arXiv subjects

Yu-Jun Zhao

Publications and source records attributed to Yu-Jun Zhao.

At least 19 recordsLinked to original sources

arXiv2026

Unified Bonding Entropy Model for Kekul\'{e} Graphene Nanoflakes

The open-shell character of Kekul\'{e} graphene nanoflakes (GNFs) is conventionally rationalized by the gain of Clar aromatic $\pi$-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

Stacking theory for bilayer two-dimensional magnets

Two-dimensional unconventional magnetism has recently attracted growing interest due to its intriguing physical properties and promising applications in spintronics. However, existing studies on stacking-induced unconventional magnetism mainly focus on specific materials and stacking configurations. Here, we develop a general symmetry-based stacking theory for two-dimensional magnets. We first introduce spin layer groups as the fundamental symmetry framework, providing the essential magnetic symmetry information for the stacking theory. Based on this framework, we construct the complete set of 448 collinear spin layer groups for describing two-dimensional collinear magnets. Subsequently, we develop a general magnetic stacking theory applicable to arbitrary magnetic systems and derive its general solutions. Using CrF$_3$ as an illustrative example, we show how this theory enables designs of two-dimensional unconventional magnetism, as validated by first-principles calculations. We realize two-dimensional fully compensated ferrimagnetism through our stacking theory. Our work provides a general symmetry-guided platform for discovering and designing stacking-induced unconventional magnetism.

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

Symmetry-guided prediction of magnetic-ordered ground states

Given the scarcity of experimentally confirmed magnetic structures, the prediction of magnetic ground states is crucial yet remains a long-sought challenge due to the complex potential energy landscape. Here, we propose a symmetry-guided framework that systematically generates magnetic configurations without requiring any experimental input or prior assumptions. Within a symmetry-breaking scenario, we incorporate the recently developed oriented spin space group formalism, which captures symmetry-breaking induced by both magnetic ordering and spin-orbit coupling. By performing nonrelativistic and relativistic first-principles calculations, we establish the energy ladder of the generated magnetic configurations. Exemplified by three prominent unconventional magnets, we demonstrate that only a few dozen calculations are sufficient to identify the ground-state magnetic structure. To demonstrate the universality and robustness of our approach, we conduct large-scale benchmark tests on the MAGNDATA database. Our framework successfully reproduces experimentally reported magnetic geometries for 78% of the surveyed materials, among which 93% have their spin orientations successfully generated when considering SOC. Furthermore, in a large-scale first-principles benchmark involving 305 compounds, 82% of experimentally reported magnetic structures are accurately captured within an energy tolerance of 5 meV per magnetic atom. Beyond reproducing known magnetic configurations, our framework further predicts a variety of low-energy metastable phases, including altermagnets, spin-orbit magnets, and noncollinear antiferromagnets with spin splitting or geometric Hall effect. Our work establishes a general and efficient route toward large-scale prediction of magnetic structures and unconventional magnets, and offers insight into the origins of magnetic interactions across diverse material systems.

cond-mat.mtrl-sci↗

arXiv2025

Slow growth of quantum magic in disorder-free Stark many-body localization

Disorder-free quantum many-body localization can strongly suppress transport while still enabling the dynamical buildup of computationally costly non-Clifford resources. In a tilted transverse-field Ising chain realizing disorder-free Stark many-body localization, we use the stabilizer R\'enyi entropy to quantify quantum magic (nonstabilizerness) and find that it remains finite and grows anomalously slowly over extended time windows before saturating to a size-dependent plateau deep in the strong-tilt regime, with pronounced initial-state selectivity. Upon increasing the Stark gradient, the long-time magic and half-chain entanglement exhibit consistent finite-size crossing behavior, indicating a crossover from ergodic dynamics to constrained localization. These results establish stabilizer-based magic as a practical complexity diagnostic of disorder-free ergodicity breaking and constrained dynamics, and provide an experimentally accessible route to benchmarking and designing near-term quantum simulators.

