arXiv subjects

Xiaoqun Wang

Publications and source records attributed to Xiaoqun Wang.

At least 37 records · Page 2Linked to original sources

arXiv2025

Universal Entanglement Pattern Formation via a Quantum Quench

We identify a universal short-time structure in symmetry-resolved entanglement dynamics -- the entanglement channel wave (ECW) -- arising from the decomposition of entanglement into conserved-quantum-number sectors that host robust, channel-specific patterns. Focusing on domain-wall melting, we conduct a systematic investigation across three paradigmatic classes of many-body systems: U(1) fermions, U(1) bosons, and SU(2) spinful fermions. For each class, we explore four distinct regimes defined by the presence or absence of interactions and disorder, employing both the Krylov-subspace iterative method and the correlation matrix approach. The ECW emerges universally across all cases, establishing its independence from particle statistics, interaction strength and disorder. In free fermions, the ECW formalism further enables analytical determination of the correlation matrix spectrum. The subsequent melting of the ECW exhibits symmetry- and statistics-dependent signatures, revealing finer structures in the growth of symmetry-resolved entanglement.

cond-mat.str-el↗

arXiv2025

Emerging kinetic-exchange for the enhanced metallic ferromagnetism in CrGeTe$_3$ under pressure

The microscopic origin of ferromagnetism in correlated materials remains heavily debated, particularly for the competing mechanisms governing insulating versus metallic phases. In this work, we theoretically study the electronic structure evolution of CrGeTe$_{3}$ under pressure and provide a consistent explanation to three unique features of this system, i.e. the semiconducting ferromagnetism at low pressure, the metallic ferromagnetism at high pressure, and the enhanced Curie temperature in the metallic phase. We propose that it is the reduced electronic correlation and enhanced $d$-$p$ hybridization that universally drive the continuous evolution of CrGeTe$_{3}$ under pressure and glue the three distinct experimental observations. Central to our discovery is the dual role of metallicity -- it simultaneously establishes kinetically driven exchange via $d$-$p$ hybridization and enables Stoner-type magnetic instability, with the contribution also from the residual super-exchange. Our analyses reveal that {\it intraband} excitations dominate the pressure-enhanced $ω_p^2$ and $T_c$ correlation. These findings establish $d$-$p$ hybridization and electronic correlation as the bridge between localized and itinerant magnetism, at least, in CrGeTe$_{3}$.

cond-mat.str-el↗

arXiv2025

Resilient cluster Mott states in layered Nb$_3$Cl$_8$ against pressure-induced symmetry breaking

In this work, by combining density-functional theory (DFT) with dynamical mean-field calculations (DMFT), we compare the crystal and electronic structures of the prototype cluster Mott insulator Nb$_{3}$Cl$_8$ at ambient and high-pressure. We explain the finite but significantly reduced charge gap experimentally observed at $P=9.7$ GPa. We reveal a local symmetry breaking of the Nb$_{3}$ trimer under pressure, reducing its symmetry from $C_{3v}$ to $C_{s}$. This leads to a strong bandwidth enhancement and a lift of band degeneracy. Crucially, despite the significant change of band details, the cluster Mott insulating state is robust against local symmetry breaking. We show that the experimental observed gap under pressure is still a cluster Mott gap and its reduced value stems from both increased bandwidth and reduced Coulomb interactions under pressure. Our study provides the first systematic theoretical elucidation of how pressure-induced symmetry breaking dictates the cluster Mott state, establishing a robust foundation for understanding the intricate relationship between symmetry, local/non-local correlations, and emergent quantum states in correlated cluster compounds.

cond-mat.str-el↗

arXiv2025

Density-matrix renormalization group algorithm for non-Hermitian systems

A biorthonormal-block density-matrix renormalization group algorithm is proposed to accurately compute properties of large-scale non-Hermitian many-body systems, in which a renormalized-space partition of the non-Hermitian reduced density matrix is implemented to fulfill the prerequisite for the biorthonormality of the renormalization group (RG) transformation and to optimize the construction of saved Hilbert spaces. A redundancy in saved spaces of the reduced density matrix is exploited to reduce a condition number resulting from the non-unitarity of the left and right transformation matrices, in order to ensure the numerical stability of the RG procedure. The algorithm is successfully applied to an interacting fermionic Su-Schrieffer-Heeger model with nonreciprocal hoppings and staggered complex chemical potential, exhibiting novel many-body phenomena.

