arXiv subjects

Xiao-Bao Yang

Publications and source records attributed to Xiao-Bao Yang.

At least 19 recordsLinked to original sources

arXiv2026

Unified Bonding Entropy Model for Kekulé Graphene Nanoflakes

The open-shell character of Kekulé graphene nanoflakes (GNFs) is conventionally rationalized by the gain of Clar aromatic $π$-sextets upon electron unpairing. While this rule successfully explains many quinoidal diradicaloids, it treats only the maximum number of sextets and neglects the multiplicity and spatial distribution of resonance configurations that realize the same Clar count. Here, we identify a second route to open-shell stabilization in which the maximum Clar-sextet number remains unchanged while the number of accessible Clar resonators increases substantially. We term this mechanism \emph{Clar-number-invariant resonance-space expansion}. By enumerating closed-shell and open-shell Clar resonators and combining this analysis with a bonding entropy model (BEM), we show that electron unpairing can release closed-shell pairing constraints, enlarge the resonance manifold, and redistribute C--C bond occupancies away from localized single- and double-bond limits. The BEM-predicted number and spatial distribution of unpaired electrons correlate strongly with density-functional-theory diradical character, local magnetic moments, optimized C--C bond lengths, and relative energies across a broad set of GNFs. The resulting framework offers a graph-based and physically transparent route for screening open-shell carbon nanostructures and for designing tunable molecular spins without requiring an increase in the maximum Clar number.

cond-mat.mtrl-sci↗

arXiv2026

Classification and design of two-dimensional altermagnets

Altermagnets -- newly identified collinear antiferromagnets -- carry zero net moment with non-relativistic, spin-polarized bands, distilling the best of ferromagnets and antiferromagnets into a single spintronic platform. Shrunking to the two-dimensional limit, they inherit the tunability of two-dimensional crystals while adding symmetry-protected spin splitting, a combination now driving intense experimental interest. Here, we review the symmetry classification of two-dimensional altermagnets based on spin-group theory and survey the growing list of candidate materials, emphasizing those with large spin splitting for experimental realization. We then examine strategies for engineering two-dimensional altermagnetism. This Review aims to consolidate theoretically proposed candidate materials and realization strategies for two-dimensional altermagnets, providing insights for future experimental efforts in this emerging field.

cond-mat.mtrl-sci↗

arXiv2025

Low-temperature series expansion of square lattice Ising model: A study based on Fisher zeros

Low-temperature expansion of Ising model has long been a topic of significant interest in condensed matter and statistical physics. In this paper we present new results of the coefficients in the low-temperature series of the Ising partition function on the square lattice, in the cases of a zero field and of an imaginary field $i(π/2)k_BT$. The coefficients in the low-temperature series of the free energy in the thermodynamic limit are represented using the explicit expression of the density function of the Fisher zeros. The asymptotic behaviour of the sequence of the coefficients when the order goes to infinity is determined exactly, for both the series of the free energy and of the partition function. Our analytic and numerical results demonstrate that, the convergence radius of the sequence is dependent on the accumulation points of the Fisher zeros which have the smallest modulus. In the zero field case this accumulation point is the physical critical point, while in the imaginary field case it corresponds to a non-physical singularity. We further discuss the relation between the series coefficients and the energy state degeneracies, using the combinatorial expression of the coefficients and the subgraph expansion.

cond-mat.stat-mech↗

arXiv2025

Free-fermion models and the two-dimensional Ising models under the zero field and imaginary field $i(π/2){k_B}T$

