arXiv subjects

Xiangang Wan

Publications and source records attributed to Xiangang Wan.

At least 19 recordsLinked to original sources

arXiv2026

Generalized Spin Groups and Applications to Magnons

Spin groups provide the natural symmetry framework for the isotropic Heisenberg model, in which spin and lattice degrees of freedom are separable. They have recently found broad applications in describing electronic band structure, magnon band topology, and transport phenomena. However, a typical spin Hamiltonian describing magnons contains not only isotropic Heisenberg exchange but also anisotropic interactions induced by spin-orbit coupling (SOC), including the Dzyaloshinskii-Moriya (DM) interaction, the Gamma exchange, and the single-ion anisotropy (SIA), which strongly influence magnon properties. More recently, some studies have also revealed relativistic magnon transport phenomena in altermagnets. Such systems typically exhibit partial spin-lattice locking, but are commonly assumed to be described by conventional magnetic groups. Here, we develop a systematic symmetry framework, generalized spin groups, for magnons. We first classify the symmetries of Heisenberg, DM, Gamma, and SIA interactions. The resulting classification contains 6 spin-only groups, together with a total of 161 nontrivial spin groups. The symmetries of magnetically ordered phases are then obtained through spontaneous symmetry breaking of the corresponding parent groups for spin Hamiltonians. We also identify 6 spin-only groups for ordered states, which generate 90, 264, 157, 264, 387, and 598 nontrivial spin groups, respectively. Finally, we apply generalized spin groups to the magnon thermal conductivity and spin conductivity tensors and derive all the symmetry-allowed forms. Our results provide a unified symmetry framework for analyzing magnon transport both with and without SOC, yielding more refined constraints on magnon responses.

cond-mat.other↗

arXiv2026

Heat-current-induced nonreciprocal phonon damping

Time-reversal symmetry enforces equal equilibrium linewidths for time-reversed phonon modes at opposite wave vectors. Here we show that a steady heat current can produce nonreciprocal anharmonic damping even in centrosymmetric nonmagnetic crystals. The effect arises from nonequilibrium occupations of scattering partners, which change the statistical weights of time-reversed three-phonon channels while their squared matrix elements remain equal. Within the single-mode relaxation-time approximation, we derive a linewidth correction that is linear in the temperature gradient and odd under wave-vector reversal. The response depends on scattering-partner velocities and equilibrium lifetimes and retains signed channel weights absent from the equilibrium linewidth. First-principles calculations for centrosymmetric MgO and diamond yield nonzero, mode-dependent linewidth corrections at selected finite wave vectors, demonstrating the mechanism in both ionic and covalent crystals. Reversing the heat current at fixed local temperature exchanges the broadening and narrowing of paired modes. This current-reversible linewidth contrast provides a spectral signature of occupation feedback, with potential implications for nonlinear phonon transport.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic Structures Database from Symmetry-aided High-Throughput Calculations

Magnetic structures-the spatial arrangement of magnetic moments-underpin the diverse functionalities of magnetic materials, yet they have been experimentally determined for only a small fraction of known compounds. Conventional first-principles prediction requires candidate magnetic configurations as input, but the number of possible configurations is in principle infinite. Although the classic Landau-theory based symmetry analysis is widely used to classify magnetic ordering, their direct application to generate input magnetic configurations remains a major challenge due to unavoidable residual degrees of freedom. Here we turn this classification language into a practical predictive framework by resolving these freedoms through a two-step symmetry-aided strategy: an irrep-compatible maximal-magnetic-subgroup selection, followed by the physically motivated high-symmetry-direction construction. This highly efficient scheme is justified within the newly developed oriented spin space group framework. Benchmarked against 1,043 Landau-compatible experimental structures in MAGNDATA, our candidate sets reproduce 90% of the experimentally reported magnetic structures in the four high-symmetry crystal systems with only ~51 candidates per material on average, and subsequent first-principles calculations with material-specific Hubbard U parameters recover the experimental ground state with 83% accuracy. Applying the validated workflow to 19,808 transition-metal compounds, we construct a database of more than 5,000 magnetic materials-nearly twice the size of the experimental MAGNDATA. The database reveals statistical regularities accessible only at scale, including the occurrence rate of magnetism, the prevalence of competing magnetic states, and enables the identification of hundreds of magnetic topological materials, altermagnets, and other emerging magnetic phases and functionalities.

