Infrared absorption cross sections and corresponding oscillator strengths of several intracenter transitions of double donors in silicon, interstitial magnesium (Mg; group IIA) and substitutional chalcogens (Ch = S; Se; group VI), were determined for impurity densities in the ranges $1\times {10}^{14}–1.6\times {10}^{15}\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^3$ for Mg and $2\times {10}^{13}–2\times {10}^{16}\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^3$ for chalcogens. The concentrations of electrically active atomic and diatomic donor centers were derived from the Hall effect measurements. The experimental integrated cross sections were obtained from low-temperature impurity absorption spectra. The oscillator strengths of related donor transitions were derived and compared with those for shallow single donors in silicon, both determined experimentally and predicted theoretically. The transitions of oscillator strengths of double donors follow the decreasing trend with decreasing radius of donor ground states and increasing an impurity binding energy.

Boron-rich compounds have attracted significant attention due to their promising and diverse physical properties, which include ultrahardness, resistance to oxidation and corrosion, and even superconductivity. Here, using a crystal structure search method based on first-principles calculations, we find a boron-rich silicon compound $\mathrm{Si}{\mathrm{B}}_{12}$ that is stable under moderate pressure of around 20 GPa, and which we predict is recoverable to ambient pressure. This silicon boride, with space group Pnnm, is structurally related to the $\gamma \text{−}{\mathrm{B}}_{28}$ boron phase. Specifically, the $\mathrm{Si}{\mathrm{B}}_{12}$ structure is formed by replacing the ${\mathrm{B}}_2$ pairs in $\gamma \text{−}{\mathrm{B}}_{28}$ with silicon atoms. Our calculations show that this Pnnm $\mathrm{Si}{\mathrm{B}}_{12}$ phase exhibits good thermal stability at moderate pressures above 20 GPa and temperatures to 900 K. We suggest this structure has dynamic stability at ambient pressure and remains stable to temperatures as high as 2000 K. Impressively, this $\mathrm{Si}{\mathrm{B}}_{12}$ phase possesses good light absorption and thermoelectrical properties, which are enhanced by its small and indirect band gap, doubly degenerate bands, and low lattice thermal conductivity. Our predictions should stimulate further investigations of this class of boron-rich semiconductors, especially in view of their superior photovoltaic and thermoelectric properties which may be beneficial in energy applications.

The recent discovery of the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) material family offers an exciting opportunity to investigate the interplay of correlations, topology, and superconductivity in kagome metals. The low-energy physics of these materials is dominated by an unusual charge density wave phase, but little is understood about the true nature of the order parameter. In this work, we use a combination of ultrafast coherent phonon spectroscopy and first-principles density functional theory calculations to investigate the charge density wave order in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. We find that the charge density wave is the result of a simultaneous condensation of three optical phonon modes at one $M$ and two $L$ points. This distortion can be described as tri-hexagonal ordering with an interlayer modulation along the $c$ axis. It breaks the $C_6$ rotational symmetry of the crystal and may offer a natural explanation for reports of uniaxial order at lower temperatures in this material family.

Based on simulation results and theoretical analysis according to an atomistically informed continuum-scale model for the surface morphological response of tungsten plasma-facing component (PFC) in nuclear fusion devices, we demonstrate that the onset of the well-known surface nanostructure (known as “fuzz”) formation in PFC tungsten is the outcome of a stress-induced surface morphological instability. Specifically, we show that formation and growth of nanotendrils emanating from the exposed surface of PFC tungsten, a precursor to fuzz formation, is caused by a long-wavelength surface morphological instability triggered by compressive stress in the PFC near-surface layer due to over-pressurized helium (He) bubbles forming in this near-surface region through implantation of low-energy He ions from the plasma. Using linear stability theory (LST), we predict the onset of surface growth in response to low-amplitude perturbations from a planar PFC surface morphology and calculate the average spacing between nanotendrils growing from the PFC surface, in excellent agreement with self-consistent dynamical simulations of surface morphological response according to the fully nonlinear surface evolution model starting with random fluctuations from the planar surface morphology that result in nanoscale surface roughness. In addition, we examine the morphological response of the PFC surface to low-amplitude perturbations of very long wavelengths from its planar morphology and interpret fully the simulation results on the basis of a weakly nonlinear tip-splitting instability theory, which predicts a post-instability nanotendril pattern formation with nanotendril separation consistent with the LST predictions regardless of the initial surface perturbation. Finally, we compare our simulation predictions for surface nanotendril growth with experimental measurements of fuzz layer growth on the exposed surface of PFC tungsten. Our simulation results are in very good agreement with the surface growth measurements at the early stages of fuzz growth, further establishing the onset of fuzz formation in PFC tungsten as the outcome of a stress-driven PFC surface morphological instability.

