arXiv subjects

Zi-Xiang Li

Publications and source records attributed to Zi-Xiang Li.

At least 37 records · Page 2Linked to original sources

arXiv2025

Skin Effect Induced Anomalous Dynamics from Charge-Fluctuating Initial States

Non-equilibrium dynamics in non-Hermitian systems has attracted significant interest, particularly due to the skin effect and its associated anomalous phenomena. Previous studies have primarily focused on initial states with a definite particle number. Here, we present a systematic study of non-reciprocal quench dynamics in the pairing states with indefinite particle number. Our study uncovers a range of novel behaviors. Firstly, we demonstrate a universal tendency towards half-filling of particle density at late times. At early times for certain initial states, we observe a chiral wavefront in both particle number distribution and charge inflow, associated with a sharp decrease in particle number. Furthermore, we find that non-Hermiticity could enhance the growth of entanglement in the initial stages of evolution. In the intermediate time regime, the characteristic skin effect leads to particle accumulation on one side, leading to a pronounced reduction in entanglement entropy. Moreover, our results reveal the presence of the quantum Mpemba effect during the restoration of U(1) symmetry. Our findings open new avenues for exploring exotic dynamic phenomena in quantum many-body systems arising from the interplay of symmetry breaking and non-Hermiticity.

quant-ph↗

arXiv2025

Symmetry Breaking Dynamics in Quantum Many-Body Systems

Entanglement asymmetry has emerged as a powerful tool for characterizing symmetry breaking in quantum many-body systems. In this Letter, we explore how symmetry is dynamically broken through the lens of entanglement asymmetry in two distinct scenarios: a non-symmetric random quantum circuit and a non-symmetric Hamiltonian quench, with a particular focus on U(1) symmetry. In the former case, the symmetry is initially broken and subsequently restored, whereas in the latter case, symmetry remains broken in the subsystem at late times, consistent with the principles of quantum thermalization. Notably, the growth of entanglement asymmetry exhibits unexpected overshooting behavior at early times in both contexts, contrasting with the behavior of charge variance. We also consider dynamics of non-symmetric initial states under the symmetry-breaking evolution. Due to the competition of symmetry-breaking in both the initial state and Hamiltonian, the early-time entanglement asymmetry can increase and decrease, while quantum Mpemba effects remain evident despite the weak symmetry-breaking in both settings.

quant-ph↗

arXiv2025

Finite-time scaling beyond the Kibble-Zurek prerequisite in Dirac systems

The conventional Kibble-Zurek mechanism and the finite-time scaling provide universal descriptions of the driven critical dynamics from gapped initial states based on the adiabatic-impulse scenario. Here we investigate the driven critical dynamics in two-dimensional Dirac systems, which harbor semimetal and Mott insulator phases separated by the quantum critical point triggered by the interplay between fluctuations of gapless Dirac fermions and order parameter bosons. We find that despite the existence of the gapless initial phase, the driven dynamics can still be captured by the finite-time scaling form. This leads us to propose a criterion for the validity of Kibble-Zurek mechanism with a gapless initial state. Accordingly, our results generalize the Kibble-Zurek theory to incorporate composite fluctuations and relax its requirement for a gapped initial state to systems accommodating gapless Dirac fermionic excitations. Our work not only brings fundamental perspective into the nonequilibrium critical dynamics, but also provides an approach to fathom quantum critical properties in fermionic systems.

cond-mat.str-el↗

arXiv2025

Superconductivity Favored Anisotropic Phase Stiffness in Infinite-Layer Nickelates

