Interest in substitutional disordered alloys has recently reemerged with focus on the symmetry-sensitive properties in the alloy such as topological insulation and Rashba effect. A substitutional random alloy ${\left(AX\right)}_x{\left(BX\right)}_{1-x}$ of components $AX$ and $BX$ generally manifests a distribution of local environments, whereby each $X$ site, for example, can be locally decorated by different substitutional arrangements of {$A,B,X$} atoms, thus creating an inherently polymorphous network. Electrons will then respond to the existence of different local environments and site symmetries, creating local charge transfer and atomic displacements patterns observed in experiments. While the macroscopic average structure $S_0$, as seen by probes with long coherence length, may have the original high symmetry of the constituent compounds, many observable physical properties are sensitive to local symmetry, and are hence the average $\langle P\left(S_i\right)\rangle$ of the properties $\left\{P\left(S_i\right);i=1,...,N\right\}$ of the individual microscopic configurations $\{S_i;i=1,N\}$ rather than the property $P\left(\langle S_i\rangle \right)=P\left(S_0\right)$ of the macroscopically averaged high-symmetry (monomorphous) configuration $S_0$. The fundamental difference between the polymorphous representation $\langle P\left(S_i\right)\rangle$ versus the monomorphous $P\left(S_0\right)$ in modeling substitutionally disordered alloys led to the often diverging results between methods that ‘‘see’’ atomic details and those that see only the high symmetry of the constituents, while missing the atomic-scale resolution needed in many cases to discern local symmetry-related physics. A natural approach that captures the polymorphous aspect of random alloys is the well-known supercell approach where lattice sites are occupied by the alloyed elements with a particular form of disorder and solved via periodic electronic structure methods for sufficiently large supercells. However, such approaches tend to produce complex $E$ versus $\mathit{k}$ dispersion relations (‘‘spaghetti bands’’), rendering the wave-vector $k$ information needed in theories of topology and Rashba physics and seen in angular resolved experiments, practically inaccessible. The results of such calculations have consequently been displayed as density of states. A solution that retains the polymorphous nature of the random alloy but reinstates the $E$ versus $\mathit{k}$ relation in the base Brillouin zone is to unfold the supercell bands. This yields alloy “effective band structure” (EBS), providing a three-dimensional picture of the distribution of spectral density in the whole Brillouin zone. It consists of $E$- and $k$-dependent spectral weight with coherent and incoherent features, all created naturally by the specific nature of the chemical bonding underlying the polymorphous distribution of many local environments. We illustrate this EBS approach for CdTe-HgTe, PbSe-SnSe, and PbS-PbTe alloys, showing atomic-scale effects such as formation of a distribution of $A\text{−}X$ and $B\text{−}X$ bond lengths, local charge transfer, and the creation and destruction of valley degeneracies. In CdTe-HgTe, the disorder effect is so weak that the incoherent term is negligible, and the monomorphous approaches are still feasible in this alloy. In PbSe-SnSe, the stronger disorder effect introduces significant $(\sim 150\mathrm{meV})$ band splitting of the topological band inversion, forming a sequential inversion of multiple bands which is important for the topological transition but absent in monomorphous methods. In PbS- PbTe, there is a strong disorder effect, revealing the emergence of ferroelectricity from the polymorphous network in this alloy.

Si hyperdoped with chalcogens (S,Se,Te) is well known to possess unique properties such as an insulator-to-metal transition and a room-temperature sub-band-gap absorption. These properties are expected to be sensitive to a postsynthesis thermal annealing, since hyperdoped Si is a thermodynamically metastable material. Thermal stability of the as-fabricated hyperdoped Si is of great importance for the device fabrication process involving temperature-dependent steps such as Ohmic contact formation. Here, we report on the thermal stability of the as-fabricated Te-hyperdoped Si subjected to isochronal furnace anneals from 250 to 1200 °C. We demonstrate that Te-hyperdoped Si exhibits thermal stability up to 400 °C for 10 min, which even helps to further improve the crystalline quality, the electrical activation of Te dopants, and the room-temperature sub-band-gap absorption. At higher temperatures, however, Te atoms are found to move out from the substitutional sites with a maximum migration energy of $E_{\mathrm{M}}=2.3\mathrm{eV}$ forming inactive clusters and precipitates that impair the structural, electrical, and optical properties. These results provide further insight into the underlying physical state transformation of Te dopants in a metastable compositional regime caused by postsynthesis thermal annealing. They also pave the way for the fabrication of advanced hyperdoped Si-based devices.

