arXiv subjects

Xiaoqun Wang

Publications and source records attributed to Xiaoqun Wang.

At least 19 recordsLinked to original sources

arXiv2026

Coupled Spin-Density-Wave and Bond-Order Driven Metal-Insulator Transition in Altermagnetic CsCr$_2$S$_2$O

A metal-insulator transition (MIT) driven by bond order (BO) coupled with a secondary spin-density wave (SDW) is identified in CsCr$_2$S$_2$O. Such coupling is enabled as a result of the broken time-reversal symmetry due to the pre-existing C-type antiferromagnetic (C-AFM) order. First-principles calculations reveal an orbital-selective physics that Cr-$d_{yz}$ orbitals form local moments and establish the altermagnetic order, while the Cr-$d_{xz}$ orbitals remain metallic and hybridize with S-$p_z$. Thus the low-energy physics is governed by the Cr-$d_{xz}$ and S-$p_z$ orbitals. On-site interactions then enhance a secondary SDW ($s$SDW) instability of the itinerant $d_{xz}$ electrons, which couples to the Cr-$d_{xz}$-S-$p_z$ bonding order. The resulting coupled $s$SDW-BO simultaneously produces experimentally observed structural distortion, charge disproportionation, local Cr-moment modulation, and gap opening. Our results establish an orbital-selective mechanism upon which pre-existing altermagnetism and electronic correlations cooperate to drive a structural MIT.

cond-mat.str-el↗

arXiv2026

Universal $L^2$-approximation using median digital-net algorithms

We propose a median digital-net algorithm for $L^2$-approximation of non-periodic functions over $[0,1]^s$, inspired by the recently developed median lattice algorithms for the periodic setting. The algorithm requires no smoothness or weight parameters but only a sufficiently large candidate Walsh index set $K$. It proceeds in three stages: generating multiple estimates of the Walsh coefficients in $K$ using independent randomized digital-net samples; taking the respective median of both the estimates and their absolute values; then, based on these median values, identifying the dominant coefficients and constructing a truncated Walsh series as the final approximation. We prove that if the target function has dominating mixed partial derivatives up to order $α$, all having finite Vitali variation of fractional order $λ$, then the algorithm achieves an $L^2$-error of $\mathcal{O}(M^{-α-λ+η})$ with high probability, where $M$ is the total number of function evaluations and $η>0$ is arbitrarily small. Furthermore, the implied constant grows at most polynomially in the dimension $s$ under suitable decay conditions on the ANOVA components of the target function. On the implementation side, we provide both parameter-dependent and -independent constructions of the index set $K$, and employ the fast Walsh--Hadamard transform and Gray code ordering to accelerate the algorithm. Numerical experiments support the theoretical analysis and demonstrate that the proposed algorithm remains effective in high-dimensional settings.

math.NA↗

arXiv2026

Crossover and Changeover in Spin-1 Kitaev-$Γ$ Chain with Uniaxial Single-ion Anisotropy

Recent advances in bond-directional spin chains have revealed extensive emergent phenomena and unconventional criticality. Here we investigate the spin-1 Kitaev-$Γ$ chain with uniaxial single-ion anisotropy (SIA) using large-scale density-matrix renormalization group calculations and bosonization analysis. Tuning the SIA strength reveals a crossover from the Kitaev phase to the large-$D$ phase, evidenced by the excitation gap changing from quadratic to linear, the coexistence and smooth evolution of spin-nematic and string order parameters, and the suppression of the double-peak specific heat. For negative SIA, we uncover a changeover from a first-order transition to a continuous one between the dimerized and Haldane phases. The continuous transition belongs to the \textrm{SU(2)$_2$} Wess-Zumino-Witten universality class with central charge $c=3/2$, a rare instance in a system without continuous symmetry. Our results establish the Kitaev-$Γ$ chain as a minimal platform for controlling crossover and changeover phenomena.