cond-mat.mtrl-sci↗

arXiv2026

Finite-Momentum Antiferromagnetic Magnon Dynamics across the Morin Transition in Hematite

Hematite ($α$-Fe$_2$O$_3$) is a prototypical antiferromagnetic platform for high-speed spintronics and magnonics, yet its finite-momentum magnon dynamics and spin-reorientation mechanism remain incompletely understood. Here we use low-wavenumber magneto-Raman spectroscopy to resolve both $k=0$ and finite-$k$ sub-terahertz magnons in $α$-Fe$_2$O$_3$ across the Morin transition. We find that the limiting group velocity $v_0$ remains nearly field independent, whereas the finite-$k$ group velocity $v_{\mathrm g}$ is strongly modified near the Morin and spin-flop transitions. By combining parallel- and transverse-field measurements with spin-wave modeling, we further extract the temperature-dependent Dzyaloshinskii--Moriya field $H_{\mathrm D}$ and uniaxial anisotropy fields $H_{\mathrm{K1}}$ and $H_{\mathrm{K2}}$. We find that $H_{\mathrm D}$ is weakly temperature dependent at approximately 2.0--2.3~T, while $H_{\mathrm{K1}}$ decreases rapidly and $H_{\mathrm{K2}}$ changes only weakly. This contrast drives the sign reversal of the effective anisotropy field and quantitatively accounts for the Morin transition. Our results establish low-wavenumber Raman spectroscopy as a quantitative probe of finite-momentum antiferromagnetic magnons and provide key material parameters for hematite-based magnonics.

cond-mat.mtrl-sci↗

arXiv2026

Prediction of Magnetic Topological Materials Combining Spin and Magnetic Space Groups

Recent developments in spin space groups (SSGs) have led to a new classification of magnetic order and, with it, the emergence of altermagnetism. However, an efficient and routine approach that predicts electronic band topology in magnetic materials by SSGs, and especially the evolution of band topology from negligible spinorbit coupling (SOC) to finite SOC with the symmetry lowering from SSGs to magnetic space groups (MSGs) determined by magnetic-moment directions, has not yet been established. Here, we propose a scheme combining SSG and its MSG subgroups to diagnose band topology in collinear magnets using symmetry indicators of electronic band topology, established for all the 1,421 SSGs and 3,420 MSG subgroups. Specifically, the compatibility relations from SSG to the MSG subgroups can enable a complete topological classification for all possible magnetic-moment directions from the first-principles calculated numbers of occurrences of irreducible (co-)representations at high-symmetry points without SOC with respect to SSG. This scheme can be directly applied to collinear magnets whose magnetic structures have already been determined experimentally or theoretically. Applying it to 488 collinear magnets from MAGNDATA with experimentally determined magnetic structures, we identify 26 materials that are trivial by MSG but are expected to inherit topology protected by SSG symmetry. We showcase Dirac nodal lines and boundary states predicted by SSG but invisible to MSG in FePSe$_3$. Our work demonstrates the predictive power of the approach combining SSG and MSG over the approaches based on either group alone. The results of high-throughput calculations are expected to extend the magnetic topological materials pool, facilitating future experimental realizations of more magnetic topological materials.