Magnetochiral dichroism (MChD) in chiral materials is a nonreciprocal directional dichroism producing optical absorption/emission differences for unpolarized (or linearly polarized) light propagating parallel and antiparallel to a magnetic field; this is an intriguing optical phenomenon enabled by coupling between chirality and magnetism. Since ubiquitous unpolarized light can be coupled with chirality of materials in a magnetic field, MChD has been attracting attention as a potential source for asymmetric photochemical reactions and as an application source for novel magneto-optical devices. However, it has been weakly observed in the visible light region, which is of the order of ${10}^{-2}-{10}^{-1}\%$ for the corresponding signal, and prevents further applications. In this study, we demonstrate a strong MChD for visible light emission based on the microscopic mechanism in a terbium(III) chiral complex with highly symmetrical nona-coordinated geometry. The MChD signal reaches $\sim 16\%$ at 14 T of the luminescence intensity, involving the development of magnetization under magnetic fields.

We introduce the “selPT” perturbative approach, based on ab initio molecular dynamics (AIMD), for computing accurate finite-temperature properties by efficiently using correlated wave-function methods. We demonstrate the power of the method by computing prototypical molecular enthalpies of adsorption in zeolite $({\mathrm{CH}}_4$ and ${\mathrm{CO}}_2$ on protonated chabazite at 300 K) using the random phase approximation. Results are in excellent agreement with experiment. The improved accuracy provided by selPT represents a crucial step towards the goal of truly quantitative AIMD predictions of experimental observables at finite temperature.

Fermi surfaces and band dispersion curves of a ${\mathrm{Cu}}_2\mathrm{Si}$ layer on Si(111), quasi-“$5\times 5$” were mapped by angle-resolved photoemission spectroscopy using synchrotron radiation. Two metallic bands were observed within the Si bulk band gap, which are likely assigned to the electronic bands of the ${\mathrm{Cu}}_2\mathrm{Si}$ layer. Additional bands were found in the Si bulk band gap that originate from the interactions between the substrate state and the ${\mathrm{Cu}}_2\mathrm{Si}$ states that have the $p_z$ character. The present research aims at investigating the changes in the electronic structure of an atomic layer when prepared on a substrate.

Electronic polarization, manifested in numerous physical phenomena such as ferroelectricity, piezoelectricity, and pyroelectricity, is of great importance to many technological applications. The presence of ferroelectricity in bulk elemental materials is extremely rare due to the high symmetry of the underlying lattices. However, the presence of a surface lowers the symmetry, which may provide a future avenue for achieving robust ferroelectric polarization. Using first-principles calculations, we show that a spontaneous electrical polarization can be realized on the silicon reconstructed (001) surface. The magnitude of this polarization is $0.45\times {10}^{-10}\mathrm{C}/\mathrm{m}$, comparable to those in benchmarking two-dimensional ferroelectric compounds such as SnTe and SnSe. The symmetry lowering is originated from a Peierls instability that removes the degeneracy of surface dangling bonds, whereby opening a band gap. Although flip-flop of the surface dimers may destroy the ordered phase at elevated temperatures, we find that a modest compressive strain, which increases the local stress and interdimer coupling, can drastically increase the robustness of the ferroelectric state towards room-temperature operations. The technology for processing Si has been well developed and the Si (001) surface has also been widely used in various electronic devices. Our discovery here thus holds high promise in a multitude of fields, ranging from nanoscale random access memory to low power consumption energy storage devices.

