arXiv subjects

Su-Huai Wei

Publications and source records attributed to Su-Huai Wei.

At least 19 recordsLinked to original sources

arXiv2026

A Self-Adaptive First-Principles Approach for Magnetic Excited States

The profound impact of excited magnetic states on the intricate interplay between electron and lattice behaviors in magnetic materials is a topic of great interest. Unfortunately, despite the significant strides that have been made in first-principles methods, accurately tracking these phenomena remains a challenging and elusive task. The crux of the challenge that lies before us is centered on the intricate task of characterizing the magnetic configuration of an excited state, utilizing a first-principle approach that is firmly rooted in the ground state of the system. We propose a versatile self-adaptive spin-constrained density functional theory formalism. By iteratively optimizing the constraining field alongside the electron wave function during energy minimization, we are able to obtain an accurate potential energy surface that captures the longitudinal and transverse variations of magnetization in itinerant ferromagnetic Fe. Moreover, this technique allows us to identify the subtle coupling between magnetic moments and other degrees of freedom by tracking energy variation, providing new insights into the intricate interplay between magnetic interactions, electronic band structure, and phonon dispersion curves in single-layered CrI$_3$. This new methodology represents a significant breakthrough in our ability to probe the complex and multifaceted properties of magnetic systems.

cond-mat.mtrl-sci↗

arXiv2026

Simplified criteria for identifying and designing altermagnetic crystals in real space

Altermagnetism is a compensated magnetic phase characterized by zero net magnetization and exchange-driven spin splitting. However, identifying altermagnets among collinear antiferromagnets usually requires full magnetic-space-group or spin-group analysis, which is not always intuitive. Here we formulate a simple real-space criterion based on how the crystallographic operations of the host nonmagnetic structure permute the two opposite-spin sublattices. We show that altermagnets usually exist on collinear compensated antiferromagnets whose magnetic primitive cell coincides with the host nonmagnetic crystallographic primitive cell. In this case, altermagnetic spin splitting is generally allowed unless an inversion-type operation exists that exchanges the two opposite-spin sublattices. Using chemically ordered Mn2SSe prototypes derived from zinc-blende or rocksalt parent structures, we demonstrate that these criteria can be easily used to construct the three symmetry classes by controlling chemical ordering and magnetic-sublattice permutation. Similar rules can also be applied to low-dimensional crystals or quasicrystals. Our work reduces the identification of altermagnets to a transparent real-space symmetry test and provides a practical route for designing altermagnetic crystals.

cond-mat.mtrl-sci↗

arXiv2026

Charge Symmetry Beyond Space-Group Equivalence

Crystallographic space-group symmetry $G_{\rm lat}$, determined by atomic species and their spatial arrangement, is one of the most important descriptors in solid-state physics, underlying the classification of electronic states, spectral degeneracies, order parameters, and phase transitions. Yet the symmetry $G$ of a crystal also depends on the electronic coupling network between atomic sites, including electron hopping, Coulomb interactions, and orbital hybridization. This raises a fundamental question: must $G$ reproduce every equivalence relation imposed by $G_{\rm lat}$? Equivalently, must symmetry-related atoms at the same Wyckoff position be electronically identical, while atoms at inequivalent Wyckoff positions are electronically distinct? We develop a systematic theory of interaction-controlled electronic equivalence, with site charge imbalance as an order parameter whose stability is governed by the competition between onsite charging cost and intersite Coulomb gain. Group-theoretical analysis identifies the site-exchange operations lost from or added to $G_{\rm lat}$. Sites identified as equivalent by $G_{\rm lat}$ can spontaneously develop charge imbalance, lowering the realized symmetry to $G\subset G_{\rm lat}$. Conversely, sites identified as inequivalent by $G_{\rm lat}$ can remain equivalent through a hidden low-energy gauge symmetry. Within the low-energy $(s,p_z)$ manifold, this realizes $G\supset G_{\rm lat}$ and protects near-Fermi degeneracies that appear accidental in a $G_{\rm lat}$-based analysis. First-principles calculations verify both scenarios and establish pressure as a control parameter: it destabilizes the charge-equivalent state in Type I, whereas in Type II it destroys the hidden equivalence, splits the near-Fermi doublets, and can drive a metal-insulator transition.