cond-mat.mtrl-sci↗

arXiv2026

Catalogue of Collinear Magnetic Structures in Stoichiometric Materials

Magnetic materials persistently serve as a fertile platform for exploring novel quantum emergent phenom ena and advancing information technologies. Reliably determining their magnetic structures (MSs), which is essential for understanding their emergent quantum properties, usually depends on experiments such as neutron scattering. Here, noting that around half of the MSs in MAGNDATA (a database of experimentally determined MSs) are collinear, we adopt the spin space groups (SSGs) to preset the candidate MSs for a given nonmagnetic crystal structure to be collinear from the outset. On the other hand, more than 80% of the collinear magnets in MAGNDATA are subject to maximal subgroups of nonmagnetic groups which are collinear SSGs, and we then enumerate all collinear MSs formed at Wyckoff positions by the maximal collinear SSG subgroups of the 230 nonmagnetic space groups. With such exhaustive enumeration, we categorize all symmetry-allowed collinear MSs for the 72,075 nonmagnetic stoichiometric structures in Inorganic Crystal Structure Database, obtaining 365,639 collinear MSs. We then determine the ground-state SSG symmetries of 7,824 collinear magnets among the 21,315 selected materials using first-principles calculations. These magnets then undergo a comprehensive topological classification combining SSGs and magnetic space groups, outputting thousands of new magnetic topological materials. The experimentally determined MSs in MAGNDATA represent the tip of the iceberg in our computational database of collinear magnets, expected to guide MS characterization by experimentalists and further investigation towards spintronics applications.

cond-mat.mtrl-sci↗

arXiv2026

Defect Geometry Selects Polar and Anomalous Hall Phases in Two-Dimensional Altermagnets

Point defects in altermagnets can create phases absent in the pristine host by selectively breaking crystal symmetries. Combining symmetry analysis, first-principles calculations, and Hamiltonian modeling, we identify how point impurities modify the altermagnetic phase. Using the pristine d- wave altermagnetic monolayer V2Se2O as a testbed, we identify three distinct classes of impurities: those that preserve spin-momentum locking, those that induce a hybrid-parity state associated with Edelstein spin conversion, and those that produce a metallic ferrimagnetic state with an anomalous Hall effect. We further discuss the robustness of two-dimensional altermagnets against point impurities. Results for other two-dimensional systems, such as Mn4N2 and 2H-FeBr3, reveal the same symmetry-based control across distinct lattices and parent spin harmonics, establishing defect geometry as a general route for engineering spin textures and transport properties.

cond-mat.mtrl-sci↗

arXiv2026

Chiral-Structured Superconductors TrX4 (Tr = Rh, Ir; X = Ge, Si): A Platform for Mixed-Parity Pairing and Topological States

Chiral-structured superconductors, with simultaneous broken mirror and inversion symmetries, promote unconventional superconductivity through parity-mixing mechanisms. Yet a few bulk chiral-structured superconductors are known, partly due to the difficulty in directly determining their atomic-scale chirality. Here we report three chiral-structured superconductors, , RhGe4, IrGe4, and IrSi4, synthesized under high pressure, with Tc values of about 1.6 K, 1.1 K, and 2.5 K, respectively.Using atomic resolution Cs-corrected scanning transmission electron microscopy (STEM) combined with X-ray diffraction characterizations, we directly confirm their chiral structure (space group P3121). This real space imaging approach overcomes ambiguities in traditional diffraction based methods. These materials exhibit type-II superconductivity, and the enhancement of spin-orbit coupling (SOC) leads to the emergence of mixed parity pairing. Calculations also reveal symmetry protected Weyl points near the Fermi level, which is robust against the SOC. Our work not only expands the family of chiral-structured superconductors but also demonstrates the indispensable role of STEM in directly determining chiral crystal structures. These materials thus offer a clean platform to explore the interplay among structural chirality, SOC, mixed parity superconductivity, and topological quantum phenomena.