cond-mat.str-el↗

arXiv2025

Enhanced convergence rates of Adaptive Importance Sampling with recycling schemes via quasi-Monte Carlo methods

This article investigates the integration of quasi-Monte Carlo (QMC) methods using the Adaptive Multiple Importance Sampling (AMIS). Traditional Importance Sampling (IS) often suffers from poor performance since it heavily relies on the choice of the proposal distributions. The AMIS and the Modified version of AMIS (MAMIS) address this by iteratively refining proposal distributions and reusing all past samples through a recycling strategy. We introduce the RQMC methods into the MAMIS, achieving higher convergence rates compared to the Monte Carlo (MC) methods. Our main contributions include a detailed convergence analysis of the MAMIS estimator under randomized QMC (RQMC) sampling. Specifically, we establish the $L^q$ $(q \geq 2)$ error bound for the RQMC-based estimator using a smoothed projection method, which enables us to apply the Hölder's inequality in the error analysis of the RQMC-based MAMIS estimator. As a result, we prove that the root mean square error of the RQMC-based MAMIS estimator converges at a rate of $\mathcal{O}(\bar{N}_T^{-1+ε})$, where $\bar{N}_T$ is the average number of samples used in each step over $T$ iterations, and $ε> 0$ is arbitrarily small. Numerical experiments validate the effectiveness of our method, including mixtures of Gaussians, a banana-shaped model, and Bayesian Logistic regression.

math.NA↗

arXiv2025

Altermagnetic Ground State in Distorted Kagome Metal CsCr$_3$Sb$_5$

The CsCr$_3$Sb$_5$ exhibits superconductivity in close proximity to a density-wave (DW) like ground state at ambient pressure\cite{Liu:2024aa}, however details of the DW is still elusive. Using first-principles density-functional calculations, we found its ground state to be a $4\times2$ altermagnetic spin-density-wave (SDW) at ambient pressure, with an averaged effective moment of $\sim$1.7$μ_B$/Cr. The magnetic long range order is coupled to the lattice, generating 4$a_0$ structural modulation. Multiple competing SDW phases are present and energetically close, suggesting strong magnetic fluctuation at finite temperature. The electronic states near Fermi level are dominated by Cr-3$d$ orbitals, and the kagome flat bands are closer to the Fermi level than those in the $A$V$_3$Sb$_5$ family in paramagnetic state. When external pressure is applied, the energy differences between competing orders and structural modulations are suppressed. Yet, the magnetic fluctuation remains present and important even at high pressure because the high-symmetry kagome lattice is unstable in nonmagnetic phase up to 30 GPa. Our results suggest the crucial role of magnetism to stabilize the crystal structure, under both ambient and high pressure.

cond-mat.supr-con↗

arXiv2025

Spin disorder state induced by Mg$^{2+}$ doping in a Kitaev material Na$_{3}$Co$_{2}$SbO$_{6}$

Due to the dominant Kitaev exchange interactions, the cobaltate, Na$_{3}$Co$_{2}$SbO$_{6}$, has been considered to be approximate to the Kitaev quantum spin liquid (QSL). Here, we investigate both magnetic dilution and chemical pressure effects of Na$_{3}$Co$_{2}$SbO$_{6}$ by the substitutions of Mg$^{2+}$ for Co$^{2+}$ through the structural, optical, magnetic and thermodynamic measurements. No structural transition has been observed, and the bandgaps remain constant in all doping levels. Combining with the magnetic and thermodynamic measurements, we find that the antiferromagnetic transition temperature is continuously suppressed with increasing Mg doping levels and completely disappears at $x=0.2$. Interestingly, when the doping level $x$ is larger than 0.2, neither long-range magnetic order nor spin glass state has been detected, and the specific heat has a residual linear term at zero field. All features indicate that Na$_{3}$(Co$_{2-x}$Mg$_{x}$)SbO$_{6}$ system enters into a novel spin disorder (NSD) state.

