arXiv subjects

Shengbai Zhang

Publications and source records attributed to Shengbai Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Hyperspin Altermagnets

The behavior of spin quantum in k-space is key to identifying altermagnets (AMs) as the third kind of fundamental collinear magnetism. In contrast, non-collinear magnets,though abundant in nature,lack well-defined spin quantum numbers, and the resulting spin textures are often highly complex, which limits their potential for next-generation spintronic applications. Here we propose hyperspin, which lives in a higher-dimensional space, to address these drawbacks. Through analyzing the commutation relations between spin and Hamiltonian for a class of non-collinear magnets, we reveal it is a hyperspin, rather than the usual spin, that commutes with Hamiltonian. Unexpectedly, these non-collinear magnets should also show collinear spin-split bands in k-space like collinear AMs. We therefore classify such non-collinear magnets as hyperspin altermagnets (HAMs), as opposed to the usual collinear AMs. Our theory elucidates the fundamental physics of AMs and HAMs and provides a framework for exploring the wide range of non-collinear magnets that may possess other kinds of conserved quantities.

cond-mat.mtrl-sci↗

arXiv2026

Atomic short-range order: a new degree of freedom for band engineering of GeSn semiconductor alloys

Chemical short-range order (SRO) in alloys denotes the statistical preference or avoidance between atomic species on neighboring lattice sites. Here, we highlight SRO as a powerful new mechanism for semiconductor alloy band engineering. Atom probe tomography reveals a significantly higher probability of Sn-Sn first nearest neighbors (1NNs) in thin-film GeSn alloys grown by molecular beam epitaxy (MBE) vs. chemical vapor deposition (CVD). Remarkably, although lower Sn concentration typically widens the bandgap, we find that the stronger presence of Sn-Sn 1NN pairs in MBE samples overrides this trend, resulting in a narrower bandgap despite having 2 at.\% lower Sn content than CVD samples. First-principles modeling corroborates this effect, attributing these SRO variations to distinctive surface terminations and growth temperatures between MBE and CVD. These findings establish SRO as a new degree of freedom for semiconductor band engineering beyond composition, strain, and quantum confinement, unlocking novel device mechanisms for the post-Moore era.

cond-mat.mtrl-sci↗

arXiv2026

Minimizing propagated density errors of atomic core-electron for simultaneously accurate bandgaps and lattice constants in closed-shell Copper semiconductors

Density functional theory struggles to accurately determine electron density of atoms, whose error is inevitably encoded into the pseudopotential and propagated into solid-state calculations. However, little is known about how this affects accuracy nor how to remedy it. In this work, through a systematic study of the effect of Cu atomic density on bandgap and lattice constants of over 50 Cu-containing simple closed-shell semiconductors, we find that core-electron density can drastically affect nuclear attraction to valence electrons and subsequent charge distribution and energy position of Cu 3$d$ electrons. The error can be eliminated at its source by employing modified Hartree-Fock pseudopotentials for Cu core while retaining (semi-)local functionals for valence electrons. This real-space partitioning approach leads to simultaneous high-accuracy in bandgap and lattice constants across the entire material class.

cond-mat.mtrl-sci↗

arXiv2026

Unconventional crystallization pathway bypassing the intermediate cubic phase in phase-change superlattices

The Ge-Sb-Te (GST) superlattice phase-change material is a promising candidate for overcoming the high power-consumption of phase-change memory (PCM). However, the working mechanism of the superlattice PCM remains controversial. Partial amorphization, which is currently considered the most plausible mechanism, remains hotly debated: how does the partially amorphized GST recrystallize into its superlattice phase instead of the conventionally expected cubic phase? Here, we address this issue using large-scale molecular dynamics simulations enabled by a machine-learning interatomic potential. Starting from a partially melted GST superlattice, we demonstrate that the residual crystalline regions serve as nuclei, enabling the amorphous GST to recrystallize directly into the superlattice phase without passing through the intermediate cubic phase. Moreover, the recrystallized phase is not an ideal superlattice, but rather a structurally ordered and chemically disordered defective superlattice characterized by anti-site defects and stacking faults. The defective superlattice region is also more susceptible to melting than the defect-free superlattice, and thereby can act as the active region of the PCM device. These results help to clarify the longstanding debates concerning the mechanism of superlattice-based PCM.