quant-ph↗

arXiv2025

Entanglement phases and phase transitions in monitored free fermion system due to localizations

In recent years, the presence of local potentials has significantly enriched and diversified the entanglement patterns in monitored free fermion systems. In our approach, we employ the stochastic Schr\"odinger equation to simulate a one-dimensional spinless fermion system under continuous measurement and local potentials. By averaging the steady-state entanglement entropy over many quantum trajectories, we investigate its dependence on measurement and localization parameters. We used a phenomenological model to interpret the numerical results, and the results show that the introduction of local potentials does not destroy the universality class of the entanglement phase transition, and that the phase boundary is jointly characterized by the measurement process and the localization mechanism. This work offers a new perspective on the characterization of the entanglement phase boundary arising from the combined effects of measurement and localization, and provides criteria for detecting this novel phase transition in cold atom systems, trapped ions, and quantum dot arrays.

quant-ph↗

arXiv2025

Unlocking Altermagnetism in Antiferromagnetic 2D Films via Adsorption

Altermagnets, characterized by zero net magnetization and momentum-dependent spin splitting, have recently garnered significant attention due to their potential applications in a variety of fields. Here, we propose a symmetry-engineering strategy to unlock altermagnetism in two dimensional (2D) antiferromagnetic systems via surface adsorption of atoms or molecules. By employing spin group theory, we systematically demonstrate that selectively breaking symmetry operations, specifically those protecting spin degeneracy in momentum space, enables the emergence of nonrelativistic spin-split electronic states. Meanwhile, preserving rotation or mirror symmetries connecting opposite sublattices ensures zero net magnetization. Through a comprehensive classification of all symmetry operations across 80 layer groups, we identify 63 antiferromagnetic spin point groups (SPGs) describing 2D materials and further isolate 15 groups that can host altermagnetic characteristics through surface adsorption. Exemplified with monolayer antiferromagnetic VPS_3 and MnPSe_3, we show that oxygen adsorption on VPS_3 and NH_3 adsorption on MnPSe_3 selectively disrupt PT symmetry while retaining the [C2||m] symmetry. This engineered symmetry reduction induces pronounced spin splitting in their band structures without spin-orbit coupling, as confirmed by first-principles calculations. Furthermore, adsorption energy analysis and thermal stability phase diagrams under varying coverage regimes reveal optimal configurations for experimental feasibility. Our work establishes a universal symmetry-engineering framework to expand the family of altermagnetic materials, offering a versatile pathway to tailor spin-split functionalities in two-dimensional antiferromagnets for advanced quantum applications.

cond-mat.mes-hall↗

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-Kekul\'e structures and electron delocalization-driven aromatic stabilization in Kekul\'e-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-Kekul\'e systems naturally favor high-spin configurations due to topological frustration, the BEM reveals that even Kekul\'e-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

Controllable Antiferromagnetic-Ferromagnetic phase transition in monolayer MnPSe3 via atomic adsorption of Li, O, and F

The engineering of magnetic order and electronic states in two-dimensional (2D) materials is pivotal for advanced spintronic technologies. Despite their potential, the scarcity of intrinsic 2D ferromagnets remains a critical challenge. Here, we employ density functional theory with Hubbard-U corrections to systematically investigate adsorbate-driven magnetic transitions in monolayer MnPSe3. While pristine MnPSe3 exhibits antiferromagnetic (AFM) ordering with a semiconducting gap, Li, O, and F adatom adsorption induces an AFM-to-ferromagnetic (AFM-FM) phase transition across large coverage ranges. Our calculations reveal enhanced thermodynamic stability at elevated coverages, with full-coverage configurations (Li0.5MnPSe3, MnPSe3O0.5, MnPSe3F0.5) favored energetically. Hybrid functional (HSE06) calculations show that F adsorption drives a semiconductor-to-half-metal transition, whereas Li and O adsorption preserves semiconductivity. Moreover, Li adsorption induces a valley splitting of 20.3 meV at the K1 and K2 points in the band structure of the monolayer MnPSe3. Magnetic anisotropy analysis reveals adsorbate-dependent easy-axis reorientation: Li (electron doping) switches the easy-axis from in-plane to out-of-plane, while O/F (hole doping) stabilizes in-plane easy-axis, consistent with carrier-density-modulation simulations. Crucially, carrier doping results indicate that once the electron doping concentration reaches critical concentration, the magnetic easy axis of monolayer MnPSe3 transitions from in-plane to out-of-plane. This work establishes atomic adsorption as a robust strategy for tailoring 2D magnetism, resolving discrepancies through rigorous treatment of exchange-correlation effects and configurational diversity.