cond-mat.mtrl-sci↗

arXiv2026

Floquet spin-group framework and its application to light-tailored spin splitting in collinear magnets

Altermagnets combine momentum-dependent spin splitting with vanishing net magnetization, opening new opportunities for spintronics. Recent studies have shown that periodic driving by linearly or circularly polarized light, as well as by multicolor light fields, can control spin splitting in altermagnets and conventional antiferromagnets, and can even generate spin splitting patterns absent in equilibrium. Despite this progress, a unified principle connecting the dynamical symmetry of light to the symmetry of spin-split bands remains lacking, limiting the systematic design of light-induced spin structures. Here, by combining the spin group of crystals with the dynamical group of light, we establish a unified Floquet spin group framework for the study of periodically driven collinear magnets with negligible spin-orbit coupling. This framework systematically determines the allowed parity, momentum dependence, and nodal structure of spin splitting under different driving protocols. Guided by this symmetry classification, we show that odd-parity, even-parity, and mixed-parity spin splittings can be switched within the same material by tailoring the driving field. We further uncover higher-order $h$-wave and $k$-wave spin splittings in three-dimensional systems. Our work establishes a unified symmetry framework for light-controlled spin splitting and provides general principles for engineering nonequilibrium spin-band structures in altermagnets and related magnetic materials.

cond-mat.mes-hall↗

arXiv2026

Bi-S network origin of cation-disorder stability and dispersive band edges in AgBiS2

Cation-disordered AgBiS2 is a promising lead-free optoelectronic material, but both its ordered structure and the microscopic origin of its favorable electronic properties remain debated. Theory has proposed a mixed-coordination tendency with tetrahedral AgS4 and octahedral BiS6 units, whereas experiments mainly report octahedrally coordinated ordered and cation-disordered phases, together with local cation off-centering. Here, we combine a machine-learning interatomic potential with a deep-learning Hamiltonian to resolve the coupled structural and electronic evolution of AgBiS2 at large length scales. We identify the three-dimensional Bi-S network as the central structural motif governing both disorder stability and band-edge electronic states. At weak disorder, Ag/Bi exchange competes with the off-centering tendency of the Ag sublattice, producing strongly distorted local environments and convoluted diffraction signatures that hinder the identification of the ordered phase. With increasing disorder, BiS6-like units connect into a continuous Bi-S network, which stabilizes the rocksalt-like disordered phase. Despite strong cation disorder, AgBiS2 retains clear semiconductor-like band dispersion and develops a direct band gap. The connected Bi:p-S:p states supported by the Bi-S network preserve a dispersive conduction-band edge and a small electron effective mass. In contrast, mobile Ag disrupts the long-range periodicity of Ag-S bonding, leading to strongly localized valence states. These results clarify the structural controversy in ordered AgBiS2 and establish a unified physical picture of disorder stability and optoelectronic response in nonisovalent semiconductor alloys.

cond-mat.mtrl-sci↗

arXiv2026

Floquet odd-parity collinear magnets

Altermagnets (AMs), recently discovered unconventional magnets distinct from both ferro- and antiferromagnets, have rapidly emerged as a prominent research topic in condensed matter physics. AMs are characterized by alternating collinear magnetic moments with zero net magnetization in real space, and spin splittings with even-parity symmetry in momentum space. However, their counterparts exhibiting odd-parity spin splittings are generally thought to be absent in collinear magnets. Here, we show that such unconventional odd-parity magnets can be induced from collinear antiferromagnets by symmetry engineering. Remarkably, using effective model analysis within Floquet-theory framework, we demonstrate that circularly polarized light irradiation of conventional antiferromagnetic lattices breaks a spin-preserving pseudo-time-reversal symmetry and induces both $p$- and $f$-wave magnets, realizing novel magnetic states dubbed Floquet odd-parity collinear magnets. Moreover, we also uncover light-induced antiferromagnetic Chern insulating states in the $f$-wave magnets. The proposed Floquet odd-parity magnet is confirmed by first-principles calculations of MnPSe$_{3}$ under circularly polarized light. Our work not only proposes a new class of unconventional magnets, but also opens an avenue for light-induced magnetic phenomena in spintronic applications.