cond-mat.mtrl-sci↗

arXiv2025

Multicolor groups for molecules and solids

The local magnetic moments of atoms in a molecule or solid can be designated by different colors. Magnetic groups, or 2-color groups, or black-and-white groups have been applied in crystallography to classify different magnets. Despite its successes in the past decades, the recent advents of altermagnets and p-wave magnets raise new challenges to this long-standing framework, which urges for a new and unified one. Here we develop a multicolor group classification framework to classify all kinds of molecules and solids, including nonmagnetic materials and magnets with collinear or non-collinear magnetism, and with or without spin-orbit couplings (SOC). This new scheme can unify the classifications of matters into a single framework, including the recently identified altermagnets and p-wave magnets. Especially, altermagnetic topological matters and p-wave magnets with SOC, can also be diagnosed with multicolor groups, a task which can not be accomplished by magnetic space groups and spin space groups. Moreover, insufficiencies and misconceptions of conventional magnetic group classification can be supplemented through this new framework. Multicolor group will serve as a new stage in the symmetry classification of matters.

cond-mat.mtrl-sci↗

arXiv2025

Uncovering the Fourier Structure of Wavefunctions in Semiconductors

Symmetry is at the heart of material properties. Symmetry of the Bravais lattice defines the degeneracy of planewaves, upon which atomic symmetry determines interaction potentials which may lift such degeneracies. This results in wavefunctions which are single planewaves throughout the Brillouin zone (BZ), except in the vicinity of lifted degeneracies. This great simplification allows for determination of optical properties from a handful of planewaves and a single transition. Further, it reveals that nonlinear optical response arises from higher order degeneracy along lines/points in the BZ.

cond-mat.mtrl-sci↗

arXiv2025

Semiconductor-compatible topological digital alloys

Recently, GeSn alloys have attracted much interest for direct-gap infrared photonics and as potential topological materials which are compatible with the semiconductor industry. However, for photonics, the high-Sn content required leads to low detectivity, associated with poor material quality, and the (>35%) Sn required for topological properties have been out of reach experimentally. Here, we demonstrate that by patterning the Sn distribution within Ge, the electronic properties have a far greater tunability than is possible with the random alloy. For the GeSn δ-digital alloy (DA) formed by confining Sn atoms in atomic layer(s) along the [111] direction of Ge, we show that ~10% Sn can lead to a triple-point semimetal. These findings are understood in terms of Sn ordering causing spatial separation of Sn and Ge band edges, leading to band inversion. This mechanism can also lead to a weak topological insulator, Weyl semimetal, and enables tunable direct bandgaps down to 2 meV, covering the entire infrared range. This DA induced topological properties are also identified in compound semiconductors, such as InAs1-xSbx, showing the general applicability of the DA design for realizing topological properties on conventional semiconductor platforms. Our findings not only point to a new class of currently unexplored topological systems accessible by epitaxy, but also establish the promise of low-Sn GeSn DAs for application as infrared laser diodes and photodetectors in Si photonic integrated circuits and infrared image sensors.

cond-mat.mtrl-sci↗

arXiv2024

Bond Dipole based Geometric Theory of Band Alignment

The band alignment (BA) between two materials is a fundamental property that governs the functionality and performance of electronic, as well as electrochemical, devices. However, despite decades of study, the inability to separate surface properties from those of bulk have made a deep understanding of the physics of BA illusive. Building on the theory of ideal vacuum level to separate surface from bulk [CWZ, Phys. Rev. B 103, 235202 (2021)], here we present a geometric theory for the band alignment, particularly, explaining the insensitivity of the alignment to interfacial orientation between isotropic materials. First, we adopt charge neutral polyhedron, termed Wigner-Seitz atoms (WSA), to partition the charge of atoms in a way which maintains crystal symmetry and tessellates the space. In contrast to CWZ theory, the band alignment of two materials constructed from such WSAs is independent of interface orientation. Upon electron relaxation at the interface, here we show that the interfacial charge transfer dipole can be faithfully descibed by the sum of localized point dipoles which exist between atoms at the interface (bond dipoles). For interfaces between isotropic materials, the magnitude of the bond dipole can be factored out as a multiplier, leaving only geometric factors, such as the crystal symmetry and dimension of the material, to determine band alignment, irrespective of the orientation of the interface. We considered 29 distinct interfaces and found that this bond dipole theory yields excellent agreement (RMS deviation < 30 meV) with first-principles results. Our theory can be straightforwardly applied to interface between alloys, as well as between anisotropic systems.

