arXiv subjects

Zi Cai

Publications and source records attributed to Zi Cai.

At least 19 recordsLinked to original sources

arXiv2026

Memory-driven Topological Defects and Unconventional Long-Range Order

We investigate many-body systems with time-delayed self-interactions mediated by a memory-feedback mechanism. We show that such temporal interactions generate non-equilibrium orders and unique topological defects absent in equilibrium-specifically, helical vortices wherein opposite vorticities propagate in reverse directions along domain walls. In one dimension, memory feedback stabilizes true long-range order against weak noise, thereby circumventing the Mermin-Wagner theorem and resulting in an unconventional finite-temperature phase transition with a dynamical exponent $z = 4$. Physical realizations of these memory-driven non-equilibrium systems using active mechatronic metamaterials has also been proposed. These results demonstrate engineered temporal interactions as a powerful paradigm for non-equilibrium many-body physics.

cond-mat.stat-mech↗

arXiv2026

Superradiant strongly correlated quantum states in cavity Hubbard model

In cavity quantum materials, entangling strongly correlated electrons with quantum light provides a unique opportunity to explore novel quantum phases and phase transitions absent in conventional solid-state materials. In this study, we develop a sign-problem-free fermion-photon hybrid Quantum Monte Carlo (QMC) algorithm, and use it to systematically investigate the ground-state phase diagram of a two-dimensional cavity Hubbard model. It is shown that the interplay between the electron correlation and photon condensation gives rise to intriguing quantum phases ({\it e.g.} superradiant antiferromagnetic and chiral/$\pi$-flux states), and different quantum phase transitions, such as a first-order superradiant phase transition and a continuous phase transition with Gross-Neveu universality class. The methodology can be readily generalized to more complicated cavity strongly correlated models.

cond-mat.str-el↗

arXiv2025

Floquet-induced bosonic pair condensate with unconventional symmetry

In this study, we propose a dynamical pairing mechanism other than the pair-wise interactions. Starting from a two-dimensional hard-core boson model with periodically modulated hopping amplitude, we derive an effective Floquet Hamiltonian with three-site interactions that are responsible for unconventional pairing between adjacent bosons. By performing a density matrix renormalization group study on this three-site interacting Hamiltonian, we reveal a bosonic pair condensate with $s+id$ wave symmetry, while the single-particle Bose-Einstein condensate is completely depleted. The experimental implementations of the proposed model on superconducting quantum circuit have also been discussed.

cond-mat.quant-gas↗

arXiv2025

Emergent long-tail dynamics in driven magnets with dynamical frustration

In this study, we show that dynamical frustration can spontaneously emerge in frustration-free magnetic systems under periodic driving. Specifically, we consider a classical spin system and demonstrate the emergence of spin-ice physics when drive-induced heating is well suppressed. In particular, we focus on the dynamics of magnetic monopole excitations, which, in sharp contrast to their equilibrium counterparts, exhibit a non-ergodic stochastic random-walk process with long-tailed, power-law distributed waiting times, where the power-law exponent is tunable by the system's effective temperature. Heating is accelerated at intermediate driving frequencies, and the system eventually heats up to an infinite-temperature state. However, the heating time is extremely sensitive to different initial-state realizations and also follows a long-tailed power-law distribution. We show that a drive-induced short-range attractive interaction between monopoles is responsible for the long-tailed distributions observed in both monopole and heating dynamics.

cond-mat.stat-mech↗

arXiv2025

Infinite-temperature quantum phases and phase transitions

In this study, we reveal nontrivial quantum physics in an infinite-temperature system. By performing an unbiased quantum Monte Carlo simulation, we study a hybrid model composed of hard-core bosons, whose hopping amplitude is mediated by the density of another type of soft-core bond bosons that can absorb entropy indefinitely. It is shown that the Bose-Einstein condensate can persist in three dimensions even when the temperature approaches the infinite-temperature limit. In contrast, in two dimensions, the quasi-superfluid is depleted by the fluctuations of the bond bosons, which, on the other hand, enhance the conductivity of the hard-core bosons in the normal phase. A generalization to the fermionic model has also been discussed.

cond-mat.stat-mech↗

arXiv2025

ProtoEHR: Hierarchical Prototype Learning for EHR-based Healthcare Predictions

Digital healthcare systems have enabled the collection of mass healthcare data in electronic healthcare records (EHRs), allowing artificial intelligence solutions for various healthcare prediction tasks. However, existing studies often focus on isolated components of EHR data, limiting their predictive performance and interpretability. To address this gap, we propose ProtoEHR, an interpretable hierarchical prototype learning framework that fully exploits the rich, multi-level structure of EHR data to enhance healthcare predictions. More specifically, ProtoEHR models relationships within and across three hierarchical levels of EHRs: medical codes, hospital visits, and patients. We first leverage large language models to extract semantic relationships among medical codes and construct a medical knowledge graph as the knowledge source. Building on this, we design a hierarchical representation learning framework that captures contextualized representations across three levels, while incorporating prototype information within each level to capture intrinsic similarities and improve generalization. To perform a comprehensive assessment, we evaluate ProtoEHR in two public datasets on five clinically significant tasks, including prediction of mortality, prediction of readmission, prediction of length of stay, drug recommendation, and prediction of phenotype. The results demonstrate the ability of ProtoEHR to make accurate, robust, and interpretable predictions compared to baselines in the literature. Furthermore, ProtoEHR offers interpretable insights on code, visit, and patient levels to aid in healthcare prediction.

