In this paper, the self-consistent PMMCS force fields (FFs) [Pedone et al., J. Phys. Chem. B 110, 11780 (2006)] widely used for the simulation of a large variety of silicates, aluminosilicate and phosphate crystals, and multicomponent oxide glasses have been revised and improved by the inclusion of two types of three-body interactions acting between T-O-T bridges ($T=\mathrm{Si}$ and P) and network former-network former repulsive interactions. The FFs named Bertani–Menziani–Pedone (BMP)-harm and BMP-shrm better reproduce the T-O-T bond angle distributions (BADs) and network former-oxygen distances. Consequently, the prediction of $Q^n$ distributions (Q stands for quaternary species, and $n$ is the number of bridging oxygens around it), neutron total distribution functions, solid-state nuclear magnetic resonance spectra of spin active nuclei (${}^{29}\mathrm{Si}$, ${}^{17}\mathrm{O}$, ${}^{31}\mathrm{P}$, ${}^{27}\mathrm{Al}$), and the density have also been hugely improved with respect to the previous version of our FF. These results also highlight the strong correlation between the T-O-T BADs and the other short and intermediate structural properties in oxide glasses, which have been largely neglected in the past. In addition to the improvement of the structure, the FF has been revealed to reproduce well the ionic conductivity in mixed alkali aluminosilicate glasses and the elastic properties. The systematic comparison with other interatomic potential models, including the polarizable core-shell model, carried out in this paper showed that our potential model is more balanced and effective for simulating a vast family of crystalline and amorphous oxide-based systems.

The structures of pyramidal-II $〈c+a〉$ dislocations, one of the most important defects in structural hexagonal-close-packed (HCP) metals, have not been fully characterized for many of the HCP metals in use today. Here, we employ ab initio informed phase-field dislocation dynamics to determine the minimum energy structure of pyramidal $\left\{\overline{1}\overline{1}22\right\}〈11\overline{2}3〉$ dislocations in ten HCP metals, including Be, Co, Mg, Re, Ti, Zn, Cd, Hf, Y, and Zr. As input for the simulations, we calculate, using first-principles density functional theory, the $\left\{\overline{1}\overline{1}22\right\}$ generalized stacking fault energy (GSFE) curves for all ten metals. From these calculations, it is found that magnetism in Co is necessary for achieving a local minimum in the GSFE curve. We observe in simulations that edge and screw character dislocations split into two partials separated by a low-energy intrinsic stacking fault. The splitting distance is shown to scale inversely with the local minimum energy normalized by the product of its shear modulus and Burgers vector. Interestingly, some HCP metals exhibit an asymmetric structure, with either unequal partial Burgers vectors or widths, in contrast to the symmetric configuration expected from linear elastic dislocation theory. We explain these structures by properties of the local maxima in their GSFE curves. Metals with larger degrees of elastic anisotropy result in dislocations with larger splitting distances than would be expected under the commonly used assumption of elastic isotropy. These findings on the sizes and asymmetry in the structures of pyramidal-II $〈c+a〉$ dislocations are fundamental to understanding how these dislocations glide and interact or react with other defects when these metals are mechanically strained.

The ab initio theory of low-field electronic transport properties such as carrier mobility in semiconductors is well-established. However, an equivalent treatment of electronic fluctuations about a nonequilibrium steady state, which are readily probed experimentally, remains less explored. Here, we report a first-principles theory of electronic noise for warm electrons in semiconductors. In contrast with typical numerical methods used for electronic noise, no adjustable parameters are required in the present formalism, with the electronic band structure and scattering rates calculated from first principles. We demonstrate the utility of our approach by applying it to GaAs and show that spectral features in AC transport properties and noise originate from the disparate time scales of momentum and energy relaxation, despite the dominance of optical phonon scattering. Our formalism enables a parameter-free approach to probe the microscopic transport processes that give rise to electronic noise in semiconductors.

Heterostructures of two semiconductors are at the heart of semiconductor devices with tremendous technological importance. The prediction and designing of semiconductor heterostructures of a specific type is a difficult materials science problem, posing a challenge to experimental and computational investigations. In this study, we first establish that the prediction of heterostructure type can be made with good accuracy from the knowledge of the band structure of constituent semiconductors. Following this, we apply machine learning, built on features characterizing constituent semiconductors, on a known dataset of binary semiconductor heterostructures extended by a synthetic minority oversampling technique. A significant feature of engineering made it possible to train a classifier model predicting the heterostructure type with an accuracy of $89\%$. Using the trained model, a large number (872 number) of unknown heterostructure semiconductor types involving elemental and binary semiconductors is theoretically predicted. Interestingly, the developed scheme is found to be extendable to heterojunctions of semiconductor quantum dots.