cond-mat.mes-hall↗

arXiv2025

Revisiting the theory of crystal polarization: The downside of employing the periodic boundary conditions

Periodic boundary condition (PBC) is a standard approximation for calculating crystalline materials properties. However, a PBC crystal is not the same as the real macroscopic crystal, therefore, if applied indiscriminately, it can lead to erroneous conclusions. For example, unlike other extensive observables such as total energy, the polarization of a macroscopic crystal cannot always be described by a PBC model, because polarization is inherently nonlocal and strongly dependent on surface terminations, irrespective of crystal size, and moreover, the symmetry of the macroscopic crystal can be altered when the PBC is applied to a macroscopic crystal. We demonstrate in this paper that the polarization of a macroscopic crystal receives contributions from both the repeating bulk units and the crystal surfaces, which must be treated on an equal footing. When the combined system of the bulk and its surfaces are taken into account, materials traditionally classified as nonpolar can, in fact, admit polar symmetry, thus explaining why experimentalists have observed polarization in some nominally ``nonpolar'' systems. Our study, thus, clarifies that polarization can only exist in polar group systems and that apparent violations of Neumann's principle reported in some recent works originate from misinterpreting bulk PBC crystal as intrinsic macroscopic crystal, ignoring the contribution from the surfaces. We demonstrate that when the full bulk-plus-surface system is considered, the crystal polarization and symmetry is fully consistent with Neumann's principle.

cond-mat.mtrl-sci↗

arXiv2025

Effect of Concentration Fluctuations on Material Properties of Disordered Alloys

Alloying compound AX with another compound BX is widely used to tune material properties. For disordered alloys, due to the lack of periodicity, it has been challenging to calculate and study their material properties. Special quasi-random structure (SQS) method has been developed and widely used to treat this issue by matching averaged atomic correlation functions to those of ideal random alloys, enabling accurate predictions of macroscopic material properties such as total energy and volume. However, in AxB1-x alloys, statistically allowed local concentration fluctuations can give rise to defect-like minority configurations, such as bulk-like AX or BX regions in the extreme, which could strongly affect calculation of some of the material properties such as semiconductor bandgap, if it is not defined properly, leading to significant discrepancies between theory and experiment. In this work, taking the bandgap as an example, we demonstrate that the calculated alloy bandgap can be significantly underestimated in standard SQS calculations when the SQS cell size is increased to improve the structural model and the bandgap is defined conventionally as the energy difference between the lowest unoccupied state and the highest occupied state, because the rare event motifs can lead to wavefunction localization and become the dominant factor in determining the "bandgap", contrary to experiment. To be consistent with experiment, we show that the bandgap of the alloy should be extracted from the majority configurations using a density-of-states fitting (DOSF) method. This DOSF approach resolves the long-standing issue of calculating electronic structure of disordered semiconductor alloys. Similar approaches should also be developed to treat material properties that depends on localized alloy wavefunctions.

cond-mat.mtrl-sci↗

arXiv2025

Emergent Multiferroic Altermagnets and Spin Control via Noncollinear Molecular Polarization

Altermagnets, with spin splitting and vanishing magnetization, have been attributed to many fascinating phenomena and potential applications. In particular, integrating ferroelectricity with altermagnetism to enable magnetoelectric coupling and electric control of spin has drawn significant attention. However, its experimental realization and precise spin manipulation remain elusive. Here, by focusing on molecular ferroelectrics, the first discovered ferroelectrics renowned for their highly controllable molecular polarizations and structural flexibility, we reveal that these obstacles can be removed by an emergent multiferroic altermagnets with tunable spin polarization in a large class of fabricated organic materials. Using a symmetry-based design and a tight-binding model, we uncover the underlying mechanism of such molecular ferroelectric altermagnets and demonstrate how noncollinear molecular polarization can switch the spin polarization on and off and even reverse its sign. From the first-principles calculations, we verify the feasibility of these materials in a series of well-established hybrid organic-inorganic perovskites and metal-organic frameworks. Our findings bridge molecular ferroelectrics and altermagnetic spintronics, highlighting an unexplored potential of multifunctional organic multiferroics.

