arXiv subjects

Xiangang Wan

Publications and source records attributed to Xiangang Wan.

At least 19 recordsLinked to original sources

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 {\mu}C/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 $\beta -$Cu$_{2}$Se

Having been extensively studied during last decades in the fields of thermoelectics and ionic conductors, the $\alpha $ 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 $\beta $ 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 $\beta -$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

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↗

arXiv2026

Prediction of Magnetic Topological Materials Combining Spin and Magnetic Space Groups

The scarcity of predicted magnetic topological materials (MTMs) by magnetic space group (MSG) hinders further exploration towards realistic device applications. Here, we propose a new scheme combining spin space groups (SSGs)--approximate symmetry groups neglecting spin-orbit coupling (SOC)--and MSGs to diagnose topology in collinear magnetic materials based on symmetry-indicator theory, enabling a systematic classification of the electronic topology across 484 experimentally synthesized collinear magnets from the MAGNDATA database. This new scheme exploits a symmetry-hierarchy due to SOC induced symmetry-breaking, so that nontrivial band topology can be revealed by SSG, that is yet invisible by the conventional MSG-based method, as exemplified by real triple points in ferromagnetic CaCu$_3$Fe$_2$Sb$_2$O$_{12}$, Dirac nodal lines at generic $k$-points in antiferromagnetic FePSe$_3$ and Weyl nodal lines in altermagnetic Sr$_4$Fe$_4$O$_{11}$. Notably, FePSe$_3$ is topologically trivial under MSG but hosts Dirac nodal lines within the SSG framework. Upon including SOC, these nodal lines are gapped and generate a sizable anomalous Hall conductivity. Despite a vanishing bulk net magnetism, FePSe$_3$ can host topologically protected surface states with large non-relativistic band spin-splitting. Moreover, topology in MTMs is tunable by rotating the magnetic moment direction once SOC is included, as exemplified in Sr$_4$Fe$_4$O$_{11}$.The interplay of topology with non-relativistic and SOC-induced control of properties via magnetic moment reorientation in the predicted MTMs is worthy of further studies in future.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic Structures Database from Symmetry-aided High-Throughput Calculations

Magnetic structures, which play a central role in determining their physical properties, are known for only very limited compounds. Traditional theoretical approaches to predicting magnetic structures predominantly rely on first-principles calculations. A key challenge of these methods is their requirement for initial magnetic configurations as inputs, which theoretically possess infinite possibilities. In this work, we introduce a strategy based on irreducible representation basis vectors that effectively narrows down the vast space of potential magnetic configurations to a finite set, typically comprising around 20 candidates per material. Despite this significant reduction, the compact input sets generated by our method already encompass the experimental magnetic structures for 253 out of 302 benchmark materials (83.8%) from the MAGNDATA database. These materials have propagation vectors q=0 and unit cells containing up to 40 atoms, all within the Landau framework. Subsequent first-principles calculations correctly identify the magnetic structure in 198 of these cases. We further apply our highly efficient method to 8,422 stoichiometric transition-metal compounds with fewer than 30 atoms per unit cell in the Inorganic Crystal Structure Database, and establish a magnetic structure database containing 2,906 magnetic materials. To demonstrate its utility, we use this database for the systematic exploration of magnetic topological phases and altermagnets, identifying 1,070 and 392 candidate materials, respectively.

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

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↗

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 $500\,\Omega^{-1}{\rm cm}^{-1}$. 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

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

Giant Anomalous Hall Effect in Kagome Nodal Surface Semimetal Fe$_3$Ge

It is well known that the intrinsic anomalous Hall effect (AHE) arises from the integration of the non-zero Berry curvature (BC), conventionally observed in the Dirac/Weyl and nodal-line semimetals. Moreover, nodal surface semimetals are expected to exhibit more significant BC under the prevalence of degenerate points near the Fermi level. In this work, we report the detection of a giant AHE in the Kagome magnet Fe$_3$Ge with a two-dimensional (2D) nodal surface (NS) at $k_{z}=\pi$ plane, exhibiting an anomalous Hall conductivity (AHC) of 1500 $\Omega^{-1}$cm$^{-1}$ at 160 K, the highest among all reported Kagome topological materials. This finding suggests a new platform for searching large AHC materials and facilitates potential room-temperature applications in spintronic devices and quantum computing.

cond-mat.mes-hall↗

arXiv2024

Realization of Hopf-link structure in phonon spectra: Symmetry guidance and High-throughput investigation