In unconventional superconductors such as cuprates and iron pnictides and chalcogenides, phase stiffness - a measure of the energy cost associated with superconducting phase variations - is on the same order of magnitude as the strength of Cooper pairing, translating to superconductivity governed by phase fluctuations. However, due to a lack of a direct experimental probe, there remains a fundamental gap in establishing microscopic picture between unconventional superconductivity and phase fluctuations. Here we show a vector current technique that allows for in-situ angle-resolved transport measurements, providing exclusive evidence suggesting an anisotropic nature of phase stiffness in infinite-layer nickelate superconductors. Pronounced anisotropy of in-plane resistance manifests itself in both normal and superconducting transition states, indicating crystal symmetry breaking. Remarkably, the electric conductivity of Nd0.8Sr0.2NiO2 peaks at 125° between the direction of the current and crystal principal axis, but this angle evolves to 160° near zero-resistance temperature. Further measurements reveal that the superconductivity is favored along a direction with minimized phase fluctuations, an orientation strikingly deviating from the symmetric direction imposed by both electronic anisotropy and the underlying crystal lattice. Identical measurements conducted on a prototypical cuprate superconductor yield consistent results, suggesting that this previously unknown behavior could be ubiquitous. By shielding insight into the contrasting anisotropy between electron fluid and superfluid, our findings provide clues for a unified framework for understanding unconventional superconductors

cond-mat.supr-con↗

arXiv2025

Correlation between unconventional superconductivity and strange metallicity revealed by operando superfluid density measurements

Strange-metal behavior has been observed in superconductors ranging from cuprates to pressurized nickelates, but its relationship to unconventional superconductivity remains elusive. Here, we perform operando superfluid density measurements on ion-gated FeSe films. We observe for the first time a synchronized evolution of superconducting condensate and the strange-metal phase with electron doping. A linear scaling between zero-temperature superfluid density and the strange-metal resistivity coefficient is further established, which nails down a direct link between the formation of superfluid in the superconducting state and the scattering of carriers in the strange-metal normal state. Remarkably, the scaling also applies for different iron-based and cuprate superconductors despite their distinct electronic structures and pairing symmetries. Such a correlation can be reproduced in a theoretical calculation on the two-dimensional Yukawa-Sachdev-Ye-Kitaev model by considering a cooperative effect of quantum critical fluctuation and disorder. These findings indicate a fundamental principle governing superconducting condensation and strange-metal scattering in unconventional superconductors.

cond-mat.supr-con↗

arXiv2024

Uncovering Emergent Spacetime Supersymmetry with Rydberg Atom Arrays

In the zoo of emergent symmetries in quantum many-body physics, the previously unrealized emergent spacetime supersymmetry (SUSY) is particularly intriguing. Although it was known that spacetime SUSY could emerge at the (1+1)d tricritical Ising transition, an experimental realization is still absent. In this work, we propose to realize emergent spacetime SUSY using reconfigurable Rydberg atom arrays featuring two distinct sets of Rydberg excitations, tailored for implementation on dual-species platforms. In such systems, the spacetime SUSY manifests itself in the respective correlation functions of a bosonic mode and its fermionic partner. However, the correlation function of the fermionic mode inevitably involves a string operator, making direct measurement challenging in the conventional setting. Here, we leverage the hybrid analog-digital nature of the Rydberg atom arrays, which allows for the simulation of a physical Hamiltonian and the execution of a digital quantum circuit on the same platform. This hybrid protocol offers a new perspective for uncovering the hidden structure of emergent spacetime SUSY.

cond-mat.quant-gas↗

arXiv2024

Imaginary-time Mpemba effect in quantum many-body systems

Various exotic phenomena emerge in non-equilibrium quantum many-body systems. The Mpemba effect, denoting the situation where a hot system freezes faster than the colder one, is a counterintuitive non-equilibrium phenomenon that has attracted enduring interest for more than half a century. In this Letter, we report a novel phenomenon of the Mpemba effect in the imaginary-time relaxation dynamics in quantum many-body systems, dubbed as imaginary-time Mpemba effect (ITME). Through numerically exact quantum Monte-Carlo (QMC) simulation, we unambiguously demonstrate that in different classes of interacting quantum models, the initial states with higher energy are relaxed faster than lower-energy initial states in the process of imaginary-time relaxation. The emergence of ITME is intimately associated with the low-energy excitations in quantum many-body systems. More crucially, since imaginary-time dynamics is broadly applied in numerical simulation on the quantum many-body ground states, the discovery of ITME potentially provides a new pathway to expedite the quantum many-body computation, particularly for QMC involving the sign problem.