cond-mat.str-el↗

arXiv2025

Competition between dimerization and vector chirality in the spin-$3/2$ $J_1$-$J_2$ Heisenberg chain with uniaxial single-ion anisotropy

The spin-$3/2$ chain is a versatile prototypical platform for the study of competition between different kinds of magnetic orders, with the objective of obtaining a deeper understanding of the corresponding quantum phase transitions. In this work, we investigate the spin-$3/2$ chain with nearest-neighbor $J_1$, next-nearest-neighbor $J_2$, and uniaxial single-ion anisotropy $D$ terms in the absence of a magnetic field. For positive values of $J_2/J_1$ and $D/J_1$, we find seven different phases in a rich phase diagram. Without frustration $J_2=0$, a gapless Luttinger liquid phase remains stable for all $D>0$. As $J_2$ increases, we observe three phases with distinct dimerized valence bond orders, which show an intricate competition with vector chiral order and incommensurate correlations. For large $J_2$, regions of phase coexistence between the dimerized and vector chiral orders emerge. We present large-scale numerical data for the determination of transition lines, order parameters, and the nature of the phase transitions.

cond-mat.str-el↗

arXiv2025

Spin-charge Kondo effect for a quantum dot with side coupled Majorana zero mode

We investigate a minimal system consisting of a quantum dot coupled to a Majorana zero mode and a normal lead. We identify the underlying screening process as a novel spin-charge Kondo effect, where the low-energy spin and charge degrees of freedom of the Majorana zero mode-quantum dot subsystem are fully screened by those in the normal lead, resulting in the formation of a spin-charge singlet. An effective low-energy model is derived, with charge fluctuations appropriately accounted for. This spin-charge Kondo effect is found to be consistent with the spin-dependent Andreev/normal boundary conditions induced by the Majorana zero mode. We demonstrate that the anomalous substructure in the spectrum and thermodynamic properties is closely tied to the proportion of the charge component in the screening cloud. The spin-charge screening cloud exhibits scaling behavior analogous to that of traditional Kondo systems, though the sub-leading even-odd effect is subtly modified by the boundary conditions. These findings enhance our understanding of Kondo physics and resolve key debates on quantum dot nanostructures with Majorana zero modes.

cond-mat.str-el↗

arXiv2025

Flat-Band Enhanced Antiferromagnetic Fluctuations and Superconductivity in Pressurized CsCr$_3$Sb$_5$

The spin dynamics and electronic orders of the kagome system at different filling levels stand as an intriguing subject in condensed matter physics. By first-principles calculations and random phase approximation analyses, we investigate the spin fluctuations and superconducting instabilities in kagome phase of CsCr$_3$Sb$_5$ under high pressure. At the filling level slightly below the kagome flat bands, our calculations reveal strong antiferromagnetic spin fluctuations in CsCr$_3$Sb$_5$, together with a leading $s_{\pm}$-wave and a competing ($d_{xy}$, $d_{x^2-y^2}$)-wave superconducting order. Unlike the general intuition that the flat bands are closely related to the ferromagnetic correlations, here we propose a sublattice-momentum-coupling-driven mechanism for the antiferromagnetic fluctuations enhanced from the unoccupied flat bands. The mechanism is generally applicable to kagome systems where the Fermi level intersects near the flat bands, offering a new perspective for future studies of geometrically frustrated systems.

cond-mat.supr-con↗

arXiv2025

Scatter correction based on quasi-Monte Carlo for CT reconstruction

Scatter signals can degrade the contrast and resolution of computed tomography (CT) images and induce artifacts. How to effectively correct scatter signals in CT has always been a focal point of research for researchers. This work presents a new framework for eliminating scatter artifacts in CT. In the framework, the interaction between photons and matter is characterized as a Markov process, and the calculation of the scatter signal intensity in CT is transformed into the computation of a $4n$-dimensional integral, where $n$ is the highest scatter order. Given the low-frequency characteristics of scatter signals in CT, this paper uses the quasi-Monte Carlo (QMC) method combined with forced fixed detection and down sampling to compute the integral. In the reconstruction process, the impact of scatter signals on the X-ray energy spectrum is considered. A scatter-corrected spectrum estimation method is proposed and applied to estimate the X-ray energy spectrum. Based on the Feldkamp-Davis-Kress (FDK) algorithm, a multi-module coupled reconstruction method, referred to as FDK-QMC-BM4D, has been developed to simultaneously eliminate scatter artifacts, beam hardening artifacts, and noise in CT imaging. Finally, the effectiveness of the FDK-QMC-BM4D method is validated in the Shepp-Logan phantom and head. Compared to the widely recognized Monte Carlo method, which is the most accurate method by now for estimating and correcting scatter signals in CT, the FDK-QMC-BM4D method improves the running speed by approximately $102$ times while ensuring accuracy. By integrating the mechanism of FDK-QMC-BM4D, this study offers a novel approach to addressing artifacts in clinical CT.