cs.LG↗

arXiv2025

Dimension-raising phase transitions in driven magnets and condensates

We propose a periodically driven system whose dimensionality is an emergent property that can be tunable, thus enables us to realize not only many-body phases with arbitrary dimensions, but also phase transitions, instead of crossovers, between phases with various dimensions. We study an interacting rotor model whose instantaneous Hamiltonian keeps the one-dimensional (1D) feature at any given time. Despite this, an emergent two-dimensional (2D) phase appears when the driving frequency exceeds a critical value, at which a dimension-raising phase transition takes place. We find that the nonequilibrium feature of the system could qualitatively change the finite temperature critical behavior of the emergent 2D phase and make it different from its equilibrium counterpart. A four-dimensional (4D) generalization and experimental realizations of the proposed model based on a programmable reconfiguration technique in optical tweezers setups have also been discussed.

cond-mat.stat-mech↗

arXiv2025

Dissipative quantum phase transitions monitored by current fluctuations

Dissipative phase transitions (DPT) are defined by sudden changes in the physical properties of nonequilibrium open quantum systems and they present characteristics that have no analog in closed and thermal systems. Several methods to detect and characterize DPT have been suggested in the literature, the most famous of which -- the $\textit{Liouvillian gap}$ -- can be derived from a spectral analysis of the Liouvillian super-operator that governs the complex interplay between coherent and dissipative dynamics. Here, we consider the $\textit{output current}$, defined as the average total quantum jumps per unit time between the open quantum system and the environment. We propose that output current fluctuations, and in particular their dynamical correlations, their power spectrum, and their characteristic timescale can provide valuable information about DPT, confirming a dramatic change of behavior at the critical point. We validate our proposal using the dissipative XYZ model and the nonlinear driven-dissipative Kerr model, showing good agreement with previous estimates of the location of the critical point. Compared to previous approaches, our proposal could be already experimentally tested in optical systems, providing a practical method to detect criticality in quantum open systems.

quant-ph↗

arXiv2024

Vacuum induced three-body delocalization in cavity quantum materials

In this study, we demonstrate that the vacuum itself suffices to delocalize an Anderson insulator inside a cavity. By studying a disordered one-dimensional spinless fermion system coupled to a single photon mode describing the vacuum fluctuation, we find that even though the cavity mode does not qualitatively change the localization behavior for a single-fermion system, it indeed leads to delocalization via a vacuum fluctuation-induced correlated hopping mechanism for systems with at least three fermions. A mobility edge separating the low-energy localized eigenstates and the high-energy delocalized eigenstates has been revealed. It is shown that such a one-dimensional three-fermion system in with correlated hopping can be mapped to a single-particle system with hopping along the face diagonals in a three dimensional lattice. The effect of the dissipation as well as a many-body generalization have also been discussed.

cond-mat.dis-nn↗

arXiv2024

Observation of Time Crystal in a Spin Maser System

Pair interaction potentials between atoms in a crystal are in general non-monotonic in distance, with a local minimum whose position gives the lattice constant of the crystal. A temporal analogue of this idea of crystal formation is still pending despite intensive studies on the time crystal phase. In a hybrid spin maser system with a time delay feedback, we report the observation of a time crystal induced by a retarded interaction with a characteristic time scale. This nonequilibrium phase features a self-sustained oscillation with an emergent frequency other than the intrinsic Larmor precession frequency of the spin maser system. It is shown that the amplitude of the oscillation is robust against perturbation, while its time phase randomly distributes from 0 to $2\pi$ for different realizations, a signature of spontaneous time translation symmetry breaking. This time crystal phase emerges only when the feedback strength exceeds a critical value, at which the system experiences a first order phase transition. Such a retarded interaction induced time crystal is closer to the idea of crystal, compared to other time crystal realizations.

physics.atom-ph↗

arXiv2024

Emergence of spatial patterns and synchronization in superconducting time crystals