Over the course of more than 70 years, several theoretical models for electric field-enhanced ion diffusion in crystalline insulators have been presented. However, there is no assessment of the validity of these models despite the urge to have a correct description of field-enhanced ion diffusion in several emerging technologies. Herein, an assessment of five models was carried out by computing the field-dependent migration enthalpy $\mathrm{\Delta }{H}_{\mathrm{mig}}\left(E\right)$ for the oxide ion in ${\mathrm{CeO}}_2$. The input to these models is the zero-field and zero-temperature migration pathway and/or the activation barrier obtained from an interatomic potential. $\mathrm{\Delta }{H}_{\mathrm{mig}}\left(E\right)$ from these models was compared with reference values obtained from a set of classical molecular-dynamics simulations in the temperature range of $1000\le T\le 1600\mathrm{K}$ and the electric field range of $0\le E\le 30\mathrm{MV}/\mathrm{cm}$. It is revealed that the most successful theoretical models are those that consider local polarization effects induced in the diffusing ion itself and its vicinity. However, all the models did not account for the effect of the electric field and finite temperature on the local polarization, leading to discrepancies between the predicted and the reference $\mathrm{\Delta }{H}_{\mathrm{mig}}\left(E\right)$, particularly at high fields. By comparing results from a rigid-ion potential and a polarizable core-shell potential, it is concluded that an intraionic polarization degree of freedom in the polarizable potential is an important factor in predicting $\mathrm{\Delta }{H}_{\mathrm{mig}}\left(E\right)$, regardless of the field-enhanced diffusion model particularly for low to moderate field. Future more accurate treatments should consider the field- and temperature-dependent interionic and intraionic local polarization effects.

We report on the inhomogeneity of the internal bias according to the Zr alloy concentration confirmed by the ferroelectric switching dynamics of ${\mathrm{Hf}}_{1-x}{\mathrm{Zr}}_x{\mathrm{O}}_2$ thin films. The analytic model for the internal bias was considered in terms of the dipole-dipole interaction accompanied by Zr concentration variation. The ferroelectricity and switching dynamics of ${\mathrm{Hf}}_{1-x}{\mathrm{Zr}}_x{\mathrm{O}}_2$ thin films were investigated using conventional electrical measurements and local piezoresponse microscopy. Analysis of static and dynamic polarization reversal revealed a correlation between the activation field for polarization switching and Zr concentrations through the variation in local ferroelectric coercivity and the characteristic ferroelectric switching time. The spatially inhomogeneous local built-in electric field was attributed to the defect interaction in the ${\mathrm{Hf}}_{1-x}{\mathrm{Zr}}_x{\mathrm{O}}_2$ thin films.