cond-mat.mes-hall↗

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

A parameter-free statistical model for two-dimensional carbon nanostructures

Energy degeneracy in physical systems may be induced by symmetries of the Hamiltonian, and the resonance of degeneracy states in carbon nanostructures can effectively enhance the stability of the system. Combining the octet rule, we introduce a parameter-free statistical model to determine the physical properties by lifting the energy degeneracy in carbon nanostructures. This model offers a direct path to accurately ascertain electron density distributions in quantum systems, akin to how charge density is used in density functional theory to deduce system properties. Our methodology diverges from traditional quantum mechanics, focusing instead on this unique statistical model by minimizing bonding free energy to determine the fundamental properties of materials. Applied to carbon nanoclusters and graphynes, our model not only precisely predicts bonding energies and electron density without relying on external parameters, but also enhances the prediction of electronic structures through bond occupancy numbers, which act as effective hopping integrals. This innovation offers insights into the structural properties and quantum behavior of electrons across various dimensions.

cond-mat.mes-hall↗

arXiv2024

Quantum feedback induced entanglement relaxation and dynamical phase transition in monitored free fermion chains with Wannier-Stark ladder

In recent years, measurement induced entanglement transitions (MIETs) have attracted significant attention. However, the dynamical transition associated with the feedback induced skin effect, which exhibits a wealth of intriguing phenomena, has not been fully understood. In this work, we investigate a dynamical phase transition in a tilted free-fermion chain under measurement-feedback protocols, emphasizing the particle density and entanglement entropy dynamics. We reveal a feedback induced skin effect, enhanced by the Wannier-Stark ladder potential, that creates localization at one boundary and generates an effective pseudo edge under periodic conditions. The observables show a two-stage evolution: a rapid initial logarithmic growth followed by decay into an area-law steady state. Using a rescaling analysis, we pinpoint the critical behavior and offer an intuitive physical picture that links it to the feedback-driven suppression of quantum jump fluctuations. The resulting entanglement dynamics appear to be governed by a system-size-dependent delay, followed by a size-independent relaxation process. This behavior is consistent with the ballistic propagation of free fermions toward a domain-wall-like steady state and does not exhibit any signatures of nontrivial criticality. This work provides an effective supplement to the dynamical transition. It provides valuable references for linking the dynamical understanding of the role feedback plays in MIETs.

quant-ph↗

arXiv2024

Fate of pseudo mobility-edge and multiple states in non-Hermitian Wannier-Stark lattice

The interaction between non-reciprocity and disorder-free localization has emerged as a fascinating open question. Here, we explore the effects of pseudo mobility edges (MEs) along with different types of eigenstates in a one-dimensional (1D) lattice subjected to a non-reciprocal finite-height Wannier-Stark ladder. Utilizing the transfer matrix method, we analytically investigate the pseudo mobility edges under non-reciprocity, which accurately describe the boundary between ergodic and non-ergodic states. The ergodic states, under nonreciprocity, form topological point gaps in the complex plane, with the corresponding eigenstates localized at the boundaries. The localization of mixed states induced by the skin effect and Wannier-Stark ladder is further amplified under non-reciprocity. Through similarity transformations, the fate of multiple eigenstates under non-reciprocal transitions can be captured. Finally, we use wave packet dynamics as a means to detect these emerging states. Our findings broaden the understanding of disorder-free localization in non-Hermitian systems.