Ising model is famous in condensed matter and statistical physics. In this work we present a free-fermion formulation of the two-dimensional classical Ising models on the honeycomb, triangular and Kagomé lattices. Each Ising model is studied in the cases of a zero field and of an imaginary field $i(π/2){k_B}T$. We employ the decorated lattice technique, star-triangle transformation and weak-graph expansion method to exactly map each Ising model in both cases into an eight-vertex model on the square lattice. The resulting vertex weights are shown to satisfy the free-fermion condition. In the zero field case, each Ising model is an even free-fermion model. In the case of the imaginary field, the Ising model on the honeycomb lattice is an even free-fermion model while the models on the triangular and Kagomé lattices are odd free-fermion models. We obtain the exact solution of the Kagomé lattice Ising model under the imaginary field $i(π/2){k_B}T$, a result not previously reported in the literature. We also show that the frustrated Ising models on the triangular and Kagomé lattices in the imaginary field still exhibit a non-zero residual entropy.

cond-mat.stat-mech↗

arXiv2025

A unified bonding entropy model to determine magnetic properties in graphene nanoflakes

Graphene nanoflakes (GNFs) exhibit rich magnetic behaviors arising from two primary mechanisms: geometry frustration in non-Kekulé structures and electron delocalization-driven aromatic stabilization in Kekulé-type systems. Herein, we develop a unified bonding entropy model (BEM) to quantitatively characterize the magnetic properties in GNFs within a statistical framework, providing an entropy-based criterion for understanding and predicting bond occupancy numbers and unpaired electron distributions. While non-Kekulé systems naturally favor high-spin configurations due to topological frustration, the BEM reveals that even Kekulé-type GNFs can exhibit magnetic character when the entropy gain from unpaired electrons outweighs the loss of aromatic stabilization. The model predictions show excellent agreement with density functional theory calculations in terms of spin density distributions and unpaired electron counts. Our results establish bonding entropy as a general guiding principle for designing carbon-based magentic materials with tunable magnetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Multi-component altermagnet: A general approach to generating multi-component structures with two-dimensional altermagnetism

Altermagnetism, as an unconventional antiferromagnetism, exhibits collinear-compensated magnetic order in real space and spin-splitting band structure in reciprocal space. In this work, we propose a general approach to generating multi-component structures with two-dimensional altermagnetism, based on symmetry analysis. Specifically, by analyzing the space group of the crystal structures and their subgroups, we systematically categorize equivalent atomic positions and arrange them into orbits based on symmetry operations. Chemical elements are then allowed to occupy all atomic positions on these orbits, generating candidate structures with specific symmetries. We present a general technique for generating collinear-compensated magnetic order, characterized by the symmetrical interconnection between opposite-spin sublattices, and employ first-principles calculations to determine magnetic ground states of multi-component materials. This approach integrates symmetry analysis with the screening of altermagnetic configurations to evaluate the likelihood of candidates possessing altermagnetism. To verify the methodology, we provide examples of previously unreported 2D altermagnets, such as Cr2Si2S3Se3, Fe2P2S3Se3, and V2O2BrI3, and evaluate their dynamical stability by calculating the phonon spectrum. The results demonstrate the feasibility of our approach in generating stable multi-component structures with two-dimensional altermagnetism. Our research has significantly enriched the candidate materials for 2D altermagnet and provided a reference for experimental synthesis.

cond-mat.mtrl-sci↗

arXiv2024

Entropy-driven electron density and effective model Hamiltonian for boron systems

The unique electron deficiency of boron makes it challenging to determine the stable structures, leading to a wide variety of forms. In this work, we introduce a statistical model based on grand canonical ensemble theory that incorporates the octet rule to determine electron density in boron systems. This parameter-free model, referred to as the bonding free energy (BFE) model, aligns well with first-principles calculations and accurately predicts total energies. For borane clusters, the model successfully predicts isomer energies, hydrogen diffusion pathways, and optimal charge quantity for closo-boranes. In all-boron clusters, the absence of B-H bond constraints enables increased electron delocalization and flexibility. The BFE model systematically explains the geometric structures and chemical bonding in boron clusters, revealing variations in electron density that clarify their structural diversity. For borophene, the BFE model predicts that hexagonal vacancy distributions are influenced by bonding entropy, with uniform electron density enhancing stability. Notably, our model predicts borophenes with a vacancy concentration of 1 6 to exhibit increased stability with long-range periodicity. Therefore, the BFE model serves as a practical criterion for structure prediction, providing essential insights into the stability and physical properties of boron-based systems.

