory. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K44.00009: New phase transitions and singularities in quasiperiodic quantum Ising chains Philip Crowley , Anushya Chandran , Christopher Laumann Unlike random or periodic potentials, quasi-periodic modulation can induce localisation-delocalisation transitions in one dimension. In this talk we analyse the implications of this for symmetry breaking in the quasi-periodically modulated quantum Ising model in 1D. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K44.00010: Functional renormalization group analysis of spinless fermions on the honeycomb lattice beyond half filling Stephan Hesselmann , David D. Scherer , Michael Scherer , Stefan Wessel We study spinless fermions on a honeycomb lattice with repulsive interactions (spinless $t-V$ model), which for the case of half filling is known to feature a Gross-Neveu chiral Ising quantum critical point at finite interaction strength. The phase diagram of this model for a finite chemical potential however remains elusive. We therefore perform an instability analysis using the functional renormalization group method with a basic Fermi surface patching scheme, which allows us to treat instabilities in different channels on an equal footing. The chemical potential is fixed at the beginning of the flow and defines the filling in terms of the free system. Below half filling but above the van-Hove filling the free Fermi surface is hole-like and we find a commensurate charge-density-wave to be dominant. Its characteristics are those of the half-filled case, which indicates phase separation at large interaction strength. Directly at the van-Hove filling the nesting property of the free Fermi surface stabilizes a dimerized charge-bond order phase. At lower fillings the free Fermi surface becomes electron-like and a superconducting instability with $f$-wave symmetry emerges. Finally we estimate the extend of the various phases and give expressions for their order parameters. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K44.00011: Ground states of center-of-mass preserving Hamiltonians Sanjay Moudgalya , Nicolas Regnault , Andrei Bernevig We investigate the energy spectra for a class of spinless fermion Hamiltonians that, in addition to the conservation of center-of-mass momentum, also conserve the center-of-mass position. Such models were first proposed in the context of featureless Mott insulators. At several filling factors p/q, where p and q are coprime integers, the low-energy spectrum can be described by a simpler nearest-neighbor effective Hamiltonian with a smaller Hilbert space. We find examples of both gapped and gapless effective Hamiltonians at various filling factors. We obtain the exact gapless ground state at half-filling, which is expected to fall outside the usual Luttinger liquid paradigm due to center-of-mass preservation, and we compute its properties. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K44.00012: Photo-induced enhancement of excitonic order Yuta Murakami , Denis Golez , Martin Eckstein , Philipp Werner Recently, the nonequilibrium dynamics of the excitonic insulator, which is a pair condensation of electrons and holes, starts to attract interest. In this talk, we discuss the dynamical properties of excitonic insulators coupled to phonons [1]. Without phonon couplings, the linear response is given by the damped amplitude oscillations of the order parameter with frequency equal to the minimum band gap. A phonon coupling to the interband transfer integral induces two types of long-lived collective oscillations of the amplitude, one originating from the phonon dynamics and the other from the phase mode, which becomes massive. We show that even for small phonon coupling, a photo-induced enhancement of the exciton condensation and the gap can be realized. Using the Anderson pseudo-spin picture, we argue that the origin of the enhancement is a cooperative effect of the massive phase mode and the Hartree shift induced by the photo excitation. We also discuss how the enhancement of the order and the collective modes can be observed with time-resolved photo-emission spectroscopy. In addition, we also show the possible dynamical phase transition in the excitonic insulator. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K44.00013: Entanglement in the Spin-1 AFM Heisenberg Model James Lambert , Erik Sorensen The Fisher information is an experimentally accesible mutipartite entanglement witness. It is unqiue from other entanglement witnesses as it allows for the detection of entanglement at finite temperatures. The spin 1 anit-ferromagnetic Heisenberg model exhibits a non-trivial phase with a gapped excitation spectrum. The addition of a uniaxial anisotropy term may drive two quantum phase transitions. Using the Fisher information we examine the bipartite and multipartite entanglement in a finite temperature region region around the isotropic point and as the quantum critical points are approached. We apply the single mode approximation to examine the entangelement properties of the groundstate in the Haldane gap. |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K44.00014: The Z_{3} order–disorder quantum phase transition in the chiral clock model Rhine Samajdar , Soonwon Choi , Mikhail Lukin , Hannes Pichler , Subir Sachdev The Z_{3} chiral clock model describes a particular generalization of the transverse-field Ising model where each Ising spin is replaced by one with three states. Additionally, the interactions, which are invariant under Z_{3}-symmetry, are "chiral''. This model exhibits a second-order phase transition between an ordered phase, where the Z_{3}-symmetry is spontaneously broken, and a disordered phase. We study the nature of this transition and numerically calculate its critical exponents with the density-matrix renormalization group. In particular, we use finite-size scaling to determine the dynamical critical exponent z and the correlation length exponent ν. Our analysis presents one of the first nontrivial instances of a quantum phase transition with z ≠ 1, implying an underlying non-conformal critical field theory. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K44.00015: Universal Scaling and Critical Exponents of the Anisotropic Quantum Rabi Model Maoxin Liu , Stefano Chesi , Zu-Jian Ying , Xiaosong Chen , Hong-Gang Luo , Hai-Qing Lin We investigate the quantum phase transition of the anisotropic quantum Rabi model, in which the rotating and counter rotating terms are allowed to have different coupling strengths. The model interpolates between two known limits with distinct universal properties. Through a combination of analytic and numerical approaches, we extract the phase diagram, scaling functions, and critical exponents, which determine the universality class at finite anisotropy (identical to the isotropic limit). We also reveal other interesting features, including a superradiance-induced freezing of the effective mass and discontinuous scaling functions in the Jaynes-Cummings limit. Our findings are extended to the few-body quantum phase transitions with N > 1 spins, where we expose the same effective parameters, scaling properties, and phase diagram. Thus, a stronger form of universality is established, valid from N = 1 up to the thermodynamic limit. More details please see arXiv:1702.06641 (accepted by Physcial Review Letters). |
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