arXiv subjects

Zi Cai

Publications and source records attributed to Zi Cai.

At least 37 records · Page 2Linked to original sources

arXiv2023

Space-time symmetry breaking in nonequilibrium frustrated magnetism

Spontaneous symmetry breaking is responsible for the rich phenomena in equilibrium physics. Driving a system out-of-equilibrium can significantly enrich the possibility of spontaneous symmetry breaking, which occurs not only in space, but also in time domain. This study investigates a driven-dissipative frustrated magnetic system. Results show that frustration in such a far-from-equilibrium system could lead to a wealth of intriguing non-equilibrium phases with intertwined space-time symmetry breaking, (e.g.) a discrete time crystal phase accompanied by a time-dependent spatial order oscillating between a long-range tripartite stripe and a short-range ferromagnetic order.

cond-mat.stat-mech↗

arXiv2022

Localization of matter waves in lattice systems with moving disorder

We study the localization phenomena in a one-dimensional lattice system with a uniformly moving disordered potential. At a low moving velocity, we find a sliding localized phase in which the initially localized matter wave adiabatically follows the moving potential without diffusion, thus resulting in an initial state memory in the many-body dynamics. Such an intriguing localized phase distinguishes itself from the standard Anderson localization in two aspects: it is not robust against interaction, but persists in the presence of slowly varying perturbations. Such a sliding localized phase can be understood as a consequence of interference between the wavepacket paths under moving quasi-periodic potentials with various periods that are incommensurate with the lattice constant. The experimental realization and detection were also discussed..

cond-mat.quant-gas↗

arXiv2022

Nonergodic delocalized paramagnetic states in quantum neural networks

Typically, it is assumed that a high-energy eigenstate of a generic interacting quantum many-body Hamiltonian is thermal and obeys the eigenstate thermalization hypothesis. In this work, we show that the paramagnetic phase of a quantum Hopfield neural network model is delocalized but nonergodic. The combination of permutational symmetry and frustration in this model organize its high-energy eigenstates into clusters, which can each be considered a large quantum spin and has no correlation with others. This model provides another ergodicity-breaking mechanism in quantum many-body systems.

cond-mat.dis-nn↗

arXiv2022

Thermal melting of discrete time crystals: a dynamical phase transition induced by thermal fluctuations

The stability of a discrete time crystal against thermal fluctuations has been studied numerically by solving a stochastic Landau-Lifshitz-Gilbert equation of a periodically-driven classical system composed of interacting spins, each of which couples to a thermal bath. It is shown that in the thermodynamic limit, even though the long-range temporary crystalline order is stable at low temperature, it is melting above a critical temperature, at which the system experiences a non-equilibrium phase transition. The critical behaviors of the continuous phase transition have been systematically investigated, and it is shown that despite the genuine non-equilibrium feature of such a periodically driven system, its critical properties fall into the 3D Ising universality class with a dynamical exponent ($z=2$) identical to that in the critical dynamics of kinetic Ising model without driving.

cond-mat.stat-mech↗

arXiv2022

A $\frac 13$ power-law universality class out of stochastic driving in interacting systems

In this paper, we study the mean-field dynamics of a general class of many-body systems with stochastically fluctuating interactions. Our findings reveal a universal algebraic decay of the order parameter $m(t)\sim t^{-χ}$ with an exponent $χ=\frac 13$ that is independent of most system details including the strength of the stochastic driving, the energy spectrum of the undriven systems, the initial states and even the driving protocols. It is shown that such a dynamical universality class can be understood as a consequence of a diffusive process with a time-dependent diffusion coefficient which is determined self-consistently during the evolution. The finite-size effect, as well as the relevance of our results with current experiments in high-finesse cavity QED systems are also discussed.

cond-mat.str-el↗

arXiv2021

Symmetry protected topological edge modes and emergent partial time reversal symmetry breaking in open quantum many-body systems

Symmetry-protected topological edge modes are one of the most remarkable phenomena in topological physics. Here, we formulate and quantitatively examine the effect of a quantum bath on these topological edge modes. Using the density matrix renormalization group method, we study the ground state of a composite system of spin-1 quantum chain, where the system and the bath degrees of freedom are treated on the same footing. We focus on the dependence of these edge modes on the global/partial symmetries of system-bath coupling and on the features of the quantum bath. It is shown that the time-reversal symmetry(TRS) plays a special role for an open quantum system, where an emergent partial TRS breaking will result in a TRS-protected topological mode diffusing from the system edge into the bath, thus make it useless for quantum computation.

cond-mat.str-el↗

arXiv2021

Order by disorder in frustration-free systems: a Quantum Monte Carlo study of a two-dimensional PXP model