An intact ${\mathrm{CuO}}_2$ plane is widely believed to be a prerequisite for the high-$T_c$ superconductivity in cuprate superconductors. However, an exception may exist in the superconducting ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$ materials where CuO chains play a more important role. From first-principles density functional theory calculations, we have studied the electronic and magnetic structures of ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$. The stoichiometric ${\mathrm{Ba}}_2{\mathrm{CuO}}_3$ and ${\mathrm{Ba}}_2{\mathrm{CuO}}_4$ contain quasi-one-dimensional CuO chains and intact two-dimensional ${\mathrm{CuO}}_2$ planes, respectively. In comparison with the nonmagnetic metal ${\mathrm{Ba}}_2{\mathrm{CuO}}_4, {\mathrm{Ba}}_2{\mathrm{CuO}}_3$ is found to be an antiferromagnetic (AFM) Mott insulator. It possesses a nearest-neighbor intrachain AFM coupling and a weak interchain interaction, and its lowest unoccupied band and highest occupied band are contributed by the Cu $3d_{b^2-c^2}$ orbital (or $d_{x^2-y^2}$ orbital if we denote the $bc$ plane as the $xy$ plane) and O $2p$ orbitals, respectively. Total energy calculations indicate that the oxygen vacancies in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$, in reference to conventional cuprate superconductors, prefer to reside in the planar sites rather than the apical oxygens in the CuO chains, in agreement with the experimental observation. Furthermore, we find that moderate charge doping can reduce the energy differences between the low-lying magnetic states of ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$, which may induce the spin fluctuations that are beneficial for the appearance of superconductivity. Our results also suggest that the spin fluctuations in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3+\delta }$ are anisotropic, which deserves further experimental verification.

We report on the epitaxial growth of single-layer ${\mathrm{NbS}}_2$ on Au(111) and determine both its crystalline and electronic structure by a combination of low-energy electron diffraction, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy. The layer is found to grow in the 1H structural phase with a lattice constant of $\left(3.29\pm 0.03\right)$ Å, a value comparable to the bulk 2H ${\mathrm{NbS}}_2$ lattice constant. The photoemission data reveals a metallic band structure down to a temperature of 30 K. The observed bands are rather broad and consistent with either a strong ${\mathrm{NbS}}_2$-substrate interaction or with the recently reported interplay of strong many body effects in single-layer ${\mathrm{NbS}}_2$ [van Loon et al., npj Quantum Mater. 3, 32 (2018)]. No indications of a charge density wave are observed.

Topological semimetals generally contain heavy elements. Using density-functional theoretic calculations, we predict that three dense lithium polymorphs in the pressure range 200–360 GPa display nontrivial semimetallic electronic structure. Specifically, these high-pressure phases exhibit Fermi pockets which are degenerate over a loop in $\mathit{k}$ space, around which an encircling $\mathit{k}$-space path is threaded by $\pm \pi$ Berry phase. Accordingly, these dense lithium phases are topological nodal loop semimetals involving a single light element.

The anomalous Hall effect (AHE) is a well-known fundamental property of ferromagnetic metals, commonly associated with the presence of a net magnetization. Recently, an AHE has been discovered in noncollinear antiferromagnetic (AFM) metals. Driven by nonvanishing Berry curvature of AFM materials with certain magnetic space-group symmetry, anomalous Hall conductivity (AHC) is very sensitive to the specific type of magnetic ordering. Here, we investigate the appearance of AHC in antiperovskite materials family $A\mathrm{NM}{\mathrm{n}}_3(A=\mathrm{Ga},\mathrm{Sn},\mathrm{Ni})$, where different types of noncollinear magnetic ordering can emerge. Using symmetry analyses and first-principles density-functional theory calculations, we show that with almost identical band structure the nearly degenerate noncollinear AFM ${\mathrm{\Gamma }}_{5\mathrm{g}}$ and ${\mathrm{\Gamma }}_{4\mathrm{g}}$ phases of ${\mathrm{GaNMn}}_3$ have zero and finite AHC, respectively. In a noncollinear ferrimagnetic M-1 phase, ${\mathrm{GaNMn}}_3$ exhibits a large AHC due to the presence of a sizable net magnetic moment. In the noncollinear antiperovskite magnets, transitions between different magnetic phases, exhibiting different AHC states, can be produced by doping, strain, or spin transfer torque, which makes these materials promising for novel spintronic applications.

