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.

Density functional theory struggles to accurately determine the 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 3d electrons. The error can be eliminated at its source by employing modified Hartree-Fock pseudopotentials for the 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.

Phase-change memory (PCM) displays great promise for the storage-class memory (SCM) technology due to its combination of fast speed of dynamic random-access memory and nonvolatility of Flash. Yet, to meet the high industrial requirement of write/erase speed for the SCM application, robust strategies for further accelerating phase transition, particularly from amorphous to crystalline PCM materials, are urgently needed. In this work, we propose a unique strategy of coherent-interface induced ultrafast crystallization in PCM materials. Employing rock-salt YAs/Ge2Sb2Te5 as a prototype, systematic first-principles molecular dynamics demonstrate that rapid nonstochastic crystallization behaviors can be achieved by the rock-salt-lattice-matching and high-temperature-resistant heterogeneous interface attached to the popular PCM material Ge2Sb2Te5 (GST). Further experiment shows that the YAs-incorporated GST device has a faster SET process compared with the pure GST device. Finally, to extend the strategy in the family of inorganic materials, high-throughput screening from over 150 000 structures discovers as many as 71 candidates for coherent interfaces with PCM GST. The present study establishes a promising strategy to overcome the speed bottleneck of PCM through atomic-scale interface design for future storage-class memory implementation.

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 advent of altermagnets and p-wave magnets raises new challenges to this long-standing framework, which calls for a new and unified one. Here we develop a multicolor group classification framework to classify nonmagnetic materials and magnets with collinear or non-collinear magnetism, and with or without spin-orbit coupling (SOC), including the recently identified altermagnets and p-wave magnets. Especially, altermagnetic topological matter with SOC and p-wave magnets can also be diagnosed with multicolor groups, a task which cannot 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 matter.

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.

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 Makov-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.

Manipulating electrons opens up emerging synthetic strategies. Multipolaron, as a rare quasiparticle containing multiple excess charges collectively dressed with shared local lattice distortions, provides an ideal medium for electron manipulation, yet remains elusive. Here, with scanning tunneling microscopy, we realize electron multipolarons in monolayer CrBr3. The multipolaron is crafted via assembling single monopolarons with the tip, allowing their electron numbers to increase one-by-one controllably. With added electrons, the multipolaron exhibits stronger local band bending and upward shift of the polaronic states. Notably, the apparent charge of the multipolaron can be reduced by tip manipulation. First principles calculations reveal that the multipolaron is stabilized by large diffusion barriers and screening of the substrate. The apparent charge reduction is attributed to the formation of the polaron-exciton droplet where the multipolaron captures holes from the substrate to partially offset the Coulomb repulsion. Our findings establish an approach for studying polaron interactions at the atomic limit.

First-principles calculations rely heavily on pseudopotentials, yet their impact on accuracy is hardly addressed. In this work, we show that most pseudopotentials to date introduce errors, which manifest themselves as errors of atomic energy levels, leading to a de facto deviation from the Hohenberg-Kohn theorem. We consider the atomic-level adjusted pseudopotentials, whose interplay with exchange-correlation functional provides a pragmatic correction that balances accuracy and efficiency. We benchmark our theory with bandgap calculation for 54 semiconductors containing monovalent Cu. The results, compared to those from conventional studies, not only remove all erroneous metal predictions for 11 compounds, but also reduce the mean relative error from 80% to 20%. Overall accuracy even exceeds those of standard hybrid functionals and GW methods.

Symmetry dictates the physical properties of materials. The symmetry of the Bravais lattice defines the set of points, lines, and planes over which sets of planewaves are degenerate, upon which atomic symmetry determines the interaction potentials which may lift such degeneracies. This results in wavefunctions which are single planewaves throughout the BZ, except in the vicinity of the removed degeneracies. As optical transitions between any two planewaves are forbidden, only regions of the Brillouin zone (BZ) near these lifted degeneracies contribute to optical properties. Application to optical response of Si and other semiconductors reveals that a single band transition, with only two planewaves, well describes their dielectric properties. Further, it provides a framework to understand non-linear optical response which is demonstrated to arise from higher order degeneracy existing along high symmetry lines/points of the BZ.

An electron in solid can be dressed by the lattice distortions of surroundings, forming a localized composite quasiparticle called small polaron, whose formation has been customarily attributed to the electron-phonon couplings that the ion polarization traps the excess electron. Here we present a theory of electron-polarization induced small polaron, in which the carrier localization happens spontaneously and drives subsequent ion relaxation. This mechanism of polaron formation is qualitatively different than the Mott-Stoneham picture in that there is no need to overcome a kinetic barrier for the carrier to self-trap to form a polaron.Through a combination of first-principles theory and model Hamiltonian, we show that this is the mechanism for polaron formation in the monolayer two-dimensional transition metal halides, CrI2, CoCl2 and CoBr2. These findings may explain the exceptional stability and manipulability of polarons in this class of materials by scanning tunneling microscopy.