In this study, an order by disorder mechanism has been proposed in a two-dimensional PXP model, where the extensive degeneracy of the classical ground-state manifold is due to strict occupation constraints instead of geometrical frustrations. By performing an unbias large-scale quantum monte carlos simulation, we find that local quantum fluctuations, which usually work against long-range ordering, lift the macroscopic classical degeneracy and give rise to a compressible ground state with charge-density-wave long-range order. A simple trial wavefunction has been proposed to capture the essence of the ground-state of the two-dimensional PXP model. The finite temperature properties of this model have also been studied, and we find a thermal phase transition with an universality class of two-dimensional Ising model.

cond-mat.str-el↗

arXiv2021

Constraint-induced breaking and restoration of ergodicity in spin-1 PXP models

Eigenstate Thermalization Hypothesis(ETH) has played a pivotal role in understanding ergodicity and its breaking in isolated quantum many-body systems. Recent experiment on 51-atom Rydberg quantum simulator and subsequent theoretical analysis have shown that hardcore kinetic constraint can lead to weak ergodicity breaking. In this work, we demonstrate, using 1d spin-1 PXP chains, that miscellaneous type of ergodicity can be realized by adjusting the hardcore constraints between different components of nearest neighbor spins. This includes ETH violation due to emergent shattering of Hilbert space into exponentially many subsectors of various sizes, a novel form of non-integrability with an extensive number of local conserved quantities and strong ergodicity. We analyze these different forms of ergodicity and study their impact on the non-equilibrium dynamics of a Z2 initial state. We use forward scattering approximation (FSA) to understand the amount of Z2-oscillation present in these models. Our work shows that not only ergodicity breaking but an appropriate choice of constraints can lead to restoration of ergodicity as well.

quant-ph↗

arXiv2020

An instanton-like excitation of a discrete time crystal

Spontaneous symmetry breaking and elementary excitation are two of the pillars of condensed matter physics that are closely related to each other. The symmetry and its spontaneous breaking not only control the dynamics and spectrum of elementary excitations, but also determine their underlying structures. In this paper, we study the excitation properties of a non-equilibrium quantum matter: a discrete time crystal phase that spontaneously breaks the temporal translational symmetry. It is shown that such an intriguing symmetry breaking allows an instanton-like excitation that represents a tunneling between two "degenerate" time crystal phases. Furthermore, we also observe a dynamical transition point at which the instanton "size" diverges, a reminiscence of the critical slowing down phenomenon in nonequilibrium statistic physics. A phenomenological theory has been proposed to understand the phase dynamics of the proposed system and the experimental realization and detection have also been discussed.

cond-mat.quant-gas↗

arXiv2020

Pattern formation and exotic order in driven-dissipative Bose-Hubbard systems

Modern experimental platforms such as supercoducting-circuit arrays call for the exploration of bosonic tight-binding models in unconventional situations with no counterpart in real materials. Here we investigate one of such situations, in which excitations are driven and damped by pairs, leading to pattern formation and exotic bosonic states emerged from a non-equilibrium quantum many-body system. Focusing on a two-dimensional driven-dissipative Bose-Hubbard model, we find that its steady states are characterized by the condensation of bosons around momenta lying on a "Bose surface", a bosonic analogue of the Fermi surface in solid-state systems. The interplay between instabilities generated by the driving, the nonlinear dissipative mode-coupling, and the underlaying lattice effect, allows the system to equilibrate into an exotic superfluid state of bosons condensed on a closed ring in momentum space instead of discrete points. Such an unconventional state with a spatially uniform density distribution goes beyond the traditional scope of pattern formation, and thus has no counterpart in the classical literature. In addition, it is a state connected to several open problems in modern condensed-matter physics, and here we provide the means to stabilize it, opening the way to its experimental study. Moreover, we also provide a concrete experimental implementation of our model in currently-available superconducting-circuit arrays. We also investigate the relaxation spectrum around the condensate, which shows a characteristic purely diffusive behavior.

cond-mat.quant-gas↗

arXiv2020

Dynamical universality classes towards an infinite temperature state

Dynamical universality is the observation that the dynamical properties of different systems might exhibit universal behavior that are independent of the system details. In this paper, we study the long-time dynamics of an one-dimensional noisy quantum magnetic model, and find that even though the system are inevitably driven to an infinite temperature state, the relaxation dynamics towards such featureless state can be highly nontrivial and universal. The effect of various mode-coupling mechanisms (external potential, disorder, interaction, and the interplay between them) as well as the conservation law on the long-time dynamics of the systems have been studied, and their relevance with current ultracold atomic experiments have been discussed.

cond-mat.quant-gas↗

arXiv2020

Symplectic ferromagnetism and phase transitions in multi-component fermionic systems

In this paper, we study the itinerant ferromagnetic phase in multi-component fermionic systems with symplectic (Sp(4), or isomorphically SO(5)) symmetry. Two different microscopic models have been considered and an effective field theory has been proposed to study the critical behavior of the nonmagnetism-magnetism phase transition. It has been shown that such systems exhibit intriguing ferromagnetism and critical behavior that different from those in spin-$\frac 12$ fermionic systems, or in high-spin systems with SU(N) symmetry. An extension of our results to higher spin systems with Sp(2N) symmetry has also been discussed.