Pyrite structure transition-metal disulfides exhibit diverse ground states vs $d$-band filling, spanning diamagnetic semiconducting, ferromagnetic metallic, antiferromagnetic Mott insulating, and superconducting in $\mathrm{Fe}{\mathrm{S}}_2, \mathrm{Co}{\mathrm{S}}_2, \mathrm{Ni}{\mathrm{S}}_2$, and $\mathrm{Cu}{\mathrm{S}}_2$. $\mathrm{Ni}{\mathrm{S}}_2$ is particularly interesting and poorly understood as its Mott insulating behavior is accompanied by complex antiferromagnetic ordering below ∼38 K and perplexing weak ferromagnetism below ∼30 K. Temperature-, pressure-, and composition-dependent insulator-metal transitions also occur, particularly in bandwidth-controlled $\mathrm{Ni}{\mathrm{S}}_{2-x}{\mathrm{Se}}_x$, hole-doped ${\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{S}}_2$, etc. Here, we use high-quality chemical-vapor-transport-grown $\mathrm{Ni}{\mathrm{S}}_2$ single crystals characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetometry, and extensive transport and magnetotransport measurements, to generate new insight into this system. In particular, resistivity, magnetoresistance, and Hall effect analyses vs temperature, thickness, and surface preparation, provide unequivocal evidence of surface conduction, where the more conductive surface shunts essentially all current at low temperatures. The surface transport changes from two dimensional and insulating to three dimensional and metallic as the surface preparation is varied (also displaying intriguing sensitivity to magnetic ordering), significantly clarifying literature ambiguities with respect to the electronic ground state. These results have immediate implications. First, the temperature-, pressure-, and composition-dependent insulator-metal transitions deduced in the extensive prior work on $\mathrm{Ni}{\mathrm{S}}_{2-x}{\mathrm{Se}}_x, {\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{S}}_2$, etc., must clearly be reexamined in light of rife metallic surface conduction, not previously taken into account. Second, $\mathrm{Ni}{\mathrm{S}}_2$ now joins $\mathrm{Fe}{\mathrm{S}}_2$ and $\mathrm{Co}{\mathrm{S}}_2$ as systems in which bulk and surface electronic behaviors are strikingly different, suggesting that metallic surface states could be a universal feature of pyrite structure transition-metal disulfides.

Heat conduction in metals is typically dominated by electron transport since electrons carry both charge and heat. In magnetic metals magnons, or spin waves, excitations of the magnetic order can be used to transport information. Heat conduction via magnons has been previously shown mostly for insulating magnets with low Gilbert damping and resulting long spin-wave lifetimes where conduction electrons cannot contribute. Here we show that thin films of properly optimized metallic ferromagnetic (FM) alloys show significant nonelectronic contributions to heat conduction, which furthermore depend on the direction of an applied magnetic field. These measurements are enabled by micromachined thermal isolation platforms optimized for thermal conductivity measurements of thin-film systems. Electrical conductivity measurements on exactly the same samples allow application of the Wiedemann-Franz relation, which shows large nonelectronic contributions to thermal conductivity for the cobalt-iron alloy with $25\%$ Co. This composition has been shown to have exceptionally low damping for a metallic FM. The thermal conductivity of a 75-nm-thick film of the $25\%$ Co alloy changes by more than $20\%$ at some temperatures, while a reference sample with $50\%$ cobalt that has much higher damping shows no field-direction dependence. Our measurements indicate that applied magnetic fields alter the magnon lifetimes in these films and that these magnons contribute to thermal conductivity in this metallic magnetic alloy with low Gilbert damping.

To understand the physics of hydrogen embrittlement at the atomic scale, a general-purpose neural network interatomic potential (NNIP) for the $\alpha$-iron and hydrogen binary system is presented. It is trained using an extensive reference database produced by density functional theory (DFT) calculations. The NNIP can properly describe the interactions of hydrogen with various defects in $\alpha$-iron, such as vacancies, surfaces, grain boundaries, and dislocations; in addition to the basic properties of $\alpha$-iron itself, the NNIP also handles the defect properties in $\alpha$-iron, hydrogen behavior in $\alpha$-iron, and hydrogen-hydrogen interactions in $\alpha$-iron and in vacuum, including the hydrogen molecule formation and dissociation at the $\alpha$-iron surface. These are superb challenges for the existing empirical interatomic potentials, like the embedded-atom method based potentials, for the $\alpha$-iron and hydrogen binary system. In this study, the NNIP was applied to several key phenomena necessary for understanding hydrogen embrittlement, such as hydrogen charging and discharging to $\alpha$-iron, hydrogen transportation in defective $\alpha$-iron, hydrogen trapping and desorption at the defects, and hydrogen-assisted cracking at the grain boundary. Unlike the existing interatomic potentials, the NNIP simulations quantitatively described the atomistic details of hydrogen behavior in the defective $\alpha$-iron system with DFT accuracy.