cond-mat.str-el↗

arXiv2026

Encoding complex-balanced thermalization in quantum circuits

Non-Markovian dynamics in open quantum systems often invalidates the complex-balanced thermalization framework, hindering predictive control of quantum simulation platforms designed to prepare out-of-equilibrium states at prescribed temperatures. We resolve this bottleneck by engineering reservoir qubits as modular microscopic units coupled to a target quantum system and constructing a quantum-circuit platform that enforces strictly Markovian complex-balanced thermalization. The platform exploits the non-orthogonality of reservoir qubit eigenstates to drive inhomogeneous heating through a modified Kubo-Martin-Schwinger relation, and uses tunable microscopic time-reversibility breaking to generate amplification-dissipation dynamics. We demonstrate two applications: temporally correlated dichromatic emission and Liouvillian exceptional-point-protected quantum synchronization at finite temperatures, displaying predictive control over out-of-equilibrium state preparation.

quant-ph↗

arXiv2026

ProTrain: Efficient LLM Training via Memory-Aware Techniques

Memory pressure has emerged as a dominant constraint in scaling the training of large language models (LLMs), particularly in resource-constrained environments. While modern frameworks incorporate various memory-saving techniques, they often expose low-level configuration knobs that require manual tuning and specialized system expertise. This not only adds engineering overhead but also risks suboptimal hardware utilization when misconfigured. This paper introduces ProTrain, a novel training system that automatically tailors memory management policies to the model architecture and underlying hardware resources, eliminating the need for manual intervention. The core of ProTrain is its automated memory management that abstracts complex memory management strategies into a few tunable configuration parameters, allowing searches for optimal parameter settings using cost models. ProTrain is equipped with a runtime profiler that provides precise estimates of latency, memory usage, and I/O bandwidth to build high-fidelity cost models. ProTrain does not change the training algorithm and thus does not compromise accuracy. Experiments show that ProTrain improves training throughput by 1.43$\times$ to 2.71$\times$ compared to the state-of-the-art training systems.

cs.DC↗

arXiv2026

Nested Multilevel Monte Carlo with Preintegration for Efficient Risk Estimation

Nested Monte Carlo is widely used for risk estimation, but its efficiency is limited by the discontinuity of the indicator function and high computational cost. This paper proposes a nested Multilevel Monte Carlo (MLMC) method combined with preintegration for efficient risk estimation. We first use preintegration to integrate out one outer random variable, which effectively handles the discontinuity of the indicator function, then we construct the MLMC estimator with preintegration to reduce the computational cost. Our theoretical analysis proves that the strong convergence rate of the MLMC combined with preintegration reaches -1, compared with -1/2 for the standard MLMC. Consequently, we obtain a nearly optimal computational complexity. Besides, our method can also handle the high-kurtosis phenomenon caused by indicator functions. Numerical experiments verify that the smoothed MLMC with preintegration outperforms the standard MLMC and the optimal computational cost can be attained. Combining our method with quasi-Monte Carlo further improves its performance in high dimensions. Keywords: Nested simulation, Multilevel Monte Carlo, Risk estimation, Preintegration

math.NA↗

arXiv2026

Symmetry-resolved properties of the trace distance in thermalizing SU(2) systems

We study diagnostics of thermalization in quantum many-body systems with global SU(2) symmetry, where the standard eigenstate thermalization hypothesis (ETH) is generalized to its non-Abelian form. As an eigenstate-level probe, we introduce a symmetry-resolved trace distance constructed from the block structure of the reduced density matrix. This block structure separates spin-sector probabilities from configurational fluctuations within each sector, naturally leading to a decomposition into a probability trace distance and a configurational trace distance. The microcanonical average of the former is bounded by fluctuations of the corresponding spin-sector probabilities within a microcanonical energy window, whereas the latter captures finer intra-sector fluctuations. In non-Abelian thermalizing systems, these spin-sector-probability fluctuations are constrained by the non-Abelian ETH and therefore become exponentially suppressed with system size. Numerical studies of the one-dimensional $J_1$--$J_2$ Heisenberg chain are consistent with this picture and suggest that, in the thermal regime, the trace distance is asymptotically dominated by the configurational trace distance.