cond-mat.mtrl-sci↗

arXiv2024

Residual Entropy of Ice: A Study Based on Transfer Matrices

Residual entropy of ice systems has long been a significant and intriguing issue in condensed matter physics and statistical mechanics. The exact solutions for the residual entropy of realistic three-dimensional ice systems remain unknown. In this study, we focus on two typical realistic ice systems, namely the hexagonal ice (ice Ih) and cubic ice (ice Ic). We present a transfer matrix description of the number of ice-ruled configurations for these two systems. First, a transfer matrix $M$ is constructed for ice Ic, where each element is the number of ice-ruled configurations of a hexagonal monolayer under certain condition. The product of $M$ and its transpose corresponds to a bilayer unit in ice Ih lattice, therefore is exactly a transfer matrix for ice Ih. Making use of this, we simply show that the residual entropy of ice Ih is not less than that of ice Ic in the thermodynamic limit, which was first proved by Onsager in 1960s. Furthermore, we find an alternative transfer matrix $M'$ for ice Ih, which is based on a monolayer periodic unit. Some interesting properties of $M$, $MM^T$ and $M'$ are illustrated, specifically the summation of all elements, the element in the first row and first column, and the trace. Each property is equivalent with the residual entropy of a two-dimensional ice model. Our work rediscovers the relationship between the residual entropies of ice Ih and ice Ic, and provides an effective description for various two-dimensional ice models.

cond-mat.stat-mech↗

arXiv2024

Unveiling the Impact of Sulfur Doping on Copper-Substituted Lead Apatite: A Theoretical Study

Room-temperature superconductivity represents a significant scientific milestone, with the initial report of LK-99, a copper-substituted lead apatite $\mathrm{Pb}_{10-x}\mathrm{Cu}_{x}(\mathrm{PO}_{4})_{6}\mathrm{O}$, offering a potential breakthrough. However, other researchers have encountered numerous challenges in replicating the original experimental results. In recent studies, Wang et al. successfully observed signs of a possible superconducting phase, such as smaller resistance and stronger diamagnetism, upon doping S into the samples. This indicates that the introduction of S is of significant importance for achieving an appropriate structure. To further investigate the role of S, we have considered the $\mathrm{Pb}_{10-x}\mathrm{Cu}_{x}(\mathrm{PO}_{4})_{6}\mathrm{S}$, systematically discussing its thermodynamic stability, as well as the influence of S on the distribution, concentration, and electronic properties of Cu. We find that $\mathrm{Pb}_{10-x}\mathrm{Cu}_{x}(\mathrm{PO}_{4})_{6}\mathrm{S}$ maintains thermodynamic stability, with S primarily influencing the distribution of Cu. The critical element dictating the electronic characteristics of the material post-synthesis is Cu, while the impact of S on the electronic properties is relatively minor. Our work provides valuable insights into the synthesis of potential apatite based room-temperature superconductors and the role of S in facilitating Cu doping.

cond-mat.supr-con↗

arXiv2023

Long-coherence pairing of low-mass conduction electrons in copper-substituted lead apatite

Two entangled qubits emerge as an essential resource for quantum control, which are normally quantum confined with atomic precision. It seems inhibitive that in the macroscopic scope collective qubit pairs manifest long coherence and quantum entanglement, especially at high temperature. Here, we report this exotic ensemble effect in solid-state sintering lead apatite samples with copper substitution, which have been repeatedly duplicated with superior stability and low cost. An extraordinarily low-field absorption signal of cw electron paramagnetic resonance (EPR) spectroscopy stems from low-mass conduction electrons implying the coherence of cuprate radicals can be long-termly protected. The pulsed EPR experiments exhibit triplet Rabi oscillation from paired cuprate diradicals with the coherence time exceeding 1 microsecond at 85K. We believe these appealing effects are sufficiently promising to be applied for scalable quantum control and computation.