physics.med-ph↗

arXiv2024

Interplay of Kitaev Interaction and Off-diagonal Exchanges: Exotic Phases and Quantum Phase Diagrams

Aligning with the everlasting search for quantum spin liquids (QSLs), identifying the QSL in Kitaev magnets has garnered great research interest during the past decade and remains nevertheless an enormous challenge. One of the major difficulties lies in that Kitaev QSL is typically fragile against competing interactions like off-diagonal exchanges, which are ubiquitous in real materials due to spin-orbit coupling and crystal-field effect. This, in turn, gives rise to many intriguing field-induced novel phases and thermal Hall effect. In this review, we will focus on the interplay of Kitaev interaction and off-diagonal $Γ$ and $Γ'$ exchanges from a numerical perspective. This review discusses some representative exotic phases such as $Γ$ spin liquid, nematic ferromagnet, spin-flop phase, and distinct chiral-spin states with spontaneously time-reversal symmetry breaking. It also presents quantum phase diagrams of anisotropic Kitaev-$Γ$ chains that exhibit kaleidoscopes of both ordered and disordered phases.

cond-mat.str-el↗

arXiv2024

Evidence for field induced quantum spin liquid behavior in a spin-1/2 honeycomb magnet

One of the most important issues in modern condensed matter physics is the realization of fractionalized excitations, such as the Majorana excitations in the Kitaev quantum spin liquid. To this aim, the 3d-based Kitaev material Na2Co2TeO6 is a promising candidate whose magnetic phase diagram of B // a* contains a field-induced intermediate magnetically disordered phase within 7.5 T < |B| < 10 T. The experimental observations, including the restoration of the crystalline point group symmetry in the angle-dependent torque and the coexisting magnon excitations and spinon-continuum in the inelastic neutron scattering spectrum, provide strong evidence that this disordered phase is a field induced quantum spin liquid with partially polarized spins. Our variational Monte Carlo simulation with the effective K-J1-Γ-Γ'-J3 model reproduces the experimental data and further supports this conclusion.

cond-mat.str-el↗

arXiv2024

Generalization Error Analysis of Deep Backward Dynamic Programming for Solving Nonlinear PDEs

We explore the application of the quasi-Monte Carlo (QMC) method in deep backward dynamic programming (DBDP) (Hure et al. 2020) for numerically solving high-dimensional nonlinear partial differential equations (PDEs). Our study focuses on examining the generalization error as a component of the total error in the DBDP framework, discovering that the rate of convergence for the generalization error is influenced by the choice of sampling methods. Specifically, for a given batch size $m$, the generalization error under QMC methods exhibits a convergence rate of $O(m^{-1+\varepsilon})$, where $\varepsilon$ can be made arbitrarily small. This rate is notably more favorable than that of the traditional Monte Carlo (MC) methods, which is $O(m^{-1/2+\varepsilon})$. Our theoretical analysis shows that the generalization error under QMC methods achieves a higher order of convergence than their MC counterparts. Numerical experiments demonstrate that QMC indeed surpasses MC in delivering solutions that are both more precise and stable.

math.NA↗

arXiv2024

Diagnosing thermalization dynamics of non-Hermitian quantum systems via GKSL master equations