quant-ph↗

arXiv2026

Importance sampling and active subspace in quasi-Monte Carlo

The quasi-Monte Carlo method is widely used in computational finance, whose efficiency strongly depends on the smoothness and effective dimension of the integrand. In this work, we investigate the combination of importance sampling and the active subspace method under the quasi-Monte Carlo framework and propose a three-step approach, referred to as the IS-AS-preintegration method, which sequentially applies importance sampling, active subspace, and preintegration. The proposed method is applied to the option pricing and sensitivity analysis problems in finance, and its performance is evaluated through extensive numerical experiments. The results demonstrate that the proposed method is highly competitive compared with existing popular methods. In particular, for out-of-the-money and deep out-of-the-money options, the proposed approach overcomes the limitations of the preintegration via active subspace method and achieves superior variance reduction, while maintaining comparable performance for other moneyness cases.

math.NA↗

arXiv2026

A median QMC method for unbounded integrands over $\mathbb{R}^{s}$ in weighted unanchored Sobolev spaces

This paper investigates quasi-Monte Carlo (QMC) integration of Lebesgue integrable functions with respect to a density function over $\mathbb{R}^s$. We extend the construction-free median QMC rule proposed by Goda and L'ecuyer (SIAM J. Sci. Comput., 2022) to the weighted unanchored Sobolev space of functions defined over $\mathbb{R}^s$ introduced by Nichols and Kuo (J. Complexity, 2014). By taking the median of $k = \mathcal{O}(\log N)$ independent randomized QMC estimators, we prove that for any $ε\in (0,r-\frac{1}{2}]$, our method achieves a mean absolute error bound of $\mathcal{O}(N^{-r+ε})$, where $N$ is the number of points and $r>\frac{1}{2}$ is a parameter determined by the function space. This rate matches the rate of randomly shifted lattice rules obtained via a component-by-component (CBC) construction, while our approach requires no specific CBC constructions or prior knowledge of the space's weight structure. Numerical experiments demonstrate that our method attains an accuracy comparable to the CBC construction based method, and outperforms the Monte Carlo method.

math.NA↗

arXiv2025

Emergence of quantum spin liquid and spin-flop phase in Kitaev antiferromagnets in a [111] magnetic field

Kitaev magnets have emerged as pivotal systems for investigating frustrated magnetism, providing a unique platform to explore quantum phases governed by the interplay between bond-dependent anisotropy and external magnetic fields. However, the quantum phase diagrams, particularly near the dominant antiferromagnetic Kitaev regime, remain puzzling despite extensive studies. In this work, we perform unbiased exact diagonalization calculations of the Kitaev-$Γ$ model in a [111] magnetic field on a $C_{6}$-symmetric 24-site cluster. By calculating the $\mathbb{Z}_2$ flux density and the topological entanglement entropy, we reveal multiple phase transitions and identify signatures of both scalar and vector chiral orders in the intermediate-field regime between the Kitaev spin liquid and the polarized phase. As the negative $Γ$ interaction increases, we discover a proximate quantum spin liquid featured by a three-peak specific heat and a spin-flop phase at a moderate magnetic field. Our findings provide insight into the field-induced intermediate phases in the antiferromagnetic Kitaev model and pave the way for the hunt for emergent phases in real materials.

cond-mat.str-el↗

arXiv2025

Density estimation via periodic scaled Korobov kernel method with exponential decay condition

We propose the periodic scaled Korobov kernel (PSKK) method for nonparametric density estimation on $\mathbb{R}^d$. By first wrapping the target density into a periodic version through modulo operation and subsequently applying kernel ridge regression in scaled Korobov spaces, we extend the kernel approach proposed by Kazashi and Nobile (SIAM J. Numer. Anal., 2023) and eliminate its requirement for inherent periodicity of the density function. This key modification enables effective estimation of densities defined on unbounded domains. We establish rigorous mean integrated squared error (MISE) bounds, proving that for densities with smoothness of order $α$ and exponential decay, our PSKK method achieves an $\mathcal{O}(M^{-1/(1+1/(2α)+ε)})$ MISE convergence rate with an arbitrarily small $ε>0$. While matching the convergence rate of the previous kernel approach, our method applies to non-periodic distributions at the cost of stronger differentiability and exponential decay assumptions. Numerical experiments confirm the theoretical results and demonstrate a significant improvement over traditional kernel density estimation in large-sample regimes.

math.ST↗

arXiv2025

Inner structure of the many-body localization transition and the fulfillment of the Harris criterion