We present an automated procedure for computing stacking fault energies in random alloys from large-scale simulations using moment tensor potentials (MTPs) with the accuracy of density functional theory (DFT). To that end, we develop an algorithm for training MTPs on random alloys. In the first step, our algorithm constructs a set of $\sim 10$ 000 or more training candidate configurations with 50–100 atoms that are representative for the atomic neighborhoods occurring in the large-scale simulation. In the second step, we use active learning to reduce this set to $\sim 100$ most distinct configurations—for which DFT energies and forces are computed and on which the potential is ultimately trained. We validate our algorithm for the MoNbTa medium-entropy alloy by showing that the MTP reproduces the DFT $\frac{1}{4}\left[111\right]$ unstable stacking fault energy over the entire compositional space up to a few percent. Contrary to state-of-the-art methods, e.g., the coherent potential approximation (CPA) or special quasi-random structures (SQSs), our algorithm naturally accounts for relaxation, is not limited by DFT cell sizes, and opens opportunities to efficiently investigate follow-up problems, such as chemical ordering. In a broader sense, our algorithm can be easily modified to compute related properties of random alloys, for instance, misfit volumes, or grain boundary energies. Moreover, it forms the basis for an efficient construction of MTPs to be used in large-scale simulations of multicomponent systems.

The ${\mathrm{Ce}}_{(1-x)}{\mathrm{La}}_x{\mathrm{CrGe}}_3$ $(x=0, 0.19, 0.43, 0.58, \mathrm{and} 1)$ intermetallic compound system has been investigated by magnetization measurements and neutron scattering techniques to determine the effect of La doping on the magnetic ordering and exchange interaction between Cr ions. The structural and magnetic characterization in this series was first verified by x-ray diffraction and bulk magnetization measurements. The samples exhibit the known hexagonal perovskite structure ($P{6}_3/\mathrm{mmc}$ space group) and have a single magnetic phase according to magnetization measurements. In this paper, the ferromagnetic ordering temperature for Cr evolves smoothly from a range of 68 K to 77 K for ${\mathrm{CeCrGe}}_3$ to a range of 91 K to 96 K for ${\mathrm{LaCrGe}}_3$ as La replaces Ce. Magnetization results indicate the formation of domain walls below the transition temperature for all ${\mathrm{Ce}}_{(1-x)}{\mathrm{La}}_x{\mathrm{CrGe}}_3$ systems investigated. Neutron results indicate ordered magnetic Cr moments aligned along the $c$ axis for the $x=1$ ${\mathrm{LaCrGe}}_3$ system, as well as for $x=0.19$, 0.43, and 0.58, which contrasts with the $x=0 {\mathrm{CeCrGe}}_3$ where the moments order in the ab plane.