cond-mat.quant-gas↗

arXiv2019

Imaginary time crystal of thermal quantum matter

Spontaneous symmetry breaking is responsible for rich quantum phenomena from crystalline structures to superconductivity. This concept was boldly extended to the breaking of time translation, opening an avenue to finding exotic phases of quantum matter with collective time modulation and correlation. Here we report that a thermally open quantum ensemble manifests in the dual space of imaginary time with crystalline ordering due to a bath-induced retarded interaction. Exact quantum Monte Carlo simulations are performed to show that this imaginary time crystal phase exhibits characteristic ground-state and thermal properties absent in conventional quantum manybody systems, especially the striking temperature-oscillating behavior of its physical observables.

cond-mat.quant-gas↗

arXiv2019

Persistent oscillations versus thermalization in the quench dynamics of quantum gases with long-range interactions

Searching for nonthermalized dynamics in interacting quantum systems is not only of fundamental theoretical interest in nonequilibrium quantum physics, but also of immense practical significance in quantum information processing. In this paper, we study quantum quench dynamics in an hard-core bosonic model with infinite-range interactions, which have been realized in recent high-finesse cavity experiments. We show the long-time dynamics of this model can exhibit either undamped oscillations or thermalization depending on the choice of initial states. The long-range nature of the interactions rather than conserved quantities are responsible for such nonergodic dynamical behaviors.

cond-mat.quant-gas↗

arXiv2019

Dissipative Majorana quantum wires

In this paper, we formulate and quantitatively examine the effect of dissipation on topological systems. We use a specific model of Kitaev quantum wire with an onsite Ohmic dissipation, and perform a numerically exact quantum Monte Carlo simulation to investigate this interacting open quantum system with a strong system-bath (SB) coupling beyond the scope of Born-Markovian approximation. We concentrate on the effect of dissipation on the topological features of the system (e.g. the Majorana edge mode) at zero temperature, and find that even though the topological phase is robust against weak SB couplings as it is supposed to be, it will eventually be destroyed by sufficiently strong dissipations via either a continuous quantum phase transition or a crossover depending on the symmetry of the system. The dissipation-driven quantum criticality is also discussed. In addition, using the framework of Abelian bosonization, we provide an analytical description of the interplay between pairing, dissipation and interaction in our model.

cond-mat.str-el↗

arXiv2017

Approximating quantum many-body wave-functions using artificial neural networks

In this paper, we demonstrate the expressibility of artificial neural networks (ANNs) in quantum many-body physics by showing that a feed-forward neural network with a small number of hidden layers can be trained to approximate with high precision the ground states of some notable quantum many-body systems. We consider the one-dimensional free bosons and fermions, spinless fermions on a square lattice away from half-filling, as well as frustrated quantum magnetism with a rapidly oscillating ground-state characteristic function. In the latter case, an ANN with a standard architecture fails, while that with a slightly modified one successfully learns the frustration-induced complex sign rule in the ground state and approximates the ground states with high precisions. As an example of practical use of our method, we also perform the variational method to explore the ground state of an anti-ferromagnetic $J_1-J_2$ Heisenberg model.

cond-mat.str-el↗

arXiv2017

Many-body self-localization in a translation-invariant Hamiltonian

We study the statistical and dynamical aspects of a translation-invariant Hamiltonian, without quench disorder, as an example of the manifestation of the phenomenon of many-body localization. This is characterized by the breakdown of thermalization and by information preservation of initial preparations at long times. To realize this, we use quasi-periodic long-range interactions, which are now achievable in high-finesse cavity experiments, to find evidence suggestive of a divergent time-scale in which charge inhomogeneities in the initial state survive asymptotically. This is reminiscent of a glassy behavior, which appears in the ground-state of this system, being also present at infinite temperatures.

cond-mat.str-el↗

arXiv2016

Universal long-time behavior of stochastically driven interacting quantum systems

One of the most important concepts in non-equilibrium physics is relaxation. In the vicinity of a classical critical point, the relaxation time can diverge and result in a universal power-law for the relaxation dynamics; the emerging classification of the dynamic universality class has been elucidated by Hohenberg and Halperin. In this paper, we systematically study the long-time relaxation dynamics in stochastically driven interacting quantum systems. We find that even though the stochastic forces will inevitably drive the systems into a featureless infinite temperature state, the way to approach the steady state can be highly nontrivial and exhibit rich universal dynamical behavior determined by the interplay between the stochastic driving and quantum many-body effects. We investigate the dynamical universality class by including different types of perturbations. The heating dynamics of a Hamiltonian with locally unbounded Hilbert space is also studied.

cond-mat.str-el↗