arXiv subjects

Xue-Feng Zhang

Publications and source records attributed to Xue-Feng Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Spinon Singlet Pairing: Microscopic nature of plaquettes in stripy LDOS

Scanning tunneling microscopy (STM) is a powerful tool for visualizing the local density of states (LDOS) of individual stripes in cuprates. However, the microscopic nature of the observed exotic LDOS patterns and their connection to high-$T_c$ superconductivity remain open questions. Within the framework of the quantum colored string model, we reveal that the ubiquitous $4a_0\times4a_0$ plaquettes originate from either the breaking of local spinon singlet pairs through hole insertion, or the formation of an unpaired spinon upon electron addition in a stripe. Moreover, by comparing our data with LDOS of cuprates, we identify an effect of particle-hole symmetry breaking (PHSB): a $2a_0$ shift, which is predicted and confirmed in a longer stripe ($L=18$). At last, we establish and verify a general relation between hole density and plaquette size across multiple fillings. Our work offers a fresh wavefunction-based perspective on interpreting STM signals in cuprate experiments and demonstrates that their origin may arise from spinon singlet pairing in the ground state of fluctuating stripes, the same mechanism underlying the $d$-wave sign structure [Phys. Rev. Lett. \textbf{137}, 086702 (2026)].

cond-mat.str-el↗

arXiv2026

Spinon Singlet Pairing: Origin of $d$-Wave Sign Structure in a Partially-Filled Stripe

Significant research advances have led to a consensus that the Fermi-Hubbard model and its extended variants are archetypical frameworks for elucidating the intertwined relationship between stripe orders and superconductivity in hole-doped high-$T_c$ materials. Notably, the Hubbard quantum simulator has recently achieved several remarkable breakthroughs, e.g., being cooled down to the cryogenic regime and enabling the observation of stable fluctuating stripes. However, the microscopic mechanism underlying the $d$-wave pairing of electrons in the presence of stripes at low temperatures remains poorly understood due to the intricate interplay between strongly correlated effects and non-negligible thermal fluctuations. Here, we conduct a close investigation of a partially-filled stripe in the representative $t$-$J$ and $t$-$t'$-$U$ models with both numerical and analytical methods. Analogous to quantum gas microscopy, the perfect sampling technique allows us to obtain the high-confidence-level statistics of the Fock basis states appearing in the ground-state wavefunction. In a novel physical paradigm, these data demonstrate that two spinons with opposite chiralities tend to pair spontaneously into a singlet state, thereby naturally giving rise to the $d$-wave pairing pattern. Then, using the updated effective theory of quantum colored string, we have reconstructed the wavefunction and have determined the nature of spinon pairing and its connection to the $d$-wave sign structure of pair-pair correlation. Furthermore, spinon singlet pairs enable the establishment of a long-range pair-pair correlation between the two stripes. Our work offers new insights into the microscopic physics of stripes and paves the way for further exploration of multi-stripe-mediated pairing mechanisms in the Fermi-Hubbard model.

cond-mat.str-el↗

arXiv2026

Gauge-constrained Spinon Complexes Near Deconfined Quantum Criticality

Quantum magnets provide a microscopic platform for studying confinement and gauge-constrained structures mediated by emergent gauge fields. We investigate confined spinon complexes in the columnar valence-bond-solid (VBS) phase near deconfined quantum criticality using pinned-spin defects and quantum Monte Carlo simulations. The pinned spins act as static spinon sources with controlled positions, spin projections, and VBS vorticities, enabling measurement of defect energies and direct real-space visualization of the associated VBS domain-wall strings. For matched spinon-antispinon sources, the excitation energy saturates beyond a characteristic separation as one extended dipole reorganizes into two shorter neutral dipoles, providing energetic and real-space evidence of string breaking. We further show that domain-wall connectivity is governed by lattice-scale VBS phase offsets in addition to vorticity and spin-projection neutrality. Compatible multi-pin patterns generate connected four-spinon complexes and extended domain-wall networks that retain their global connectivity under local distortions. These results establish pinned-spin defects as a controlled tool for assembling and resolving multi-spinon structures and their confining strings in a VBS phase.