The thickness dependence of the crystal structure in epitaxial $\left({\mathrm{Hf}}_x{\mathrm{Zr}}_{1-x}\right){\mathrm{O}}_2$ $\left(x=0,0.5,1\right)$ films is demonstrated. X-ray diffraction measurements suggest that the crystal phase changes from the monoclinic to the orthorhombic and tetragonal phases sequentially as film thickness decreases in $\mathrm{Hf}{\mathrm{O}}_2$ and $\left({\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}\right){\mathrm{O}}_2$ films. Both films are very thin at the ferroelectric orthorhombic phase $\left(<5\mathrm{nm}\right)$, but the $\left({\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}\right){\mathrm{O}}_2$ films are thicker than the $\mathrm{Hf}{\mathrm{O}}_2$ films. In $\mathrm{Zr}{\mathrm{O}}_2$ films, the monoclinic phase changes into the tetragonal phase as film thickness decreases. This means that the ferroelectric orthorhombic phase was not observed in the present study. However, ferroelectricity was observed for the present $\mathrm{Zr}{\mathrm{O}}_2$ films in the film thickness around the thickness boundary where the tetragonal and monoclinic phases stabilize. We confirmed the field-induced phase transition from the tetragonal to the orthorhombic phase in these $\mathrm{Zr}{\mathrm{O}}_2$ films by micro-x-ray diffraction measurement, which suggested that the phase transition takes place due to the small free energy difference between the ferroelectric orthorhombic phase and paraelectric tetragonal phase. Our experimental results agree with theoretical reports of the thickness dependence of the crystal structure in epitaxial films with a surface energy effect.

Epitaxial metal-semiconductor heterostructures with suitable Schottky barrier can lead to high thermoelectric figure-of-merit (zT) due to selective filtering of low-energy electrons as well as reduced thermal conductivity from phonon scattering at the interfaces. Lattice-matched vertical metal-semiconductor multilayer/superlattices as well as metallic nanoparticles embedded inside semiconducting hosts have been studied intensively to explore their thermoelectric properties. However, development of in-plane metal-semiconductor heterostructures and exploration of their physical properties have remained elusive primarily due to the growth and fabrication challenges. In-plane heterostructures are expected to be more suitable for planar integration and should exhibit unique properties. In this work, we demonstrate an in-plane ${\mathrm{Cr}}_2\mathrm{N}\text{−}\mathrm{CrN}$ metal-semiconductor heterostructure that exhibits an improved thermoelectric power factor. The in-plane heterostructure is deposited by controlling the Cr-flux during deposition that leads to an in-plane phase separation between the metallic-${\mathrm{Cr}}_2\mathrm{N}$ and semiconducting CrN grains. Temperature-dependent electrical transport exhibits an Arrhenius-type thermal activation behavior with an activation energy of 70 meV, and an in-plane electrical conductivity that is about two orders of magnitude higher than that of CrN. The Seebeck coefficient also remained moderately large at −150 μV/K at 700K leading to a very large power factor of $2.1\mathrm{mW}/\mathrm{m}{\mathrm{K}}^2$ at 700 K.

We report the discovery of superconductivity in noncentrosymmetric NbReSi, which crystallizes in a hexagonal ZrNiAl-type crystal structure with space group $P\overline{6}2m$ (No. 189). Bulk superconductivity, with $T_c=6.5\mathrm{K}$ was characterized via electrical-resistivity, magnetization, and heat-capacity measurements. The low-temperature electronic specific heat suggests a fully gapped superconducting state in NbReSi, while a large upper critical field of ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)\sim 12.6 \mathrm{T}$ is obtained, which is comparable to the weak-coupling Pauli limit. The electronic band-structure calculations show that the density of states at the Fermi level are dominated by Re and Nb $d$ orbitals, with a sizable band splitting induced by the antisymmetric spin-orbit coupling. NbReSi represents another candidate material for revealing the puzzle of time-reversal symmetry breaking observed in some Re-based superconductors and its relation to the lack of inversion symmetry.

We present ab initio calculations of the electronic and optical properties of hexagonal ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloys in the lonsdaleite structure. Lattice constants and electronic band structures in excellent agreement with experiment are obtained using density-functional theory. Hexagonal Si has an indirect band gap, whereas hexagonal Ge has a pseudo-direct gap, i.e., the optical transitions at the minimum direct band gap are very weak. The pseudo-direct character of pure hexagonal Ge is efficiently lifted by alloying. Already for a small admixture of Si, symmetry reduction enhances the oscillator strength of the lowest direct optical transitions. The band gap is direct for a Si content below 45 %. We validate lonsdaleite group-IV alloys to be efficient optical emitters, suitable for integrated optoelectronic applications.