cond-mat.mtrl-sci↗

arXiv2024

Revisiting the Formulation of Charged Defect in Solids

Defect physics is at the heart of microelectronics. By keeping track of the reference energy in total energy calculations, we explicitly show that the "potential alignment" correction vanishes, and the classic Markov-Payne correction yields accurate results. From linear response theory, we further formulate an accurate expression for the quadrupole correction. Application to numerous defects including anisotropic material yields accurate formation energies in small supercells and the historically slow convergence of the 2+ diamond vacancy is shown to be a result of slow varying gap levels of the defect leading to a size dependent dielectric constant.

cond-mat.mtrl-sci↗

arXiv2023

Small polarom formation by electron-electron interaction

In a solid, electrons can be scattered both by phonons and other electrons. First proposed by Landau, scattering by phonons can lead to a composite entity called a polaron, in which a lattice distortion traps an itinerant electron (or hole) such that the distortion and carrier move in unison as a single particle with larger effective mass. While this is the traditional view of polarons, the rise of 2D systems, especially strongly correlated ones, open the prospect of electron scattering taking on a larger role in spontaneous carrier localization for such material systems. Here, we show that in transition metal halides, such electron-electron interactions can lead to polaron formation even in the absence of lattice distortion. This suggests an alternative direction for polaron formation, transport, and control in solids. This new mechanism of polaron formation is confirmed by first-principles calculation of 2D transition metal halides, CrI2, CoCl2 and CoBr2. These theoretical predictions are supported by scanning tunneling microscopy/spectroscopy measurements of polarons in CrI2.

cond-mat.mtrl-sci↗

arXiv2023

Inert gas as electronic impurity in semiconductors: The case for active infrared absorption in silicon

Inert (noble gas) elements are extremely inactive to surrounding chemical environment and are frequently employed as protective gas in various semiconductor fabrication processes. In this work, we surprisingly discover that high doses of argon up to $10^{17}-10^{20} cm^{-3}$ can be measured in silicon exposed by laser pulses even after 1300 days. First-principles calculations and molecular dynamics identify a unique argon-locking-vacancy (ALV) defect atomic model in silicon. The ALV defect is dynamically robust in contrast to the frequently moving pure Si vacancy. While argon is chemically inert, it readily modulates defect states of the occupied vacancy via steric repulsion and rattling motions, leading to significant band splitting within bandgap and thus strong infrared absorptions. Moreover, the repulsion between substitutional argon and dangling bonds results in shallow donors which explains the confusion of enhanced n-type carriers in experiments. The work paves a way of using noble gas element to produce active infrared absorption source for the non-heteroepitaxy photonic detectors directly on silicon wafer at infrared communication wavelength.

cond-mat.mtrl-sci↗

arXiv2022

Enumeration of Moire Patterns of a Hexagonal Twisted Bilayer and Intercalated Transition Metals in Twisted h-BN

A real-space method using generating integers is used to classify the possible moire patterns for two equal hexagonal lattices. The result is that the rotations that take (n,m) to (m,n) with n,m relatively prime form the fundamental moire transformations, and the number of lattice coincidence areas within each supercell is given by (n-m)^2. The scheme may be extended to cases where the lattice constants differ. Additionally, we consider a system with a transition metal between the layers of a twisted bilayer of h-BN. We find that the lowest energy configurations for such an arrangement are those at aligned and anti-aligned sites of the moire pattern, depending on the transition metal, and the low-symmetry sites possess high magnetization.

physics.comp-ph↗

arXiv2022

Ultrafast Charge Transfer Enhancement in CdS-MoS2 via Linker Molecule

Hybrid systems, which take advantage of low material dimensionality, have great potential for designing nanoscale devices. Quantum dots (QDs) -- a 0D nanostructure -- can be combined with 2D monolayers to achieve success in photovoltaics and photocatalytic water splitting. In such colloidal systems, ligand molecules such as cysteine play an important role in device performance. The role of the ligand molecule in these QD heterostructures is poorly understood. In this study, time-dependent density functional theory (TD-DFT) is employed in order to explore how the ligand affect the charge transfer at the ultra-fast timescale. We study the charge transfer dynamics in CdS-MoS2 heterostructures both with and without an organic linker molecule. We find that the ligand molecule enhances the ultrafast charge transfer, and that electrons are preferentially transferred from CdS to MoS2 as band alignment would predict. The electronic dynamics and time-evolved projection character are sensitive to the ionic temperature and excitation density.