cond-mat.str-el↗

arXiv2026

Quantum String Interactions Revealed by Full Counting Statistics

How quantum strings interact is a basic question for extended objects in quantum many-body physics. Even the simplest hard-core constraint (no crossing), can generate a nontrivial effective potential, whose microscopic form is difficult to determine because the relative distance between the strings is intrinsically nonlocal. Here we show that this nonlocality is naturally captured by full counting statistics (FCS). For two hard-core quantum strings, we derive an analytic FCS expression for the emergent interaction by identifying the virtual process in which the two strings touch and hop back. Using the FCS--entanglement relation, we find the effective potential has the entanglement-controlled asymptotic form $\lnΔE(r)\sim -π^2 r^2/(12 S_\ell)$ up to subleading terms, where $S_\ell$ is the entanglement entropy between the two halves of a quantum string. We confirm the theory using high-precision numerical calculations and finite-size FCS estimates. Our results reveal FCS as a direct route to effective interactions between quantum topological line-defects, which may also be extended to higher-form charge.

cond-mat.str-el↗

arXiv2026

Frustrated Rydberg Atom Arrays Meet Cavity-QED: Emergence of the Superradiant Clock Phase

Rydberg atom triangular arrays in an optical cavity serve as an ideal platform for understanding the interplay between geometric frustration and quantized photons. Using a large-scale quantum Monte Carlo method, we obtain a rich ground state phase diagram. Around half-filling, the infinite long-range light-matter interaction lifts the ground state degeneracy, resulting in a novel order-coexisted superradiant clock phase that completely destroys the fragile order-by-disorder phase observed in classical light fields. According to the Ginzburg-Landau theory, this replacement may result from the competition between threefold and sixfold clock terms. Similar to the spin supersolid, the clear first-order phase transition at the $Z_2$ symmetry line is attributed to the nonzero photon density, which couples to the threefold clock term. Finally, we discuss the low-energy physics in the dimer language and propose that cavity-mediated nonlocal ring exchange interactions may play a critical role in the rich physics induced by the attachment of cavity-QED. Our work opens a new arena of research on the emergent phenomena of quantum phase transitions in many-body quantum optics.

cond-mat.quant-gas↗

arXiv2026

Entanglement entropy and disorder operator at kagome deconfined quantum criticality

We investigate the deconfined quantum critical point (DQCP) candidate in the extended hard-core Bose-Hubbard model on the kagome lattice, employing quantum Monte Carlo simulations to study the entanglement entropy and the $U(1)$ disorder operator. In stark contrast to findings in $J$-$Q$ models and other candidates, the universal logarithmic correction coefficients for both quantities are found to be {positive}, consistent with a unitary conformal field theory (CFT). Crucially, the current central charge $C_J$, extracted from the small-angle behavior of the disorder operator, is enhanced by a factor of approximately {4/3} compared to that of the conventional 3D $O(2)$ Wilson-Fisher fixed point. This enhancement {implies} a consistent explanation in the recently observed low-energy excitation spectrum at this DQCP, which features {two distinct linearly dispersing modes} with a velocity ratio of approximately three. Our results provide evidence that this quantum phase transition constitutes a genuine DQCP, characterized by coexisting fractionalized excitations that collectively modify its critical properties.