We treat the disordered Heisenberg model in 1D as the standard model of many-body localization (MBL). Two new and independent order parameters stemming solely from the half-chain von Neumann entanglement entropy $S_{\textrm{vN}}$ are introduced to probe the eigenstate phase transition in this model. From the symmetry-endowed entropy decomposition, they are the probability distribution deviation $|d(p_n)|$ and the von Neumann entropy $S_{\textrm{vN}}^{n}(D_n\!=\!\mbox{max})$ of the maximally dimensional symmetry subdivision. The finite-size scaling reveals that $\{p_n\}$ drives the localization transition, preceded by a thermalization breakdown transition governed by $\{S_{\textrm{vN}}^{n}\}$. For the noninteracting case, these transitions coincide, but in the interacting circumstance they separate. Such separability creates an intermediate phase regime and discriminates between the Anderson and MBL transitions. One obstacle whose solution eludes the community to date concerns the violation of the Harris criterion in most numerical investigations of MBL. Upon elucidating the mutually independent measures comprising $S_{\textrm{vN}}$, it becomes clear that the previous studies may lack the resolution to pinpoint thus potentially overlook the crucial internal structure of the transition. We show that after this necessary decomposition, the universal critical exponents for both transitions of $|d(p_n)|$ and $S_{\textrm{vN}}^{n}(D_n\!=\!\mbox{max})$ fulfill the Harris criterion: $ν\approx2\ (ν\approx1.5)$ for quench (quasirandom) disorder. Our work puts forth symmetry combined with entanglement as an organization principle for the generic eigenstate matter and phase transition.

cond-mat.dis-nn↗

arXiv2025

Randomized Quasi-Monte Carlo and Importance Sampling for Super-Fast Growing Functions with Applications to Finance

Many problems can be formulated as high-dimensional integrals of discontinuous functions that exhibit significant boundary growth, challenging the error analysis and applications of randomized quasi-Monte Carlo (RQMC) methods. This paper studies RQMC methods for super-fast growing functions satisfying generalized exponential growth conditions, with a special focus on financial derivative pricing. The main contribution of this paper is threefold. First, by combining RQMC with importance sampling (IS), we derive a new error bound for a class of integrands, whose values and derivatives are bounded by the critical growth function $e^{A|\boldsymbol{x}|^2}$ with $A = 1/2$. This result extends the existing results in the literature, which are limited to the case $A < 1/2$. We demonstrate that by imposing a light-tailed condition on the proposal distribution of IS, RQMC can achieve an error rate of $O(n^{-1 + ε})$ with a sample size n and an arbitrarily small $ε>0$. Second, we verify that the Gaussian proposals used in Optimal Drift Importance Sampling (ODIS) satisfy the required light-tailed condition, providing a rigorous theoretical guarantees for RQMC-ODIS in critical growth scenarios. Third, for discontinuous integrands from finance, we prove that the integrands after preintegration satisfy the exponential growth condition. This ensures that the preintegrated functions can be seamlessly incorporated into our RQMC-IS framework. Numerical experiments on financial derivative pricing validate our theory, showing that the RQMC-IS with preintegration is effective in handling problems with discontinuous payoffs, successfully achieving the expected convergence rates.

math.NA↗

arXiv2025

Tunable Luttinger liquid and correlated insulating states in one-dimensional moiré superlattices

Two-dimensional moiré superlattices have been extensively studied, and a variety of correlated phenomena have been observed. However, their lower-dimensional counterpart, one-dimensional (1D) moiré superlattices, remain largely unexplored. Electrons in 1D are generally described by Luttinger liquid theory, with universal scaling relations depending only on the Luttinger parameter g. In particular, at half-filling, Umklapp scattering plays a crucial role, as it can significantly change the conductance-temperature scaling relation and lead to Mott insulators. However, this prediction has never been observed since doping an empty band to half-filling was extremely difficult. Here, we show that the marriage of moiré superlattices and 1D electrons makes it possible to study the Luttinger liquid in an exceptionally wide filling region simply by electrical gating. We perform transport measurements on 1D moiré superlattices of carbon nanotubes on hexagonal boron nitride (hBN) substrates, and observe correlated insulating states at 1/4 and 1/2 fillings of the superlattice mini-band, where Umklapp scattering becomes dominant. We also observe a T-linear conductance at these commensurate fillings over a range of temperatures. Strikingly, the T-linear conductance leads to a strongly suppressed Luttinger parameter, suggesting a state of extreme correlation.