The realization of Hopf-link structure in the Brillouin zone is rather rare hindering the comprehensive exploration and understanding of such exotic nodal loop geometry. Here we first tabulate 141 space groups hosting Hopf-link structure and then investigate Phonon Database at Kyoto University consisting of 10034 materials to search for phonon realization of the Hopf-link nodal structure. It is found that almost all the investigated materials own nodal loops or nodal chains while only 113 materials can host Hopf-link structure in phonon spectra, among which 8 representative materials are manually selected to showcase relatively clean Hopf-link structure including LiGaS$_2$, LiInSe$_2$, Ca$_2$Al$_2$Si(HO$_4$)$_2$, Ca$_7$GeN$_6$, Al(HO)$_3$, NaNd(GaS$_2$)$_4$, Ga$_5$(PS)$_3$ and RbTh$_3$F$_{13}$. The visible phonon drumhead surface states corresponding to the nodal loops in the Hopf-link structure are further demonstrated using Ga$_5$(PS)$_3$ as an example.The listed 113 crystalline materials provide a good platform for experimentalists to further explore the interesting properties related to Hopf-link structure.

cond-mat.mtrl-sci↗

arXiv2024

Quasiparticle interference in altermagnets

A novel collinear magnetic phase, termed ``altermagnetism,'' has recently been uncovered, characterized by zero net magnetization and momentum-dependent collinear spin-splitting. To understand the intriguing physical effects of altermagnets and explore their potential applications, it is crucial to analyze both the geometric and spin configurations of altermagnetic Fermi surfaces. Here, we conduct a comprehensive study of the quasiparticle interference (QPI) effects induced by both nonmagnetic and magnetic impurities in metallic altermagnets, incorporating the influence of Zeeman splitting and spin-orbit coupling. By examining the QPI patterns for various spin polarizations of magnetic impurities and different spin-probe channels, we identify a series of distinctive signatures that can be used to characterize altermagnetic Fermi surfaces. These predicted signatures can be directly compared with experimental results obtained through spin-resolved scanning tunneling spectroscopy.

cond-mat.str-el↗

arXiv2024

ToMSGKpoint: A user-friendly package for computing symmetry transformation properties of electronic eigenstates of nonmagnetic and magnetic crystalline materials

The calculation of (co)irreducible representations of energy bands at high-symmetry points (HSPs) is essential for high-throughput research on topological materials based on symmetry-indicators or topological quantum chemistry. However, existing computational packages usually require transforming crystal structures into specific conventions, thus hindering extensive application, especially to materials whose symmetries are yet to be identified. To address this issue, we developed a Mathematica package, \texttt{ToMSGKpoint}, capable of determining the little groups and (co)irreducible representations of little groups of HSPs, high-symmetry lines (HSLs), and high-symmetry planes (HSPLs) for any nonmagnetic and magnetic crystalline materials in two and three dimensions, with or without considering spin-orbit coupling. To the best of our knowledge, this is the first package to achieve such functionality. The package also provides magnetic space group operations, supports the analysis of (co)irreducible representations of energy bands at HSPs, HSLs, and HSPLs using electronic wavefunctions obtained from \textit{ab initio} calculations interfaced with VASP. Designed for user convenience, the package generates results in a few simple steps and presents all relevant information in clear tabular format. Its versatility is demonstrated through applications to nonmagnetic topological insulator Bi$_2$Se$_3$ and Dirac semimetal Na$_3$Bi, as well as the antiferromagnetic topological material MnBi$_2$Te$_4$. Suitable for any crystal structure, this package can be conveniently applied in a streamlined study once magnetic space group varies with various symmetry-breakings caused by phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

Catalog of phonon emergent particles

The outcome of conventional topological materials prediction scheme could sensitively depend on first-principles calculations parameters. Symmetry, as a powerful tool, has been exploited to enhance the reliability of predictions. Here, we establish the relationship between the Wyckoff positions (WYPOs) and the phonon wavefunctions at each high-symmetry point (HSP) in all 230 space groups (SGs). Based on this, on one hand, we obtain a complete mapping from WYPO to the occurrence of emergent particles (EMPs) at each HSP in 230 SGs, and establish several rules of enforcing EMPs for phonons; on the other hand, we determine the contribution of the WYPO to the phonon angular momentum. Then we unambiguously identify 20,516,167 phonon EMPs in 111,872 materials in two databases. The purely symmetry-determined wavefunctions generalize the conventional Bloch theorem, could find a wide scope of application to physical properties related with basis functions of irreducible representations.

cond-mat.mtrl-sci↗