cond-mat.str-el↗

arXiv2026

Interplay of Unidirectional Quantum Strings in Kagome Rydberg Atom Array

Leveraging the rapid development of quantum simulators, the intriguing phenomena of quantum string are observed across various quantum simulation platforms. However, the complex interplay between the quantum strings cannot be well analyzed due to the limited system size in real quantum simulators. Here, with the help of a newly developed quantum Monte Carlo method, we can simulate a larger-scale Kagome Rydberg atom array, providing an ideal playground for studying quantum strings. By introducing a novel edge pinning method, the ends of a quantum string can be attached to edges so that the flexible manipulation of the quantum string becomes possible. Due to the geometric constraint, the quantum strings are unidirectional, which strongly complicates their interplay. To quantitatively describe the quantum string, we built a one-dimensional effective model. With both analytic and numerical methods, rich physics can be found, including ``geometric breaking", heart-like superposition state of quantum strings, and the attractive inter-string interactions. This work can benefit the comprehension of quantum strings and may also shed light on the simulation of high-energy physics.

cond-mat.quant-gas↗

arXiv2025

Loop Algorithm for Quantum Transverse Ising Model in a Longitudinal Field

The quantum transverse Ising model and its extensions play a critical role in various fields, such as statistical physics, quantum magnetism, quantum simulations, and mathematical physics. Although it does not suffer from the sign problem in most cases, the corresponding quantum Monte Carlo algorithm performs inefficiently, especially at a large longitudinal field. The main hindrance is the lack of loop update method which can strongly decrease the auto-correlation between Monte Carlo steps. Here, we successfully develop a loop algorithm with a novel merge-unmerge process. It demonstrates a great advantage over the state-of-the-art algorithm when implementing it to simulate the Rydberg atom chain and Kagome qubit ice. This advanced algorithm suits various systems such as Rydberg atom arrays, trapped ions, quantum materials, and quantum annealers.

cond-mat.str-el↗

arXiv2025

Quantum phase transitions of the anisotropic Dicke-Ising model in driven Rydberg arrays

We study the properties of a generalized Dicke-Ising model realized with an array of Rydberg atoms, driven by microwave electric fields and coupled to an optical cavity. As this platform allows for a precisely tunable anisotropy parameter, the model exhibits a rich landscape of phase transitions and critical phenomena, induced by the interplay of rotating-wave, counter-rotating-wave, and Ising interactions. We develop an improved quantum Monte Carlo algorithm based on the stochastic series expansion that explicitly tracks the Fock state of the quantum cavity. In the superradiant (SR) phase, this allows us to determine, through data collapse, the scaling laws of the photon number. We also demonstrate the vanishing of parity symmetry in finite-size simulations and show that the Rydberg blockade leads to a significant suppression of cavity occupation. Notably, stronger quantum fluctuations induced by the counter-rotating wave terms slightly favor the superradiant solid (SRS) phase over the Solid-1/2 state. Finally, we confirm that the SR phase transition and the transition from the Solid-1/2 to the SRS are second-order. In contrast, the transitions from the Solid-1/2 or SRS to the SR phase are both first-order for any value of the normalized anisotropy parameter.

cond-mat.quant-gas↗

arXiv2025

Quantum colored strings in the hole-doped $t$-$J_z$ model

The stripe phase, an intertwined order observed in high-temperature superconductors, is regarded as playing a key role in elucidating the underlying mechanism of superconductivity, especially in cuprates. Following Jan Zaanen's early scenario, the filled charge stripe, with one hole per unit cell of the charge order, can be taken as the interactive elastic quantum strings of holes, stabilized by $π$-phase shifts between neighboring magnetic domains. However, this scenario is challenging to explain, particularly in terms of electron pairing, which necessitates hole pairs. In this work, we propose a new effective model for describing the stripe phase in the hole-doped $t$-$J_z$ model. With respect to the antiferromagnetic background, the model comprises three types of color-labeled point-defects coupling to an effective spin field, so named as ``colored string". Comparing with numerical results from large-scale density matrix renormalization group (DMRG) simulations, we find semi-quantitative agreement in local hole density, magnetic moment, and the newly proposed spectrum features of the effective spin field. By systematically analyzing the hole-density distribution and the scaling of groundstate energy at different system sizes, we determine the effective core radius and the effective hopping amplitude of the quantum string. Furthermore, the local pinning field can be finely adjusted to drag the quantum string, offering a potential method for detecting it in optical lattices. At last, we further demonstrate the partially-filled stripe with less than one hole per unit cell of the charge order can also be well described by the effective theory.