cond-mat.mtrl-sci↗

arXiv2025

Cooperative Suppression Strategy for Dual Thermal Transport Channels in Crystalline Materials

We propose a novel design principle for achieving ultralow thermal conductivity in crystalline materials via a "heavy-light and soft-stiff" structural motif. By combining heavy and light atomic species with soft and stiff bonding networks, both particle-like ($κ_p$) and wave-like ($κ_c$) phonon transport channels are concurrently suppressed. First-principles calculations show that this architecture induces a hierarchical phonon spectrum: soft-bonded heavy atoms generate dense low-frequency modes that enhance scattering and reduce $κ_p$, while stiff-bonded light atoms produce sparse high-frequency optical branches that disrupt coherence and lower $κ_c$. High-throughput screening identifies Tl$_4$SiS$_4$ ($κ_p$ = 0.10, $κ_c$ = 0.06 W/mK) and Tl$_4$GeS$_4$ ($κ_p$ = 0.09, $κ_c$ = 0.06 W/mK) as representative candidates with strongly suppressed transport in both channels. A minimal 1D triatomic chain model further demonstrates the generality of this mechanism, offering a new paradigm for phonon engineering beyond the conventional $κ_p$-$κ_c$ trade-off.

cond-mat.mtrl-sci↗

arXiv2025

Mechanism of $E'_γ$ Defect Generation in Ionizing-irradiated $a$-SiO$_2$: The Nonradiative Carrier Capture-Structural Relaxation Model

The total ionizing dose (TID) effect of semiconductor devices stems from radiation-induced $E'_γ$ defects in the $a$-SiO$_2$ dielectrics, but the conventional ``hole transport-trapping'' model of defect generation fails to explain recent basic experiments. Here, we propose an essentially new ``nonradiative carrier capture-structural relaxation'' (NCCSR) mechanism that can consistently explain the puzzling temperature/electric-field dependence, based on spin-polarized HSE06 hybrid functional calculations and existing experimental alignment of defect formation energies and charge capture cross-sections of large-sample oxygen vacancies in $a$-SiO$_2$. It is revealed that, the long-assumed $V_{Oγ}$ precursors with high formation energy cannot survive in high temperature-grown $a$-SiO$_2$; whereas the stable $V_{Oδ}$ can capture irradiation-induced holes via strong electron-phonon coupling, generating metastable $E'_δ$ that most relax into stable $E'_γ$. A fractional power-law (FPL) dynamic model is derived based on the mechanism and the Kohlrausch-Williams Watts (KWW) decay function. It can uniformly describe nonlinear data over a wide dose and temperature range. This work not only provides a solid cornerstone for prediction and hardening of TID effects of SiO$_2$-based semiconductor devices, but also offers a general approach for studying ionizing radiation physics in alternative dielectrics with intrinsic electronic metastability and dispersion.

cond-mat.mtrl-sci↗

arXiv2025

Bipolar doping in van der Waals semiconductor through Flexo-doping

Doping plays a key role in functionalizing semiconductor devices, yet traditional chemical approaches relying on foreign-atom incorporation suffer from doping-asymmetry, pronounced lattice disorder and constrained spatial resolution. Here, we demonstrate a physical doping technique to directly write nanoscale doping patterns into layered semiconductors (MoS2). By applying localized tensile and compressive stress via an atomic force microscopy probe, p and n type conductance are simultaneously written into the designed area with sub-100-nm resolution, as verified by spatially resolved capacitance and photocurrent experiments. Density functional theory calculations reveal strain-driven shifts of donor and acceptor levels, as large as several hundreds of meV, linking mechanical stress to semiconductor doping. Fabricated strain-engineered junction efficiently rectifies the current flow and performs logic operations with stable dynamic response. This strain-driven approach enables spatially precise doping in van der Waals materials without degrading crystallinity, offering a versatile platform for nanoscale semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2025

Carrier Emission and Capture Competition mediated A(n)BC Recombination Model in Semiconductors with Multi-Level Defects