Understanding and manipulating properties emerging at a surface or an interface require a thorough knowledge of structure-property relationships. We report a study of a prototype oxide system, $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ grown on $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$, by combining in situ angle-resolved x-ray photoelectron spectroscopy, ex situ x-ray diffraction, and scanning transmission electron microscopy/spectroscopy with electric transport measurements. We find that $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ films thicker than 20 unit cells (u.c.) exhibit a universal behavior with no more than one u.c. intermixing at the interface but at least 3 u.c. of Sr segregation near the surface, which is (La/Sr)O terminated. The conductivity vs film thickness shows the existence of nonmetallic layers with thickness $\sim 6.5\pm 0.9\mathrm{u}.\mathrm{c}.$, which is independent of film thickness but mainly relates to the deviation of Sr concentration near the surface region. Below 20 u.c., the surface of the films appears mixed, (La/Sr)O with ${\mathrm{MnO}}_2$ termination. Decreasing film thickness to less than 10 u.c. leads to the enhanced deviation of chemical composition in the films and eventually drives the film insulating. Our observation offers a natural explanation for the thickness-driven metal-nonmetal transition in thin films based on the variation of film stoichiometry.

We investigated the electrical transport properties and magnetic properties of oxygen-containing fluorite-type neodymium dideuteride, $\mathrm{Nd}{\mathrm{O}}_x{\mathrm{D}}_y$, epitaxial thin films $\left(0.05\le x\le 1.4\right)$ grown on $\mathrm{Ca}{\mathrm{F}}_2\left(111\right)$ substrate by a pulsed laser deposition technique. The $\mathrm{Nd}{\mathrm{O}}_x{\mathrm{D}}_y$ thin films with $x\le 0.3$ were crystallized in a pure fluorite phase, while the films with $x\ge 0.6$ were mixtures composed of fluorite and hexagonal $\mathrm{N}{\mathrm{d}}_2{\mathrm{O}}_3$-type phases. In the fluorite phase, ferromagnetism was observed from a magnetization loop and anomalous Hall effect. The films with $x<1.4$ showed metallic transport properties with an abrupt drop of resistivity below Curie temperature $T_C$. With increasing $x$, $T_C$ was raised to 10.0 K, which was higher than that of $\mathrm{Nd}{\mathrm{H}}_{2+x}$ with comparable carrier density.

Nuclear magnetic resonance (NMR) spin diffusion measurements have been widely used to estimate domain sizes in a variety of polymer materials. In cases where the domains are well-described as regular, repeating structures (e.g., lamellar, cylindrical channels, monodispersed spherical domains), the domain sizes estimated from NMR spin diffusion experiments agree with the characteristic length scales obtained from small-angle x-ray scattering and microscopy. In our laboratory, recent NMR spin diffusion experiments for hydrated sulfonated Diels Alder poly(phenylene) (SDAPP) polymer membranes have revealed that assuming a simple structural model can often misrepresent or overestimate the domain size in situations where more complex and disordered morphologies exist. Molecular dynamics simulations of the SDAPP membranes predict a complex heterogeneous hydrophilic domain structure that varies with the degree of sulfonation and hydration and is not readily represented by a simple repeating domain structure. This heterogeneous morphology results in NMR-measured domain sizes that disagree with length scales estimated from the ionomer peak in scattering experiments. Here we present numerical NMR spin diffusion simulations that show how structural disorder in the form of domain size distributions or domain clustering can significantly impact the spin diffusion analysis and estimated domain sizes. Simulations of NMR spin diffusion with differing domain size distributions and domain clustering are used to identify the impact of the heterogeneous domain structure and highlight the limitations of using NMR spin diffusion techniques for irregular structures.