quant-ph↗

arXiv2023

Strain-induced interlayer magnetic coupling spike of two-dimensional van der Waals material Fe$_5$GeTe$_2$

A stronger interlayer magnetic coupling (ILMC) can open up new opportunities in spintronics devices for Fe$_5$GeTe$_2$ (F5GT), a demonstrated two-dimensional (2D) van der Waals (vdW) material with high Currie temperature. Here we observe an extraordinary ILMC spike in F5GT, jumping from 1.15 to 12.79 meV/f.u, by applying a 3% in-plane strain. This spike is mainly ascribed to a significant increase in the magnetic moment of the Fe5 ion. Moreover, the applied in-plane strain can also significantly enhance the magnetic anisotropy energy (MAE) of the system, triggering the transition between the in/off-plane configurations in multi-layer F5GT.

cond-mat.mtrl-sci↗

arXiv2022

Analysis of Transition Path Ensemble in the Exactly Solvable Models via Overdamped Langevin Equation

Transition of a system between two states is an important but difficult problem in natural science. In this article we study the transition problem in the framework of transition path ensemble. Using the overdamped Langevin method, we introduce the path integral formulation of the transition probability and obtain the equation for the minimum action path in the transition path space. For the effective sampling in the transition path ensemble, we derive a conditional overdamped Langevin equation. In two exactly solvable models, the free particle system and the harmonic system, we present the expression of the conditional probability density and the explicit solutions for the conditional Langevin equation and the minimum action path. The analytic results demonstrate the consistence of the conditional Langevin equation with the desired probability distribution in the transition. It is confirmed that the conditional Langevin equation is an effective tool to sample the transition path ensemble, and the minimum action principle actually leads to the most probable path.

cond-mat.stat-mech↗

arXiv2022

Residual Entropy of a Two-dimensional Ising Model with Crossing and Four-spin Interactions

We study the residual entropy of a two-dimensional Ising model with crossing and four-spin interactions, both for the case that in zero magnetic field and that in an imaginary magnetic field i(π/2)kT. The spin configurations of this Ising model can be mapped into the hydrogen configurations of square ice with the defined standard direction of the hydrogen bonds. Making use of the equivalence of this Ising system with the exactly solved eight-vertex model and taking the low temperature limit, we obtain the residual entropy. Two soluble cases in zero field and one soluble case in imaginary field are examined. In the case that the free-fermion condition holds in zero field, we find the ground states in low temperature limit include the configurations disobeying the ice rules. In another case in zero field that the four-spin interactions are -{\infty}, and the case in imaginary field that the four-spin interactions are 0, the residual entropy exactly agrees with the result of square ice determined by Lieb in 1967. In the solutions of the latter two cases, we have shown alternative approaches to the residual entropy problem of square ice.

cond-mat.stat-mech↗

arXiv2022

Exact Results for the Residual Entropy of Ice Hexagonal Monolayer

Since the problem of the residual entropy of square ice was exactly solved, exact solutions for two-dimensional realistic ice models have been of interest. In this paper, we study the exact residual entropy of ice hexagonal monolayer in two cases. In the case that the external electric field along the z-axis exists, we map the hydrogen configurations into the spin configurations of the Ising model on the Kagomé lattice. By taking the low temperature limit of the Ising model, we derive the exact residual entropy, which agrees with the result determined previously from the dimer model on the honeycomb lattice. In another case that the ice hexagonal monolayer is under the periodic boundary conditions in the cubic ice lattice, we employ the six-vertex model on the square lattice to represent the hydrogen configurations obeying the ice rules. The exact residual entropy in this case is obtained from the solution of the equivalent six-vertex model. Our work provides more examples of the exactly soluble two-dimensional models.

cond-mat.stat-mech↗

arXiv2022

On the Numerical Stationary Distribution of Overdamped Langevin Equation in Harmonic System