The ABC model has been widely used to describe the carrier recombination rate, in which the rate of non-radiative recombination assisted by deep-level defects is assumed to depend linearly on excess carrier density $Δn$, leading to a constant recombination coefficient A. However, for multi-level defects that are prevalent in semiconductors, we demonstrate here that the rate should depend nonlinearly on $Δn$. When $Δn$ varies, the carrier capture and emission of defects can change the defect density distribution in different charge states, which can further change the carrier capture and emission rates of the defects and thus make the recombination rate depend non-linearly on $Δn$, leading to an $A(n)$ function. However, in many recent calculation studies on carrier recombination rate of multi-level defects, only carrier capture was considered while carrier emission from defect levels was neglected, causing incorrect charge-state distribution and misleading linear dependence of the rate on $Δn$. For $\text{V}_{\text{Ga}}$-$\text{O}_{\text{N}}$ in GaN and $\text{Pb}_\text{I}$ in CsPbI$_3$, our calculations showed that neglecting the carrier emission can cause the recombination rate underestimation by more than 8 orders of magnitude when $Δn$ is $10^{15}$ cm$^{-3}$. Our findings suggest that the recent studies on carrier recombination assisted by multi-level defects should be revisited with carrier emission considered, and the widely-used $ABC$ model should be reformed into the $A(n)BC$ model.

cond-mat.mtrl-sci↗

arXiv2024

Is p-type doping in TeO2 feasible?

Wide-bandgap two-dimensional (2D) beta-TeO2 has been reported as a high-mobility p-type transparent semiconductor (Nat. Electron. 2021, 4, 277-283), attracting significant attention. This "breakthrough" not only challenges the conventional characterization of TeO2 as an insulator but also conflicts with the anticipated difficulty in hole doping of TeO2 by established chemical trends. Notably, the reported Fermi level of 0.9 eV above the valence band maximum (VBM) actually suggests that the material is an insulator, contradicting the high hole density obtained by Hall effect measurement. Furthermore, the detected residual Se and the possible reduced elemental Te in the 2D beta-TeO2 samples introduces complexity, considering that elemental Se, Te, and Te1-xSex themselves are high-mobility p-type semiconductor. Therefore, doubts regarding the true cause of the p-type conductivity observed in the 2D beta-TeO2 samples arise. In this work, we employ density functional theory calculations to illustrate that TeO2, whether in its bulk forms of alpha-, beta-, or gamma-TeO2, or in 2D beta-TeO2, inherently exhibits insulating properties and poses challenges in carrier doping due to its shallow conduction band minimum and deep valence band maximum (VBM). Our findings shed light on the insulating electrical properties and doping difficulty of TeO2, contrasting with the claimed p-type conductivity in the 2D beta-TeO2 samples, prompting inquiries into the true origin of the p-type conductivity.

cond-mat.mtrl-sci↗

arXiv2023

Theoretical understanding of correlation between magnetic phase transition and the superconducting dome in high-Tc cuprates

Many issues concerning the origin of high-temperature superconductivity (HTS) are still under debate. For example, how the magnetic ordering varies with doping and its relationship with the superconducting temperature; and why the maximal Tc always occurs near the quantum critical point. In this paper, taking hole-doped La2CuO4 as a classical example, we employ the first-principles band structure and total energy calculations and Monte Carlo simulations to explore how the symmetry-breaking magnetic ground state evolves with hole doping and the origin of a dome-shaped superconductivity region in the phase diagram. We demonstrate that the local antiferromagnetic ordering and doping play key roles in determining the electron-phonon coupling, thus Tc. Initially, the La2CuO4 possesses a checkerboard local antiferromagnetic ground state. As the hole doping increases, Tc increases with the increase of the density of states at the Fermi surface. But as the doping increases further, the strength of the antiferromagnetic interaction weakens. At the critical doping level, a magnetic phase transition occurs that reduces the local antiferromagnetism-assisted electron-phonon coupling, thus diminishing the Tc. The superconductivity disappears in the heavily overdoped region when the antiferromagnetic ordering disappears. These observations could account for why cuprates have a dome-shaped superconductivity region in the phase diagram. Our study, thus, contributes to a fundamental understanding of the correlation between doping, local magnetic ordering, and superconductivity of HTS.