We predict a new three-dimensional boron allotrope based on systematic first-principles electronic structure calculations. This allotrope can be derived by substituting each carbon atom in a hexagonal diamond lattice with a ${\mathrm{B}}_4$ tetrahedron and possesses the same space group $P{6}_3/mmc$ as hexagonal diamond, hence it is termed as H-boron. We show that H-boron has good stability and excellent mechanical property. Remarkably, we find that H-boron is a topological metal with rich types of spin-orbit-free emergent fermions, including semi-Dirac fermion, quadratic and linear triple-point fermion, nodal-line fermion, and nodal-surface fermion. We clarify their symmetry protections and characterize them by constructing the corresponding low-energy effective models. Our work not only discovers a new boron allotrope with excellent properties, it also offers a platform to explore interesting physics of new kinds of emergent fermions.

Molecular dynamics simulations of the temperature-dependent crystal growth rates of the salts, NaCl and ZnS, from their melts are reported, along with those of a number of pure metals. The growth rate of NaCl and the fcc-forming metals show little evidence of activated control, while that of ZnS and Fe, a bcc forming metal, exhibit activation barriers similar to those observed for diffusion in the melt. Unlike ZnS and Fe, the interfacial inherent structures of NaCl and Cu and Ag are found to be crystalline. We calculate the median displacement between the interfacial liquid and crystalline states and show that this distance is smaller than the “cage” length, demonstrating that crystal growth in the fast crystallizers can occur via local vibrations and by doing so largely avoid the activated kinetics associated with the larger displacements associated with particle transport.

In titanium alloys, the $\omega$(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal $\omega$ transformation that can occur even above the $\omega$ transformation temperature. In body-centered cubic $\beta$-titanium alloyed with $\beta$-stabilizing elements, there are locally unstable regions having fewer $\beta$-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal $\omega$ transformation occurs even when the entire $\beta$ region is stable on average so that athermal $\omega$ transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of $2/3{\left[111\right]}_{\beta }$ longitudinal phonon, which cannot be suppressed by the stabilization of $\beta$ phase on average. In the diffusionless isothermal and athermal $\omega$ transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of ${\left\{111\right\}}_{\beta }$ pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the $\omega$-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent $\beta /\omega$ interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the $\beta /\omega$ interface energy is the rate-determining process in the diffusionless $\omega$ transformations.

We present an extension of the ‘learn on the fly’ method to the study of the motion of dislocations in metallic systems, developed with the aim of producing information-efficient force models that can be systematically validated against reference QM calculations. Nye tensor analysis is used to dynamically track the quantum region centered at the core of a dislocation, thus enabling quantum mechanics/molecular mechanics simulations. The technique is used to study the motion of screw dislocations in Ni-Al systems, relevant to plastic deformation in Ni-based alloys, at a variety of temperature/strain conditions. These simulations reveal only a moderate spacing ($\sim 5\AA$) between Shockley partial dislocations, at variance with the predictions of traditional molecular dynamics (MD) simulation using interatomic potentials, which yields a much larger spacing in the high stress regime. The discrepancy can be rationalized in terms of the elastic properties of an hcp crystal, which influence the behavior of the stacking fault region between Shockley partial dislocations. The transferability of this technique to more challenging systems is addressed, focusing on the expected accuracy of such calculations. The bcc $\alpha \text{−}\mathrm{Fe}$ phase is a prime example, as its magnetic properties at the open surfaces make it particularly challenging for embedding-based QM/MM techniques. Our tests reveal that high accuracy can still be obtained at the core of a dislocation, albeit at a significant computational cost for fully converged results. However, we find this cost can be reduced by using a machine learning approach to progressively reduce the rate of expensive QM calculations required during the dynamical simulations, as the size of the QM database increases.