Chalcogenide perovskite semiconductors, with their excellent optical absorption, chemical stability, and lack of toxicity, have emerged as a promising alternative to traditional halide perovskites. Through first-principles density functional theory, we show that despite the large lattice mismatch between the prototypical BaZrS3 and BaZrO3 chalcogenide perovskites, BaZr(S1−xOx)3 can form low-energy ordered lattices that significantly reduce strain. The bandgap dependence of the resulting ordered compound on x is found to exhibit double Vegard's law behavior, having two distinct linear regions, associated with an underlying distorted or undistorted perovskite structures.

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 delta-digital alloy (DA) formed by confining Sn atoms in atomic layer(s) along the [111] direction of Ge, we show that similar to 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.

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 \delta-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. Our findings are generally applicable to other semiconductors DAs and point to a new class of currently unexplored topological systems accessible by epitaxy and 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.

The band alignment (BA) between two materials is a fundamental property that governs the functionality and performance of electronic and electrochemical devices. However, despite decades of study, the inability to separate surface properties from those of the bulk has made a deep understanding of the physics of BAs elusive. Building on the theory of the ideal vacuum level to separate surface from bulk [Choe et al., Phys. Rev. B 103, 235202 (2021)], here we present a geometric theory for the band alignment, specifically explaining the insensitivity of the alignment to interfacial orientation between isotropic materials. First, we adopt a neutral polyhedron, termed Wigner-Seitz atoms (WSA), to partition the charge of atoms in a way that maintains crystal symmetry and tessellates the space. In contrast to the CWZ theory, the band alignment of two materials constructed from such WSAs is independent of interface orientation. Upon electron relaxation at the interface, we show that the interfacial charge transfer dipole can be accurately described by the sum of localized point dipoles that 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 crystal symmetry and dimension of the material, to determine band alignment, regardless 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 easily applied to interface between alloys, as well as between anisotropic systems.

α-In2Se3 is a promising two-dimensional (2D) ferroelectric semiconductor with unique phase transition behaviors and intrinsic n-type conductivity. However, the origin of this conductivity and the impact of defects on the phase transition remain unclear. In this study, we employed the WLZ method to calculate vacancies' formation energy and ionization energy in monolayer α-In2Se3 and identified the defect-bound band edge states. Our results reveal a strong polarization-defect coupling effect, where the bottom-layer selenium vacancy drives intrinsic n-type conductivity in the sample with upward polarization while reversing the polarization-induced deep p-type defect. Furthermore, we demonstrate that a vacancy stabilizes the ferroelectric phase and reduces the phase transition rate to the paraelectric phase. Finally, we propose a defect-engineered ferroelectric field-effect transistor model that controls the resistance by leveraging the polarization-defect coupling effect. This work highlights the significant roles of vacancy defects in 2D α-In2Se3, offering strategies to design In2Se3 electronic devices at the nanoscale.

Chalcogenide perovskites, particularly BaZrS3, hold promise for optoelectronic devices owing to their exceptional light absorption and inherent stability. However, thin films obtained at lower processing temperatures typically result in small grain sizes and inferior transport properties. Here we introduce an approach employing co-sputtering elemental Ba and Zr targets followed by CS2 sulfurization, with a judiciously applied NaF capping layer. NaF acts as a flux agent during sulfurization, leading to marked increase in grain size and improved crystallinity. This process results in near-stoichiometric films with enhanced photoresponse. Terahertz spectroscopy further reveals a carrier mobility more than two orders of magnitude higher than those obtained from field-effect transistor measurements, suggesting that bulk transport is limited by grain boundary scattering. Our results demonstrate flux-assisted sulfurization as an effective strategy to improve the crystallinity of chalcogenide perovskite thin films for optoelectronic applications.

Recently, short-range order (SRO) has attracted significant attention, challenging the conventional view of the atomic positions in alloys being random. Furthermore, the presence of SRO has been predicted to have profound effects on the electronic and topological properties of group-IV alloys, offering a different direction in designing group-IV materials for photoelectronic and quantum devices. However, due to the limited understanding of the formation mechanisms, developing effective methods to manipulate SRO in epitaxy is still challenging. To address this, we propose a mechanism for the GeSn alloy, revealing that surface diffusion plays a key role in SRO formation. Building on this mechanism, we show that the distinct surface conditions in MBE and CVD lead to the formation of SRO with enhanced Sn–Sn pairing in MBE-grown samples, while CVD-grown samples remain random alloys. Our findings provide an initial understanding of the kinetic process of SRO formation, providing guidance for the design of experiments to manipulate SRO.

Just like we classify lions and tigers into the same panthera, but into different species, we can classify molecules and solids by their spatial and spin space symmetries. In magnetic molecules and solids, local moments of the atoms can be represented by different colors; we can therefore classify these materials by playing with colors. The advantage of the multicolor group scheme is that it unifies the classifications into a single framework through multicolor group theory, including the recently identified altermagnets and p-wave magnets. In particular, non-magnetic compounds fall into 0-color groups, ferro/ferrimagnets fall into 1-color groups, antiferromagnets, as usual, fall into 2-color groups, and non-collinear magnets fall into the simple and complex multicolor groups. Moreover, confusions brought about by conventional magnetic-group classification can be resolved through this new framework.