cond-mat.str-el↗

arXiv2025

Quantum Simulation of Two-Dimensional $\mathrm{U(1)}$ Gauge Theory in Rydberg and Rydberg-Dressed Atom Arrays

Simulating $\mathrm{U(1)}$ quantum gauge theories with spatial dimension greater than one is of great physical significance yet has not been achieved experimentally. Here we propose a simple realization of $\mathrm{U(1)}$ gauge theory on triangular lattice Rydberg atom arrays. Within experimentally accessible range, we find that the effective model well simulates various aspects of the $\mathrm{U(1)}$ gauge theory, such as emergence of topological sectors, incommensurability, and the deconfined Rokhsar-Kivelson point. Our proposal is easy to implement experimentally and exhibits pronounced quantum dynamics compared with previous proposals realizing $\mathrm{U(1)}$ and $\mathbb Z_2$ gauge theories.

quant-ph↗

arXiv2024

Spinon Singlet in Quantum Colored String: Origin of $d$-Wave Pairing in a Partially-Filled Stripe

Although both experimental observations and numerical simulations have reached a consensus that the stripe phase is intertwined with superconductivity in cuprates, the microscopic mechanism behind $d$-wave pairing in the presence of stripes remains unclear. Using the effective theory of quantum colored strings, we derive the wavefunction in Fock space. Our results show that two spinons with opposite chiralities tend to pair into a spinon singlet, which in turn facilitates the formation of negative pair-pair correlations between distant $x$-bonds and $y$-bonds, a hallmark of the $d$-wave pairing pattern. The same pair-pair correlation pattern is observed across various models, as confirmed by large-scale density matrix renormalization group calculations. Based on these results, we conclude that the spinon singlet is the origin of $d$-wave superconductivity in a fluctuating, partially-filled stripe, and this mechanism may also extend to multi-stripe configurations.

cond-mat.str-el↗

arXiv2024

Deconfined quantum phase transition on the kagome lattice: Distinct velocities of spinon and string excitations

Deconfined quantum phase transition (DQPT) provides an extraordinary possibility of the quantum phase transition beyond the Ginzburg-Landau paradigm, which is interwoven with numerous exotic phenomena of the strongly correlated quantum many-body system, e.g. fractional excitation, emergent symmetries, and gauge field. However, various candidates of DQPT have been demonstrated to be weakly first-order, and the conformal field theory (CFT) has to be altered into a non-unitary one. Here we numerically found two linear dispersions with different velocities in one of the few survivors of DQPT -- the extended hard-core Bose-Hubbard model on the Kagome lattice. Such counterintuitive results directly lead to the negation of possible emergent Lorentz symmetry, and the breakdown of conventional theory of DQPT. Furthermore, the snapshots of boson configuration hint that these two velocities may correspond to the dynamics of the fractional excitations and quantum strings, respectively. Our work will inspire the revisit of the theory of DQPT and benefit the field of quantum materials and quantum simulations.

cond-mat.str-el↗

arXiv2024

Analysis of Pseudo-Random Number Generators in QMC-SSE Method

In the quantum Monte Carlo (QMC) method, the Pseudo-Random Number Generator (PRNG) plays a crucial role in determining the computation time. However, the hidden structure of the PRNG may lead to serious issues such as the breakdown of the Markov process. Here, we systematically analyze the performance of the different PRNGs on the widely used QMC method -- stochastic series expansion (SSE) algorithm. To quantitatively compare them, we introduce a quantity called QMC efficiency that can effectively reflect the efficiency of the algorithms. After testing several representative observables of the Heisenberg model in one and two dimensions, we recommend using LCG as the best choice of PRNGs. Our work can not only help improve the performance of the SSE method but also shed light on the other Markov-chain-based numerical algorithms.