cond-mat.dis-nn↗

arXiv2024

Description of two-dimensional altermagnetism: Categorization using spin group theory

Altermagnetism, recently spotlighted in condensed matter physics, presents captivating physical properties and holds promise for spintronics applications. This study delves into the theoretical description and categorization of two-dimensional altermagnetism using spin group theory. Employing spin-group formalism, we establish seven distinct spin layer groups, extending beyond the conventional five spin Laue groups, to describe two-dimensional altermagnetism. Utilizing these findings, we classify previously reported two-dimensional altermagnets and identify novel materials exhibiting altermagnetism. Specifically, monolayer MnTeMoO$_6$ and VP$_2$H$_8$(NO$_4$)$_2$ are predicted to be two-dimensional altermagnets. Furthermore, we scrutinize their spin-momentum locking characteristics through symmetry analysis and density functional theory calculations, substantiating their altermagnetic properties.

cond-mat.mtrl-sci↗

arXiv2024

Bilayer stacking A-type altermagnet: A general approach to generating two-dimensional altermagnetism

In this article, we propose a new concept of bilayer stacking A-type altermagnet (BSAA), in which two identical ferromagnetic monolayers are stacked with antiferromagnetic coupling to form a two-dimensional A-type altermagnet. By solving the stacking model, we derive all BSAAs for all layer groups and draw three key conclusions: (1) Only 17 layer groups can realize intrinsic A-type altermagnetism. All 2D A-type altermagnets must belong to these 17 layer groups, which will be helpful to search for 2D A-type altermagnet. (2) It is impossible to connect the two sublattices of BSAA using $S_{3z}$ or $S_{6z}$, a constraint that is also applicable to all 2D altermagnets. (3) $C_{2\alpha}$ is a general stacking operation to generate BSAA for an arbitrary monolayer. Our theory not only can explain the previously reported twisted-bilayer altermagnets, but also can provide more possibilities to generate A-type altermagnets. Our research has significantly broadened the range of candidate materials for 2D altermagnets. Based on conclusion (1), the bilayer NiZrCl$_6$ is predicted to exhibit intrinsic A-type altermagnetism. Additionally, we use twisted-bilayer NiCl$_2$, previously reported in the literature, as the second example of BSAA. Furthermore, utilizing symmetry analysis and first-principles calculation, we scrutinize their spin-momentum locking characteristic to substantiate their altermagnetic properties.

cond-mat.mtrl-sci↗

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↗

arXiv2024

Room temperature magnetic phase transition in an electrically-tuned van der Waals ferromagnet

Finding tunable van der Waals (vdW) ferromagnets that operate at above room temperature is an important research focus in physics and materials science. Most vdW magnets are only intrinsically magnetic far below room temperature and magnetism with square-shaped hysteresis at room-temperature has yet to be observed. Here, we report magnetism in a quasi-2D magnet Cr1.2Te2 observed at room temperature (290 K). This magnetism was tuned via a protonic gate with an electron doping concentration up to 3.8 * 10^21 cm^-3. We observed non-monotonic evolutions in both coercivity and anomalous Hall resistivity. Under increased electron doping, the coercivities and anomalous Hall effects (AHEs) vanished, indicating a doping-induced magnetic phase transition. This occurred up to room temperature. DFT calculations showed the formation of an antiferromagnetic (AFM) phase caused by the intercalation of protons which induced significant electron doping in the Cr1.2Te2. The tunability of the magnetic properties and phase in room temperature magnetic vdW Cr1.2Te2 is a significant step towards practical spintronic devices.

cond-mat.mtrl-sci↗