cond-mat.supr-con↗

arXiv2026

Colossal Type-II Multiferroic Polarization Driven by Collinear Spin Orders

Achieving strong magnetoelectric coupling (MEC) together with large ferroelectric polarization remains a central challenge in type-II multiferroics. In conventional spin-driven multiferroics, the induced polarization is usually mediated by spin-orbit coupling (SOC) or spin-lattice coupling (SLC). Since many representative systems are based on 3d transition-metal ions, where SOC is relatively weak and SLC-induced lattice distortions are often limited, their polarizations are typically much smaller than those of proper ferroelectrics. Moreover, electric polarizations in type-II multiferroics are generally induced by spiral spin orders stabilized by competing magnetic interactions, which often leads to relatively low magnetic transition temperatures. In this Letter, using spin-group symmetry, we propose an SOC- and SLC-independent route to MEC in collinear 3d magnetic systems. We show that, even for a noncentrosymmetric lattice structure, different collinear magnetic configurations can either forbid or allow electric polarization, indicating direct magnetic control of polarization and hence strong MEC. The first-principles calculations excluding SOC on monolayer 2H-VS2 support this picture: a collinear stripy antiferromagnetic order induces an in-plane ferroelectric polarization up to 25.00 μC/cm2, about two orders of magnitude larger than that of typical type-II multiferroics. Furthermore, our microscopic model suggests that the induced polarization originates from SOC-independent p-d hybridization governed by electronic hopping. Our results suggest a possible route toward type-II multiferroics combining strong MEC with large electronic polarization in collinear 3d magnetic systems.

cond-mat.mtrl-sci↗

arXiv2026

Dirac Semimetal Phase in Rhombohedral $β-$Cu$_{2}$Se

Having been extensively studied during last decades in the fields of thermoelectics and ionic conductors, the $α$ phase of Cu$_{2}$Se with antfluoride crystal structure has recently emerged as a topological zero-gap semimetal with a quadratic contact point which exists at the Fermi surface of its bulk electronic spectrum. Here we argue based on density functional electronic structure calculation that the $β$ phase of Cu$_{2}$Se realized in a recently discovered rhombohedral structure shows a Dirac semimetal behavior of the electrons near the Fermi level. These topological semimetals are currently generating a lot of interest due to unusual transport phenomena, such as strong quantum oscillations, large magnetoresistance effect and ultrahigh carrier mobilities with their Fermi velocities potentially exceeding graphene. We show that there exist Fermi arc states at the surface spectrum of $β-$Cu$_{2}$Se that are topologically protected by the bulk Dirac points. Their shape and spin properties should be resilient to the back- and side scattering effects in the surface transport, suggesting new ways for realizing high-mobility electronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Altermagnetic Spin Precession and Spin Transistor

Altermagnets hold great potential for spintronic applications, yet their intrinsic spin dynamics and associated transport properties remain largely unexplored. Here, we investigate spin-resolved quantum transport in a multi-terminal setup based on a $d$-wave altermagnet. It is found that the altermagnetic spin splitting in momentum space induces an interesting spin precession in two-dimensional real space, giving rise to characteristic spin patterns. This altermagnetic spin precession manifests as a spatial modulation of the transverse Hall-like voltage, whose oscillation period provides a direct measure of the spin-splitting strength. When the altermagnetism is electrically tunable, the proposed setup functions as a prototype for a highly efficient spin transistor. The key physical effects are shown to be robust against dephasing and crystalline warping. Our work not only identifies a fingerprint signature of altermagnets, offering a direct probe of the altermagnetic spin splitting, but also represents an important step toward bridging their fundamental physics with practical spintronic applications.

cond-mat.mes-hall↗

arXiv2026

Catalog of phonon emergent particles and chiral phonons: Symmetry-based classification and materials database investigation

Chirality and topology are fundamental and ubiquitous in nature. Symmetry has proven to be a powerful tool for predicting topological phonons. However, to date, topological phonon emergent particles (EMPs) have not been systematically cataloged in material databases. Moreover, traditional symmetry methods are often inadequate for predicting chiral phonons, because realistic calculations can yield negative results even when symmetry analysis permits phonon chirality. Here, we first establish a complete symmetry-based classification: given any space group and Wyckoff positions (WYPOs) occupied by atoms, the number of occurrences of all (co-)irreducible representations ((co-)irreps) (that can host EMPs) can be unambiguously known without omission prior to expensive and parameter-dependent calculation. Moreover, whether a phonon mode (belonging to one (co-)irrep) is chiral can also be determined from the occupied WYPOs. We then perform a materials database investigation identifying over 25 million EMPs at high-symmetry points and along high-symmetry lines and computing the concrete value of phonon angular momentum for each mode. We demonstrate two main applications: identifying ideal materials with surface chirality momentum locking and identifying materials with giant phonon magnetic moment. All computational data are compiled into a website: http://phonon.nju.edu.cn, which is expected to stimulate future studies on topological and chiral phonons.