Efficient numerical algorithm for stochastic differential equation has been an important object in the research of statistical physics and mathematics for a long time. In this paper we study the highly accurate numerical algorithm of the overdamped Langevin equation. In particular, our interest is the behaviour of the numerical schemes for solving the overdamped Langevin equation in the harmonic system. Three algorithms are obtained for overdamped Langevin equation, from the large friction limit of the schemes for underdamped Langevin dynamics. We derive the explicit expression of the stationary distribution of each algorithm by analysing the discrete time trajectory, for both one-dimensional and multi-dimensional cases. The accuracy of the stationary distribution of each algorithm is illustrated by comparing to the exact Boltzmann distribution. Our results demonstrate that, the "BAOA-limit" algorithm generates the exact distribution for the harmonic system in the canonical ensemble, within the stable regime of the time interval. The other algorithms do not produce the exact distribution of the harmonic system.

cond-mat.stat-mech↗

arXiv2022

Five-fold Symmetry in Au-Si Metallic Glass

The first metallic glass of Au-Si alloy has been discovered for over half a century, but its atomic structure is still puzzling. Herein, Au 8 Si dodecahedrons with local five-fold symmetry are revealed as building blocks in Au-Si metallic glass, and the interconnection modes of Au 8 Si dodecahedrons determine the medium-range order. With dimensionality reduction, the surface ordering is attributed to the motif transformation of Au 8 Si dodecahedrons into planar Au 5 Si pyramids with five-fold symmetry, and thus the self-assembly of Au 5 Si pyramids leads to the formation of the ordered Au 2 Si monolayer with the lowest energy. Furthermore, the structural similarity analysis is performed to unveil the physical origin of structural characteristics in different dimensions. The amorphism of Au-Si is due to the smooth energy landscape around the global minimum, while the ordered surface structure occurs due to the steep energy landscape.

cond-mat.mtrl-sci↗

arXiv2022

Determining ground states of alloy by a symmetry-based classification

Reducing the number of candidate structures is crucial to improve the efficiency of global optimization. Herein, we demonstrate that the generalized Hamiltonian can be described by the atom classification model (ACM) based on symmetry, generating competent candidates for the first-principles calculations to determine ground states of alloy directly. The candidates can be obtained in advance through solving the convex hull step by step, because the correlation functions of ACM can be divided into various subspace according to the defined index $l$. As an important inference, this index can be converted to the number of Wyckoff positions, revealing the dominant effect of geometry symmetry on structural stability. Taking Ni-Pt, Ag-Pd, Os-Ru, Ir-Ru and Mo-Ru as examples, we not only identify the stable structures in previous theoretical and experimental results, but also predict a dozen of configurations with lower formation energies, such as Ag$_{0.5}$Pd$_{0.5}$ ($Fd$-$3m$), Os$_{0.5}$Ru$_{0.5}$ ($Pnma$), Ir$_{1/3}$Ru$_{2/3}$ ($P6_{3}/mmc$), and Mo$_{0.25}$Ru$_{0.75}$ ($Cmcm$).

cond-mat.mtrl-sci↗

arXiv2018

Insights into the Unusual Semiconducting Behavior in Low-Dimensional Boron

Elementary semiconductors are rare and attractive, especially for low-dimensional materials. Unfortunately, most of boron nanostructures were found to be metallic, despite of their typical semiconducting bulk structure. Herein, we propose a general recipe to realize low-dimensional semiconducting boron. This unusual semiconducting behavior is attributed to charge transfer and electron localization, induced by the symmetry breaking that divides boron atoms into cations and anions. In addition, it is feasible to accomplish band gap engineering by rationally designing various structures. Importantly, the low-dimensional semiconducting boron are predicted to be an excellent solar-cell material with the power conversion efficiency of higher than 20%, paving the way for their promising optoelectronic applications.

cond-mat.mes-hall↗