physics.comp-ph↗

arXiv2022

A Universal Description of Workfunction

At the surfaces of materials, the bulk symmetry of the charge density is broken and electron spill-out into the vacuum region creates a surface dipole. Such spill-out has been historically calculated by Lang and Kohn [Phys. Rev. B \textbf{3}, 1215 (1971)] using average electron density to sucessfully explain the workfunction in metals. However, despite its initial success, in the fifty years since it has not been extended beyond simple metals. Here we show that the degree of charge spill-out is largely controlled by the innate bulk workfunction $ϕ_I$, which is the Fermi level position of $\it bulk$ relative to the ideal vacuum. By incorporating the contribution of $ϕ_I$ to the surface dipole we show that Lang-Kohn's $\it jellium$ based approach can be broadly expanded to understand the workfunction over a wide range of metals, semiconductors, and insulators.

cond-mat.mtrl-sci↗

arXiv2022

Manipulating single excess electrons in monolayer transition metal dihalide

Polarons are entities of excess electrons dressed with local response of lattices, whose atomic-scale characterization is essential for understanding the many body physics arising from the electron-lattice entanglement, but yet difficult to achieve. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we show the visualization and manipulation of single polarons with different origin, i.e., electronic and conventional polarons, in monolayer CoCl2, that are grown on HOPG substrate via molecular beam epitaxy. Four types of polarons are identified, all inducing upward local band bending, but exhibiting distinct appearances, lattice occupations, polaronic states and local lattice distortions. First principles calculations unveil three types of polarons are stabilized by electron-electron interaction. The type-4 polaron, however, are driven by conventional lattice distortions. All the four types of polarons can be created, moved, erased, and moreover interconverted individually by the STM tip, allowing precise control of single polarons unprecedently. This finding identifies the rich category of polarons and their feasibility of manipulation in CoCl2, which can be generalized to other transition metal halides.

cond-mat.mes-hall↗

arXiv2022

Dative epitaxy of commensurate monocrystalline covalent-van der Waals moiré supercrystal

Realizing van der Waals (vdW) epitaxy in the 80s represents a breakthrough that circumvents the stringent lattice matching and processing compatibility requirements in conventional covalent heteroepitaxy. However, due to the weak vdW interactions, there is little control over film qualities by the substrate. Typically, discrete domains with a spread of misorientation angles are formed, limiting the applicability of vdW epitaxy. Here we report the epitaxial growth of monocrystalline, covalent Cr5Te8 2D crystals on monolayer vdW WSe2 by chemical vapor deposition, driven by interfacial dative bond formation. The lattice of Cr5Te8, with a lateral dimension of a few ten microns, is fully commensurate with that of WSe2 via 3 x 3 (Cr5Te8)-7 x 7 (WSe2) supercell matching, forming a single crystalline moire superlattice. Our work has established a conceptually distinct paradigm of thin film epitaxy termed dative epitaxy, which takes full advantage of covalent epitaxy with chemical bonding for fixing the atomic registry and crystal orientation, while circumventing its stringent lattice matching and processing compatibility requirements; conversely, it ensures the full flexibility of vdW epitaxy, while avoiding its poor orientation control. Cr5Te8 2D crystals grown by dative epitaxy exhibit square magnetic hysteresis, suggesting minimized interfacial defects that can serve as pinning sites.

cond-mat.mtrl-sci↗

arXiv2021

Polarization at the Nanoscale

Modern polarization theory yields surface bound charge associated with spontaneous polarization of bulk. However, understanding polarization in nano systems also requires a proper treatment of charge transfer between surface dangling bonds. Here, we develop a real-space approach for total polarization and apply it to wurtzite semiconductors and BaTiO3 perovskite. First-principles calculations utilizing this approach not only yield spontaneous bulk polarization in agreement with Berry phase calculations, but also uncover phenomena specific to nano systems. As an example, we show surface passivation leads to a complete quenching of the piezoelectric effect, which reemerges only at larger length scale and/or spontaneous polarization.

cond-mat.mtrl-sci↗

arXiv2021

Bilayer twisting as a mean to isolate connected flat bands in a Kagome lattice through Wigner crystallization

The physics of flat band is novel and rich but difficult to access. In this regard, recently twisting of bilayer van der Waals (vdW)-bounded two-dimensional (2D) materials has attracted much attention, because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy. Alternatively, one may start with a 2D Kagome lattice, which already possesses flat bands at the Fermi level, but unfortunately these bands connect quadratically to other (dispersive) bands, leading to undesirable effects. Here, we propose, by first-principles calculation and tight-binding modeling, that the same bilayer twisting approach can be used to isolate the Kagome flat bands. As the starting kinetic energy is already vanishingly small, the interlayer vdW potential is always sufficiently large irrespective of the twisting angle. As such the electronic states in the (connected) flat bands become unstable against a spontaneous Wigner crystallization, which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.

cond-mat.mes-hall↗