cond-mat.mtrl-sci↗

arXiv2025

Lattice-decoupled rotatable stripe-like charge order within the strange metal phase of 2M-WS2

In quantum materials, charge orders typically stabilize in specific crystallographic orientations, though their formation mechanisms may vary. Here, using low-temperature scanning tunneling microscopy (STM), we discover a lattice-decoupled rotatable stripe-like charge order coexisting with superconductivity in 2M-WS2. The charge order manifests five distinct orientations across different sample regions, yet maintains an identical wavelength. This directional decoupling from host lattice challenges existing paradigms. First-principles calculations of phonon spectra and nesting function fail to explain the ordering mechanism. Intriguingly, the transition temperature of the charge orders exhibits spatial variations (21-46 K), coinciding with the temperature range of the recently reported strange metal phase in this material. This correlation suggests that the interplay between strong electronic correlations and electron-phonon coupling must be critically evaluated to elucidate the emergence of this unconventional charge order.

cond-mat.supr-con↗

arXiv2025

Revealing Spin and Spatial Symmetry Decoupling: New Insights into Magnetic Systems with Dzyaloshinskii-Moriya Interaction

It is widely accepted that spin-orbit coupling (SOC) generally locks spin and spatial degrees of freedom, as a result, the spin, despite being an axial vector, is fixed and cannot rotate independently, and the magnetic system should be described by magnetic space groups (MSGs). While as a new type of group, spin space groups (SSGs) have been introduced to approximately describe the symmetry of magnetic systems with negligible SOC, and received significant attention recently. In this work, we prove that in two cases of coplanar spin configurations, there are spin-only operations that strictly hold even with considerable Dzyaloshinskii-Moriya interaction (DMI), and the symmetry of their spin models could be described by the spin-coplanar SSG. In addition, we also find that for spin-collinear cases, regardless the strength of DMI, the magnon systems within the framework of linear spin wave theory (LSWT) also preserve the decoupled spin and spatial rotations, but the symmetry does not belong to the conventional definitions of collinear spin groups. We discuss the potential realization of these novel symmetries in rod, layer, and three-dimensional (3D) space groups. Our work extends the applicability of SSGs to magnetic materials with heavy elements, and reveals that the coexistence of DMI and SSG symmetries provides new opportunity for exploring novel magnon transport phenomena, and potential material realization had also been discussed.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry Analysis of Magnetoelectric Coupling Effect in All Point Groups

Symmetry analysis provides crucial insights into the magnetoelectric coupling effect in type-II multiferroics. In this Letter, we comprehensively investigate couplings between electric polarization and inhomogeneous magnetization across all 32 crystallographic point groups using a phenomenological Landau theory. Our theory successfully explains the ferroelectric polarizations in all known type-II multiferroics characterized by incommensurate magnetic orders. In addition, we predict 12 promising type-II multiferroic candidates with the highest magnetic transition temperature of 84 K through systematic screening of MAGNDATA database. Furthermore, we find that the collinear spin-sinusoidal texture emerges as a previously unrecognized source of ferroelectric polarization. We also demonstrate that topological ferroelectric vortex states can be induced by ferromagnetic vortex configurations in uniaxial point groups, opening a route to realizing coexisting multiple-vortex states in multiferroics.