cond-mat.str-el↗

arXiv2024

Non-integer Floquet Sidebands Spectroscopy

In the quantum system under periodical modulation, the particle can be excited by absorbing the laser photon with the assistance of integer Floquet photons, so that the Floquet sidebands appear. Here, we experimentally observe non-integer Floquet sidebands (NIFBs) emerging between the integer ones while increasing the strength of the probe laser in the optical lattice clock system. Then, we propose the Floquet channel interference hypothesis (FCIH) which surprisingly matches quantitatively well with both experimental and numerical results. With its help, we found both Rabi and Ramsey spectra are very sensitive to the initial phase and exhibit additional two symmetries. More importantly, the height of Ramsey NIFBs is comparable to the integer one at larger $g/ω_s$ which indicates an exotic phenomenon beyond the perturbative description. Our work provides new insight into the spectroscopy of the Floquet system and has potential application in quantum technology.

cond-mat.quant-gas↗

arXiv2023

Quantum optimization within lattice gauge theory model on a quantum simulator

Simulating lattice gauge theory (LGT) Hamiltonian and its nontrivial states by programmable quantum devices has attracted numerous attention in recent years. Rydberg atom arrays constitute one of the most rapidly developing arenas for quantum simulation and quantum computing. The $\mathbb{Z}_2$ LGT and topological order has been realized in experiments while the $U(1)$ LGT is being worked hard on the way. States of LGT have local constraint and are fragmented into several winding sectors with topological protection. It is therefore difficult to reach the ground state in target sector for experiments, and it is also an important task for quantum topological memory. Here, we propose a protocol of sweeping quantum annealing (SQA) for searching the ground state among topological sectors. With the quantum Monte Carlo method, we show that this SQA has linear time complexity of size with applications to the antiferromagnetic transverse field Ising model, which has emergent $U(1)$ gauge fields. This SQA protocol can be realized easily on quantum simulation platforms such as Rydberg array and D-wave annealer. We expect this approach would provide an efficient recipe for resolving the topological hindrances in quantum optimization and the preparation of quantum topological state.

quant-ph↗

arXiv2023

Asymmetric scattering behaviors of spin wave dependent on magnetic vortex chirality

In this letter, an asymmetric spin wave scattering behaviors caused by vortex chirality are investigated in cross-shaped ferromagnetic system. In the system, four scattering behaviors are found, 1) asymmetric skew scattering, depending on the polarity of vortex core, 2) back scattering (reflection), depending on the vortex core stiffness, 3) side deflection scattering, depending on structural symmetry of the vortex circulation, and 4) geometrical scattering, depending on waveguide structure. The first and second scattering behaviors are attributed to nonlinear topological magnon spin Hall effect related to magnon spin-transfer torque effect, which has value for magnonic exploration and application.

cond-mat.mtrl-sci↗

arXiv2023

Magnon-bandgap controllable artificial domain wall waveguide

In this paper, a magnon-bandgap controllable artificial domain wall waveguide is proposed by means of micromagnetic simulation. By the investigation of the propagation behavior and dispersion relationship of spin waves in artificial domain wall waveguides, it is found that the nonreciprocal propagation of spin waves in the artificial domain walls are mainly affected by the local effective exchange field, and the magnon bandgap can be controlled by changing the maximum value of the effective exchange field. In addition, it is observed that the artificial domain wall waveguides are structurally more stable than the natural domain wall waveguides under the same spin wave injection conditions, and the magnon bandgap of the artificial domain wall waveguides can be adjusted by its width and magnetic anisotropy parameters. The bandgap controllable artificial domain wall scheme is beneficial to the miniaturization and integration of magnon devices and can be applied to future magnonic technology as a novel frequency filter.

cond-mat.mtrl-sci↗