The application of the eigenstate thermalization hypothesis to non-Hermitian quantum systems has become one of the most important topics in dissipative quantum chaos, recently giving rise to intense debates. The process of thermalization is intricate, involving many time-evolution trajectories in the reduced Hilbert space of the system. By considering two different expansion forms of the density matrices adopted in the biorthogonal and right-state time evolutions, we have derived two versions of the Gorini-Kossakowski-Sudarshan-Lindblad master equations describing the non-Hermitian systems coupled to a bosonic heat bath in thermal equilibrium. By solving the equations, we have identified a sufficient condition for thermalization under both time evolutions, resulting in Boltzmann biorthogonal and right-eigenstate statistics, respectively. This finding implies that the recently proposed biorthogonal random matrix theory needs an appropriate revision. Moreover, we have exemplified the precise dynamics of thermalization and thermodynamic properties with test models.

quant-ph↗

arXiv2024

Chiral spin state and nematic ferromagnet in the spin-1 Kitaev-$Γ$ model

The higher-spin Kitaev magnets, in which the Kitaev interaction and off-diagonal exchange couplings are overwhelmingly large, have emerged as a fertile avenue to explore exotic phases and unusual excitations. In this work, we study the quantum phase diagram of the spin-1 Kitaev-$Γ$ model on the honeycomb lattice using density-matrix renormalization group. It harbours six distinct phases and the intriguing findings are three magnetically ordered phases in which both time-reversal symmetry and lattice symmetry albeit of different sort are broken spontaneously. The chiral spin state originates from the order-by-disorder effect and exhibits an almost saturated scalar spin chirality at the quantum level. Depending on the relative strength of the two interactions, it also features columnar-like or plaquette-like dimer pattern as a consequence of the translational symmetry breaking. In parallel, the nematic ferromagnets are situated at ferromagnetic Kitaev side and possess small but finite ferromagnetic ordering. The lattice-rotational symmetry breaking enforces nonequivalent bond energy along one of the three bonds. Although the intrinsic difference between the two nematic ferromagnets remains elusive, the discontinuities in the von Neumann entropy, hexagonal plaquette operator, and Wilson loop operator convincingly suggest that they are separated via a first-order phase transition.

cond-mat.str-el↗

arXiv2024

Error analysis for empirical risk minimization over clipped ReLU networks in solving linear Kolmogorov partial differential equations

Deep learning algorithms have been successfully applied to numerically solve linear Kolmogorov partial differential equations~(PDEs). A recent research shows that if the initial functions are bounded, the empirical risk minimization (ERM) over clipped ReLU networks generalizes well for solving the linear Kolmogorov PDE. In this paper, we propose to use a truncation technique to extend the generalization results for polynomially growing initial functions. Specifically, we prove that under an assumption, the sample size required to achieve an generalization error within $\varepsilon$ with a confidence level $\varrho$ grows polynomially in the size of the clipped neural networks and $(\varepsilon^{-1},\varrho^{-1})$, which means that the curse of dimensionality is broken. Moreover, we verify that the required assumptions hold for Black-Scholes PDEs and heat equations which are two important cases of linear Kolmogorov PDEs. For the approximation error, under certain assumptions, we establish approximation results for clipped ReLU neural networks when approximating the solution of Kolmogorov PDEs. Consequently, we establish that the ERM over artificial neural networks indeed overcomes the curse of dimensionality for a larger class of linear Kolmogorov PDEs.

math.NA↗

arXiv2024

Deep Learning Based on Randomized Quasi-Monte Carlo Method for Solving Linear Kolmogorov Partial Differential Equation

Deep learning algorithms have been widely used to solve linear Kolmogorov partial differential equations~(PDEs) in high dimensions, where the loss function is defined as a mathematical expectation. We propose to use the randomized quasi-Monte Carlo (RQMC) method instead of the Monte Carlo (MC) method for computing the loss function. In theory, we decompose the error from empirical risk minimization~(ERM) into the generalization error and the approximation error. Notably, the approximation error is independent of the sampling methods. We prove that the convergence order of the mean generalization error for the RQMC method is $O(n^{-1+ε})$ for arbitrarily small $ε>0$, while for the MC method it is $O(n^{-1/2+ε})$ for arbitrarily small $ε>0$. Consequently, we find that the overall error for the RQMC method is asymptotically smaller than that for the MC method as $n$ increases. Our numerical experiments show that the algorithm based on the RQMC method consistently achieves smaller relative $L^{2}$ error than that based on the MC method.

math.NA↗