We identify a time crystal phase characterized by a frequency half of the driving frequency in disordered superconductors by employing the time dependent Bogoliubov-de Gennes formalism at zero temperature with a periodically driven coupling constant. After a period of exponential increase of spatial inhomogeneities and exponential suppression of the order parameter amplitude, the time crystal develops islands of different sizes. Each of these islands is a time crystal with the same frequency albeit with a phase shift $\pi$ with respect to the homogeneous time crystal. After its emergence, the island gradually becomes smaller, though the phase shift persists, until it is abruptly synchronized at a time that it depends on its initial size. We find a critical disorder strength, still deep in the metallic phase, at which the time crystal phase terminates. For even stronger disorder, the order parameter oscillates with the driving frequency in regions where localization effects are not important.

cond-mat.supr-con↗

arXiv2024

Universal critical dynamics of quantum geometry

In this study, we prove that the quantum critical point in the ground state of quantum many-body systems, can also govern the universal dynamical behavior when the systems are driven far from equilibrium, which can be captured by the evolution of the quantum geometry of the systems. By investigating quantum quench dynamics in quadratic fermionic models, we prove that the quantum volume of these systems typically grows linearly over time, with a growth velocity demonstrating universal behavior: its first derivative over the control parameter exhibits a discontinuity at the quantum critical point, with an universal jump value that is independent of specific models, but is crucially determined by the system dimension. This result reveals universal dynamical properties of non-equilibrium quantum many-body systems

cond-mat.quant-gas↗

arXiv2023

Quantum slush state in Rydberg atom arrays

In this study, we propose an exotic quantum state which does not order at zero temperature in a Rydberg atom array with antiblockade mechanism. By performing an unbiased large-scale quantum Monte Carlo simulation, we investigate a minimal model with facilitated excitation in a disorder-free system. At zero temperature, this model exhibits a heterogeneous structure of liquid and glass mixture. This state, dubbed quantum slush state, features a quasi-long-range order with an algebraic decay for its correlation function, and is different from most well-established quantum phases of matter.

cond-mat.quant-gas↗

arXiv2023

Holographic dissipative space-time supersolids

Driving a system out of equilibrium enriches the paradigm of spontaneous symmetry breaking, which could then take place not only in space but also in time. The interplay between temporal and spatial symmetries, as well as symmetries from other internal degrees of freedom, can give rise to novel nonequilibrium phases of matter. In this study, we investigate a driven-dissipative superfluid model using holographic methods and reveal the existence of a space-time supersolid (STS) phase which concomitantly breaks the time translation, spatial translation, and the internal U(1) symmetry. The holographic methods naturally include finite temperature effects, which enables us to explore the complex phase diagram of this model and observe a cascade of out-of-equilibrium phase transitions from the STS phase to a synchronized superfluid phase, and finally to a normal fluid phase, by increasing the temperature.

hep-th↗

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↗

arXiv2023

Prethermal time-crystalline spin ice and monopole confinement in a driven magnet

Studies on systems far from equilibrium open up new avenues for investigating exotic phases of matter. A driven-dissipative frustrated spin system is examined in this study, and we suggest an out-of-equilibrium non-magnetic phase where the spins do not order but adhere to the ice rule in space and establish a long-range crystalline order in time. In contrast to the conventional spin ice, the dynamics of monopoles is confined due to the nonequilibrium feature of our model. Possible experimental realizations of our model has been discussed.

cond-mat.str-el↗

arXiv2022

Feedback-induced interactive dynamics: unitary but dissipative evolution

The time evolution of a physical system is generally described by a differential equation, which can be solved numerically by adopting a difference scheme with space-time discretization. This discretization, as a numerical artifact, results in accumulated errors during evolution thus usually plays a negative role in simulations. In a quantum circuit, however, the ``evolution time'' is represented by the depth of the circuit layer, thus is intrinsically discrete. Hence, the discretization-induced error therein is not a numerical artifact, but a physical observable effect responsible for remarkable nonequilibrium phenomena absent in conventional quantum dynamics. In this paper, we show that the combination of measurement feedback and temporal discretization can give rise to a new type of quantum dynamics characterized by unitary but dissipative evolution. As physical consequences of such an unitary but dissipative evolution, a nonequilibrium steady state with spontaneous symmetry breaking is revealed in a zero-dimensional (single-qubit) system. A localization mechanism distinct from that in the well-established Anderson localization has also been proposed in an one-dimensional interactive quantum system.

quant-ph↗

arXiv2022

Emergent non-Hermitian physics in generalized Lotka-Volterra model

In this paper, we study the non-Hermitian physics emerging from a predator-prey ecological model described by a generalized Lotka-Volterra equation. In the phase space, this nonlinear equation exhibits both chaotic and localized dynamics, which are separated by a critical point. These distinct dynamics originate from the interplay between the periodicity and non-Hermiticity of the effective Hamiltonian in the linearized equation of motion. Moreover, the dynamics at the critical point, such as algebraic divergence, can be understood as an exceptional point in the context of non-Hermitian physics.

cond-mat.stat-mech↗