cond-mat.str-el↗

arXiv2024

Nonequilibrium Critical Dynamics with Emergent Supersymmetry

Proposed as an elegant symmetry relating bosons and fermions, spacetime supersymmetry (SUSY) has been actively pursued in both particle physics and emergent phenomena in quantum critical points (QCP) of topological quantum materials. However, how SUSY casts the light on nonequilibrium dynamics remains open. In this letter, we investigate the Kibble-Zurek dynamics across a QCP with emergent $\mathcal{N}=2$ spacetime SUSY between the Dirac semimetal and a superconductor through large-scale quantum Monte Carlo simulation. The scaling behaviors in the whole driven process are uncovered to satisfy the full finite-time scaling (FTS) forms. More crucially, we demonstrate that the emergent SUSY manifests in the intimate relation between the FTS behaviors of fermionic and bosonic observables, namely the fermions and bosons acquire the identical anomalous dimensions. Our work not only brings a fundamental new ingredient into the critical theory with SUSY, but also provide the theoretical guidance to experimental detect of QCP with emergent SUSY from the perspectives of Kibble-Zurek mechanism and FTS.

cond-mat.str-el↗

arXiv2024

Quantum spin liquid from electron-phonon coupling

A quantum spin liquid (QSL) is an exotic insulating phase with emergent gauge fields and fractionalized excitations. However, the unambiguous demonstration of the existence of a QSL in a "non-engineered" microscopic model (or in any material) remains challenging. Here, using numerically-exact sign-problem-free quantum Monte Carlo simulations, we show that a QSL arises in a non-engineered electron-phonon model. Specifically, we investigate the ground-state phase diagram of the bond Su-Schrieffer-Heeger (SSH) model on a 2D triangular lattice at half filling (one electron per site) which we show includes a QSL phase which is fully gapped, exhibits no symmetry-breaking order, and supports deconfined fractionalized holon excitations. This suggests new routes for finding QSLs in realistic materials and high-$T_c$ superconductivity by lightly doping them.

cond-mat.str-el↗

arXiv2024

Theory of an infinitely anisotropic phase of a two-dimensional superconductor

We propose a simple model of a two-dimensional ``locally smectic superconductor'' that exhibits power-law non-linear I-V characteristics with different powers for current applied in two orthogonal directions. We discuss the potential relevance of this model to recent experimental findings on interfacial superconductivity in EuO/KTaO$_3$(110) (Nat. Phys. 20, 957 (2024)), where the apparent critical temperature ($T_c$) has been shown experimentally to depend on the current direction.

cond-mat.supr-con↗

arXiv2024

Universal scaling behavior of resistivity under two-dimensional superconducting phase fluctuations

In superconductors with relatively low superfluid density, such as cuprate high-$T_c$ superconductors, the phase fluctuations of the superconducting order parameter are remarkable, presumably playing a nonnegligible role in shaping many distinctive physical properties. This work systematically investigates the electrical transport properties arising from thermal superconducting phase fluctuations in two-dimensional superconductors. Employing the Monte Carlo procedure, we access the numerically exact properties of a microscopic model of superconductivity, in which the classical XY model governs the thermal phase fluctuations of the superconducting order parameter. For both $s$-wave and $d_{x^2-y^2}$-wave pairings, the electrical resistivity exhibits a universal scaling behavior in the temperature range above $T_c$. Our numerical results demonstrate that the scaling behavior of the quasiparticle lifetime is associated with the correlation length of the superconducting order parameter, yielding the universal scaling behavior of electrical resistivity determined by the Berezinskii-Kosterlitz-Thouless critical scaling of the correlation length. Furthermore, we discuss the dependence of the electrical resistivity coefficient on the pairing amplitude and the possible implication on recent transport experiments.