In the 1960s, it was discovered in the former Soviet Union that short pulses of intense electric currents through metallic wires brought about temporary drops in the flow stress of the metal as it is deformed plastically. This became known as the electroplastic effect, or electroplasticity. After more than 50 years of experimental and theoretical research, no consensus has emerged as to the mechanism of the effect. Following a brief review of the principal experimental results, we show that when a current flows through a metal the ionic cores of atoms of the metal experience a force equal to the Lorentz force on the conduction electrons, which arises from the magnetic field created by the current. This is the origin of the pinch effect in metals. We then present a new theory of electroplasticity based on mechanical stresses created by pulsing the current as a result of electromagnetic induction. Unlike earlier theories, the rate of change of the current is treated explicitly in a dynamic version of the pinch effect. Pulses of normal and shear stresses arise with a magnitude that depends on the rate at which the current changes during a pulse and a small number of other well defined variables. Unlike earlier theories which focused on the maximum of the current during a pulse, this new theory highlights the time dependence of the current pulse as well as its maximum value. Experiments to test the theory are suggested. Four mechanisms proposed earlier for the electroplastic effect are reviewed critically in the appendices. They are dislocation unpinning in a magnetic field, electromigration of dislocations, Joule heating, and the static pinch effect. We show that the physics of the first two mechanisms is unsound and the second two cannot explain most experimental observations.

Muffin-tin methods have been instrumental in the design of honeycomb lattices that show, in contrast to graphene, separated $s$ and in-plane $p$ bands, a $p$ orbital Dirac cone, and a $p$ orbital flat band. Recently, such lattices have been experimentally realized using the two-dimensional electron gas on Cu(111). A possible next avenue is the introduction of spin-orbit coupling to these systems. Intrinsic spin-orbit coupling is believed to open topological gaps and create a topological flat band. Although Rashba coupling is straightforwardly incorporated in the muffin-tin approximation, intrinsic spin-orbit coupling has only been included either for a very specific periodic system, or only close to the Dirac point. Here, we introduce effective intrinsic and Rashba spin-orbit terms in the Hamiltonian for both periodic and finite-size systems. We observe a strong band opening over the entire Brillouin zone between the $p$ orbital flat band and the Dirac cone hosting a pronounced edge state, robust against the effects of Rashba spin-orbit coupling.

We performed molecular dynamics simulations of the high-temperature oxidation of metal alloys composed of Al, Cr, and Fe and compared their behavior with that of pure Fe. The metal alloy elements (Al and Cr) segregated to the surface during oxidation, producing a lower stress gradient at the metal/environment interface compared to pure Fe. We have found that the lowered stress gradients produced in the alloy material appear to play a key role in the development of corrosion. Interfaces with lower stress gradients have reduced rates of ${\mathrm{H}}_2\mathrm{O}$ adsorption, especially for the ferritic (bcc) alloys. The diffusivity of oxygen and hydrogen drops more rapidly for the interfaces with reduced stress gradients. The stress gradient is also diminished when the gas pressure is increased, indicating that the Fe-Cr-Al alloy system is more resistant to oxidation than pure Fe at higher pressures. Therefore, we conclude that the lower stress gradients at the alloy/environment interface reduce the stress concentration and can slow down the rate of the initial oxide scale growth. We also compared bcc and fcc alloys with pure Fe based on our three evaluation criteria (peak stress, stress gradient, and summation of stress in the oxide scale). We found that the alloys have lower values under the three criteria compared to pure Fe. The bcc alloy has the best score under a water rich environment and the fcc alloy is proven to be better for peak stress and summation of stress in the oxide scale under an oxygen rich environment. For surface segregation to occur, we find that a minimum content of Al or Cr in the near-surface region must be achieved. We also learned that the role of Al is more important than that of Cr in terms of corrosion resistant behavior at relatively higher temperatures for the Fe-Cr-Al ternary alloys.