cond-mat.supr-con↗

arXiv2023

Graph deep learning accelerated efficient crystal structure search and feature extraction

Structural search and feature extraction are a central subject in modern materials design, the efficiency of which is currently limited, but can be potentially boosted by machine learning (ML). Here, we develop an ML-based prediction-analysis framework, which includes a symmetry-based combinatorial crystal optimization program (SCCOP) and a feature additive attribution model, to significantly reduce computational costs and to extract property-related structural features. Our method is highly accurate and predictive, and extracts structural features from desired structures to guide materials design. As a case study, we apply our new approach to a two-dimensional B-C-N system, which identifies 28 previously undiscovered stable structures out of 82 compositions; our analysis further establishes the structural features that contribute most to energy and bandgap. Compared to conventional approaches, SCCOP is about 10 times faster while maintaining a comparable accuracy. Our new framework is generally applicable to all types of systems for precise and efficient structural search, providing new insights into the relationship between ML-extracted structural features and physical properties.

cond-mat.mtrl-sci↗

arXiv2022

Designing Ultra-Flat Bands in Twisted Bilayer Materials at Large Twist Angles without specific degree

Inter-twisted bilayers of two-dimensional (2D) materials can host low-energy flat bands, which offer opportunity to investigate many intriguing physics associated with strong electron correlations. In the existing systems, ultra-flat bands only emerge at very small twist angles less than a few degrees, which poses challenge for experimental study and practical applications. Here, we propose a new design principle to achieve low-energy ultra-flat bands with increased twist angles. The key condition is to have a 2D semiconducting material with large energy difference of band edges controlled by stacking. We show that the interlayer interaction leads to defect-like states under twisting, which forms a flat band in the semiconducting band gap with dispersion strongly suppressed by the large energy barriers in the moire superlattice even for large twist angles. We explicitly demonstrate our idea in bilayer alpha-In2Se3 and bilayer InSe. For bilayer alpha-In2Se3, we show that a twist angle -13.2 degree is sufficient to achieve the band flatness comparable to that of twist bilayer graphene at the magic angle -1.1 degree. In addition, the appearance of ultra-flat bands here is not sensitive to the twist angle as in bilayer graphene, and it can be further controlled by external gate fields. Our finding provides a new route to achieve ultra-flat bands other than reducing the twist angles and paves the way towards engineering such flat bands in a large family of 2D materials.

physics.comp-ph↗

arXiv2022

Effective Lifetime of Non-Equilibrium Carriers in Semiconductors from Non-Adiabatic Molecular Dynamics Simulations

The lifetime of non-equilibrium electrons and holes in semiconductors is crucial for solar cell and optoelectronic applications. Non-adiabatic molecular dynamics (NAMD) simulations based on time-dependent density functional theory (TDDFT) are widely used to study excited-state carrier dynamics. However, the calculated carrier lifetimes are often different from experimental results by orders of magnitude. In this work, by revisiting the definition of carrier lifetime and considering different recombination mechanisms, we report a systematic procedure for calculating the effective carrier lifetime in realistic semiconductor crystals that can be compared directly to experimental measurements. The procedure shows that considering all recombination mechanisms and using reasonable densities of carriers and defects are crucial in calculating the effective lifetime. When NAMD simulations consider only Shockey-Read-Hall (SRH) defect-assisted and band-to-band non-radiative recombination while neglect band-to-band radiative recombination, and the densities of non-equilibrium carriers and defects in supercell simulations are much higher than those in realistic semiconductors under solar illumination, the calculated lifetimes are ineffective and thus differ from experiments. Using our procedure, the calculated effective lifetime of the halide perovskite CH3NH3PbI3 agrees with experiments. It is mainly determined by band-to-band radiative and defect-assisted non-radiative recombination, while band-to-band non-radiative recombination is negligible. These results indicate that it is possible to calculate carrier lifetimes accurately based on NAMD simulations, but the directly calculated values should be converted to effective lifetimes for comparison to experiments. The revised procedure can be widely applied in future carrier lifetime simulations.

cond-mat.mtrl-sci↗