cond-mat.str-el↗

arXiv2023

Fermi surface symmetric mass generation: a quantum Monte-Carlo study

The symmetric mass generation (SMG) phase is an insulator in which a single-particle gap is intrinsically opened by the interaction, without involving symmetry spontaneously breaking or topological order. Here, we perform unbiased quantum Monte-Carlo simulation and systematically investigate a bilayer fermionic model hosting Fermi surface SMG in the strongly interacting regime. With increasing interaction strength, the model undergoes a quantum phase transition from an exciton insulator to an SMG phase, belonging to the (2+1)-dimensional O(4) universality class. We access the spectral properties of the SMG phase, resembling a Mott insulating phase with relatively flat dispersion and pronounced spectral broadening. The dispersion of Green's function zeros is extracted from spectral function, featuring a surface at zero frequency precisely located at the original non-interacting Fermi surface, which constitutes a hallmark of the Fermi surface SMG phase. The bilayer model we study is potentially relevant to the newly discovered high-$T_c$ superconductor $\rm{La}_3 \rm{Ni}_2 \rm{O}_7$. Our results in SMG phase qualitatively capture the salient features of spectral function unveiled in recent ARPES experiments, shedding new insight on the underlying physics of $\rm{La}_3 \rm{Ni}_2 \rm{O}_7$.

cond-mat.str-el↗

arXiv2023

Evidence for Quantum Stripe Ordering in a Triangular Optical Lattice

Understanding strongly correlated quantum materials, such as high $T_\textrm{c}$ superconductors, iron-based superconductors, and twisted bilayer graphene systems, remains to be one of the outstanding challenges in condensed matter physics. Quantum simulation with ultra-cold atoms in particular optical lattices, which provide orbital degrees of freedom, is a powerful tool to contribute new insights to this endeavor. Here, we report the experimental realization of an unconventional Bose-Einstein condensate of $^{87}$Rb atoms populating degenerate $p$-orbitals in a triangular optical lattice, exhibiting remarkably long coherence times. Using time-of-flight spectroscopy, we observe that this state spontaneously breaks the rotational symmetry and its momentum spectrum agrees with the theoretically predicted coexistence of exotic stripe and loop current orders. Like certain strongly correlated electronic systems with intertwined orders, as high-$T_\textrm{c}$ cuprate superconductors, twisted bilayer graphene, and the recently discovered chiral density-wave state in kagome superconductors $\textrm{AV}_3 \textrm{Sb}_5$ (A=K, Rb, Cs), the newly demonstrated quantum state, in spite of its markedly different energy scale and the bosonic quantum statistics, exhibits multiple symmetry breakings at ultralow temperatures. These findings hold the potential to enhance our comprehension of the fundamental physics governing these intricate quantum materials.

cond-mat.quant-gas↗

arXiv2023

High-temperature superconductivity induced by the Su-Schrieffer-Heeger electron-phonon coupling

Experimental quest for high-temperature and room-temperature superconductivity (SC) at ambient pressure has been a long-standing research theme in physics. It has also been desired to construct reliable microscopic mechanisms that may achieve high-temperature SC. Here we systematically explore SC in the Su-Schrieffer-Heeger (SSH) electron-phonon coupling models by performing numerically-exact quantum Monte-Carlo simulations. Our results reliably showed that superconducting $T_c$ of the SSH models is high, remarkably higher than those in the Holstein models, particularly in strong electron-phonon coupling regime. This is mainly because SSH phonons can not only induce strong pairing between electrons but also help the phase coherence of Cooper pairs, thus realizing higher $T_c$. As mechanism of higher-$T_c$ of the SSH models could be potentially relevant to realistic materials, it paves a promising way to find higher-temperature SC in the future.

cond-mat.str-el↗

arXiv2023

Bogoliubov Quasiparticle on the Gossamer Fermi Surface in Electron-Doped Cuprates