cond-mat.mtrl-sci↗

arXiv2025

Generation of Pure Spin Current with Insulating Antiferromagnetic Materials

The generation of pure spin currents is critical for low-dissipation spintronic applications, yet existing methods relying on spin-orbit coupling or ferromagnetic interfaces face challenges in material compatibility and operational robustness. We propose a paradigm-shifting approach to generate symmetry-protected pure spin currents by applying mechanical stress on insulating antiferromagnetic materials, i.e., the pure piezospintronic effect. We first classify magnetic point groups enabling pure piezospintronic effects. A novel first-principles method is developed to compute the spin dipole moments and coefficients of the piezospintronic effect. Integrating these methodologies with high-throughput screening, we identify FeOOH, Cr2O3 and NaMnX (X=As, Bi, P, Sb) with significant pure piezospintronic effects. Interestingly, we reveal that the ionic displacement contribution dominates the piezospintronic effect, in contrast to the piezoelectric effect. Our study not only provides first-principles approach for investigating spin dipole moment related phenomena (e.g., ferrotoroidicity, fractional quantum spin dipole moment, piezospintronics), but also provide promising piezospintronic materials for experimental verification and industrial applications.

cond-mat.mtrl-sci↗

arXiv2025

Large Berry curvature effects induced by extended nodal structures: Rational design strategy and high-throughput materials predictions

Berry curvature can drastically modify the electron dynamics, thereby offering an effective pathway for electron manipulation and novel device applications. Compared to zero-dimensional nodal points in Weyl/Dirac semimetals, higher-dimensional extended nodal structures, such as nodal lines and nodal surfaces, are more likely to intersect the Fermi surface, leading to large Berry curvature effects without fine-tuning the chemical potential. In this work, we propose a strategy that utilizes straight nodal lines (SNLs) and flat nodal surfaces (FNSs) to design large Berry curvature effects, and we exhaustively tabulate SNLs and FNSs within the 1651 magnetic space groups (MSGs). We demonstrate that SNLs and FNSs can generate large Berry curvature widely distributed in the Brillouin zone. As an application, we identify 158 MSGs that host FNSs, SNLs, or both and allow for nonvanishing anomalous Hall conductivity (AHC). Based on these 158 MSGs, we screen materials from the MAGNDATA magnetic material database for high-throughput calculations, identifying 60 materials with AHC values exceeding $500\,Ω^{-1}{\rm cm}^{-1}$. We select the candidate materials $\rm SrRuO_3$ and $\rm Ca_2NiOsO_6$ to demonstrate the contributions of FNSs and SNLs to one and two nonvanishing AHC components, respectively. We also investigate the tuning of AHC through symmetry breaking, outlining all possible symmetry-breaking pathways, and select the candidate material HoNi to demonstrate this approach by applying an external magnetic field. Additionally, we identify Berry curvature quadrupoles in the candidate materials, indicating that our strategy can be generalized to Berry curvature multipole effects. Our work will guide both the theoretical and experimental design of materials with large Berry curvature effects, with significant implications for a wide range of device applications.

cond-mat.mtrl-sci↗

arXiv2025

Diagnosing Altermagnetic Phases through Quantum Oscillations

The recently delimited altermagnetic phase is characterized by zero net magnetization but momentum-dependent collinear spin-splitting. To explore the intriguing physical effects and potential applications of altermagnets, it is essential to analyze their Fermi surface properties, encompassing both configurations and spin textures. Here, we conduct a Fermiology study on metallic altermagnets and demonstrate that the collinear spin-split features of their Fermi surfaces can be clearly revealed through quantum oscillation measurements. By introducing a transverse Zeeman field to remove the spin-degenerate lines in the momentum space, the Fermi surface undergoes a Lifshitz transition, giving rise to spin-flipped cyclotron motion between orbits with opposite spins. Accordingly, the Lifshitz-Onsager quantization yields two sets of Landau levels, leading to frequency splitting of the Shubnikov-de Haas oscillations in conductivity. In the presence of spin-orbit coupling, the Zeeman field causes two separate cyclotron orbits to merge at the Lifshitz transition point before splitting again. This results in the two original frequencies discontinuously changing into a single frequency equal to their sum. Our work unveils a unique and universal signature of altermagnetic Fermi surfaces that can be probed through quantum oscillation measurements.

cond-mat.str-el↗