cond-mat.str-el↗

arXiv2025

Many-body localization and particle multioccupancy in the disordered Bose-Hubbard model

We study the potential influence of the particle multi-occupations on the stability of many-body localization in the disordered Bose-Hubbard model. Within the higher-energy section of the dynamical phase diagram, we find that there is no apparent finite-size boundary drift between the thermal phase and the many-body localized regime. We substantiate this observation by introducing the Van Vleck perturbation theory into the field of many-body localization. The appropriateness of this method rests largely on the peculiar Hilbert-space structure enabled by the particles' Bose statistics. The situation is reversed in the lower-energy section of the dynamical phase diagram, where the significant finite-size boundary drift pushes the putative many-body localized regime up to the greater disorder strengths. We utilize the algebraic projection method to make a connection linking the disordered Bose-Hubbard model in the lower-energy section to an intricate disordered spin chain model. This issue of the finite-size drift could hence be analogous to what happens in the disordered Heisenberg chain. Both trends might be traced back to the particles' intrinsic or emergent single-occupancy constraint like the spin-$1/2$, hard-core boson, or spinless fermion degrees of freedom.

cond-mat.dis-nn↗

arXiv2025

Spinons, solitons and random singlets in the spin-chain compound copper benzoate

The $S=1/2$ antiferromagnetic Heisenberg chain is a paradigmatic quantum system hosting exotic excitations such as spinons and solitons, and forming random singlet state in the presence of quenched disorder. Realizing and distinguishing these excitations in a single material remains a significant challenge. Using nuclear magnetic resonance (NMR) on a high-quality single crystal of copper benzoate, we identify and characterize all three excitation types by tuning the magnetic field at ultra-low temperatures. At a low field of 0.2 T, a temperature-independent spin-lattice relaxation rate ($1/T_1$) over more than a decade confirms the presence of spinons. Below 0.4 K, an additional relaxation channel emerges, characterized by $1/T_1 \propto T$ and a spectral weight growing as $-\ln(T/T_0)$, signaling a random-singlet ground state induced by weak quenched disorder. At fields above 0.5 T, a field-induced spin gap $Δ\propto H^{2/3}$ observed in both $1/T_1$ and the Knight shift signifies soliton excitations. Our results establish copper benzoate as a unique experimental platform for studying one-dimensional quantum integrability and the interplay of disorder and correlations.

cond-mat.str-el↗

arXiv2025

Symmetry- and energy-resolved entanglement dynamics in a disordered Bose-Hubbard model

Using numerical quantum quenches with the integration of both symmetry and energy resolutions, we comprehensively study the dynamics of symmetry-resolved entanglement in a disordered Bose-Hubbard (dBH) model, concentrating on the two types of inhomogeneous initial states to target the lower- and higher-energy sections of its dynamical phase diagram. (i) Motivated by the recent experiment [A. Lukin et al., Science 364, 256 (2019)] which focused on the lower-energy dynamic behaviors of the dBH chain, we first show that, at low energies, for a thermalizing state, although the second law of thermodynamics prohibits the decrease of the total entropy over time, for part of the channel-resolved entropies, a long-term entropic reduction may arise at weak disorder. (ii) A companion channel-resolved analysis at strong disorder further hints that the priorly observed double-log growth of the number entropy might not directly indicate the breakdown of MBL in spin or fermion chains, providing a refreshing perspective on this major controversy in the community. (iii) From time-evolving the line-shape low-energy product state, we subsequently reveal an abrupt formation of a novel "entropy imbalance pattern" across the different symmetry channels. Intriguingly, this imbalance melts in the strong-disorder limit. We conjecture that the melting of the entropic pattern, together with the freezing of a concurrent particle-density wave, embodies a dual trait inherent to MBL. (iv) Specifically, we find a cluster MBL regime, unique to the Bose statistics, emerging from the higher-energy section. This cluster MBL regime realizable even at weak disorder appears not suffer from the finite-size drift and is distinguished by its absence of the hallmark of MBL - the unbounded growth of the entanglement entropy. Our theoretical predictions are by and large testable via the present experimental facilities.

cond-mat.dis-nn↗

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↗