In contrast to hole-doped cuprates, electron-doped cuprates consistently exhibit strong antiferromagnetic correlations with a commensurate (π, π) ordering wave vector, leading to the prevalent belief that antiferromagnetic spin fluctuations mediate Cooper pairing in these unconventional superconductors. However, early investigations produced two paradoxical findings: while antiferromagnetic spin fluctuations create the largest pseudogap at "hot spots" in momentum space, Raman scattering and angle-resolved photoemission spectroscopy measurements using the leading-edge method seem to suggest the superconducting gap is also maximized at these locations. This presented a dilemma for spin-fluctuation-mediated pairing: Cooper pairing is strongest at momenta where normal state low energy spectral weight is most suppressed. Here we investigate this dilemma in Nd2-xCexCuO4 using angle-resolved photoemission spectroscopy under significantly improved experimental conditions. The unprecedented signal-to-noise ratio and resolution allow us to directly observe the Bogoliubov quasiparticles, demonstrating the existence and importance of two sectors of states: 1. The reconstructed main band and the states gapped by the antiferromagnetic pseudogap around the hot spots. 2. The gossamer Fermi surface states with distinct dispersion inside the pseudogap, from which Bogoliubov quasiparticle coherence peaks emerge below Tc. Supported by numerical results, we propose that the non-zero modulus of the antiferromagnetic order parameter causes the former, while fluctuations in the antiferromagnetic order parameter orientation are responsible for the latter. Our revelations of the gossamer Fermi surface reconcile the paradoxical observations, deepening our understanding of superconductivity in electron-doped cuprates in particular, and unconventional superconductivity in general.

cond-mat.supr-con↗

arXiv2023

Emergent normal fluid in the superconducting ground state of overdoped cuprates

The microscopic mechanism for the disappearance of superconductivity in overdoped cuprates is still under heated debate. Here we use scanning tunneling spectroscopy to investigate the evolution of quasiparticle interference phenomenon in $\rm Bi_2Sr_2CuO_{6+δ}$ over a wide range of hole densities. We find that when the system enters the overdoped regime, a peculiar quasiparticle interference wavevector with quarter-circle pattern starts to emerge even at zero bias, and its intensity grows with increasing doping level. Its energy dispersion is incompatible with the octet model for d-wave superconductivity, but is highly consistent with the scattering interference of gapless normal carriers. The weight of the gapless quasiparticle interference is mainly located at the antinodes and is independent of temperature. We propose that the normal fluid emerges from the pair-breaking scattering between flat antinodal bands in the quantum ground state, which is the primary cause for the reduction of superfluid density and suppression of superconductivity in overdoped cuprates.

cond-mat.supr-con↗

arXiv2023

Boosting quantum Monte Carlo and alleviating sign problem by Gutzwiller projection

Here we develop a new scheme of projective quantum Monte-Carlo (QMC) simulation combining unbiased zero-temperature (projective) determinant QMC and variational Monte-Carlo based on Gutzwiller projection wave function, dubbed as ``Gutzwiller projection QMC''. The numerical results demonstrate that employment of Gutzwiller projection trial wave function with minimum energy strongly speed up the convergence of computational results, thus tremendously reducing computational time in the simulation. More remarkably, we present an example that sign problem is enormously alleviated in the Gutzwiller projection QMC, especially in the regime where sign problem is severe. Hence, we believe that Gutzwiller projection QMC paves a new route to improving the efficiency, and alleviating sign problem in QMC simulation on interacting fermionic systems.

cond-mat.str-el↗

arXiv2023

Non-Hermitian strongly interacting Dirac fermions: a quantum Monte-Carlo study

Exotic quantum phases and phase transition in the strongly interacting Dirac systems has attracted tremendous interests. On the other hand, non-Hermitian physics, usually associated with dissipation arising from the coupling to environment, emerges as a frontier of modern physics in recent years. In this letter, we investigate the interplay between non-Hermitian physics and strong correlation in Dirac-fermion systems. We develop a sign-problem-free projector quantum Monte-Carlo (QMC) algorithm for the non-Hermitian interacting fermionic systems. Employing state-of-the-art projector QMC simulation, we decipher the ground-state phase diagram of the Honeycomb Hubbard model in the presence non-Hermitian asymmetric spin resolved hopping processes. Intriguingly, the antiferromagnetic ordering induced by Hubbard interaction is enhanced by the non-Hermitian asymmetric hopping. More remarkably, our study reveals that critical properties of the quantum phase transition between Dirac semi-metal and AF ordered phases are consistent with the XY universality class in Hermitian system, implying Hermiticity is emergent at the quantum critical point. The numerically-exact QMC approach utilized in this study is easily applied to other non-Hermitian interacting fermionic models, hence paving a new avenue to investigating quantum many-body physics in non-Hermitian systems.

cond-mat.str-el↗