Tunneling atomic force microscopy (TUNA) was used at ambient conditions to measure the current-voltage $(I\text{−}V)$ characteristics at clean surfaces of highly oriented graphite samples with Bernal and rhombohedral stacking orders. The characteristic curves measured on Bernal-stacked graphite surfaces can be understood with an ordinary self-consistent semiconductor modeling and quantum mechanical tunneling current derivations. We show that the absence of a voltage region without measurable current in the $I\text{−}V$ spectra is not a proof of the lack of an energy band gap. It can be induced by a surface band bending due to a finite contact potential between tip and sample surface. Taking this into account in the model, we succeed to obtain a quantitative agreement between simulated and measured tunnel spectra for band gaps $(12...37)$ meV, in agreement with those extracted from the exponential temperature decrease of the longitudinal resistance measured in graphite samples with Bernal stacking order. In contrast, the surface of relatively thick graphite samples with rhombohedral stacking reveals the existence of a maximum in the first derivative $dI/dV$, a behavior compatible with the existence of a flat band. The characteristics of this maximum are comparable to those obtained at low temperatures with similar techniques.

Using density functional theory, we determined that $2\times 2$ and $\left(\sqrt{3}\times \sqrt{3}\right)\mathrm{R}{30}^{\circ }$ surface reconstructions of bulk-terminated GaN (0001) are degenerate in energy but differ in their electronic structure. Consistent with previous reports, our study of the adsorption energy of Ga adatoms shows that a laterally contracted Ga bilayer is the most energetically favorable surface arrangement under Ga-rich conditions, albeit with a different atomic arrangement of the Ga bilayer than previously reported. We also determined the potential energy surfaces for La and Ga adatoms on bulk-terminated GaN (0001) surface and for a La adatom on a bulk-terminated GaN (0001) covered by a contracted Ga bilayer and discuss possible adatom surface migration paths. An exchange reaction of a La adatom with surface Ga was found to be energetically favorable for both the bulk-terminated and the Ga bilayer-covered surfaces, indicating formation of LaN. A careful study of the diffusion of a La adatom through the Ga bilayer toward the bulk-terminated GaN surface suggests that a lower energy arrangement is achieved when the La adatom exchanges positions with a surface Ga atom. This indicates that ${\mathrm{LaGa}}_2$ and LaN are potential materials to form a transition layer to facilitate epitaxial integration of oxides on GaN.

We performed systematic transport measurements on FeSe single crystals with applying in-plane biaxial strain $\varepsilon$ ranging from $-0.96\%$ to 0.23%. Biaxial strain was introduced by firmly gluing samples to various substrate materials with different thermal expansion. With increasing $\varepsilon$, structural and superconducting transition temperatures monotonically increased and decreased, respectively. We analyzed magnetotransport results using a compensated three-carrier model. The evaluated densities of hole and electron carriers systematically changed with strain. This indicates that we succeeded in controlling the band structure of single-crystalline FeSe.

Blended organic thin films have been studied during the last decades due to their applicability in organic solar cells. Although their optical and electronic features have been examined intensively, there is still a lack of detailed knowledge about their growth processes and resulting morphologies, which play a key role in the efficiency of optoelectronic devices such as organic solar cells. In this study, pure and blended thin films of copper phthalocyanine (CuPc) and the Buckminster fullerene $\left({\mathrm{C}}_{60}\right)$ were grown by vacuum deposition onto a native silicon oxide substrate at two different substrate temperatures, 310 and 400 K. The evolution of roughness was followed by in situ real-time x-ray reflectivity. Crystal orientation, island densities, and morphology were examined after the growth by x-ray diffraction experiments and microscopy techniques. The formation of a smooth wetting layer followed by rapid roughening was found in pure CuPc thin films, whereas ${\mathrm{C}}_{60}$ shows a fast formation of distinct islands at a very early stage of growth. The growth of needlelike CuPc crystals losing their alignment with the substrate was identified in co-deposited thin films. Furthermore, the data demonstrate that structural features become larger and more pronounced and that the island density decreases by a factor of four when going from 310 to 400 K. Finally, the key parameters roughness and island density were well reproduced on a smaller scale by kinetic Monte Carlo simulations of a generic, binary lattice model with simple nearest-neighbor interaction energies. A weak molecule-substrate interaction caused a fast island formation and weak interaction between molecules of different species was able to reproduce the observed phase separation. The introduction of different same-species and cross-species Ehrlich-Schwoebel barriers for interlayer hopping was necessary to reproduce the roughness evolution in the blend and showed the growth of CuPc crystals on top of the thin film in agreement with the experiment.

$A$-site doping of $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ (SRO) has revealed that the evolution of ferromagnetism in SRO cannot be explained only by the $A$-site ion radii. Recently, ${\mathrm{Pb}}^{2+}$ doping, which introduces a lone-pair-electrons on $A$-site, has been found to weaken the ferromagnetism dramatically, indicating that a hybridization between Ru-$4d$ and Pb-$6s$ orbitals would play an important role in this system. Here we perform magnetotransport measurements for single crystalline ${\mathrm{Sr}}_{1-x}{\mathrm{Pb}}_x\mathrm{Ru}{\mathrm{O}}_3$ ($0\le x\le 1$) thin films grown on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ (001) substrate for elucidating the change of electronic structure and ferromagnetism. Sign inversion of AHE typically observed in SRO disappears as doping ${\mathrm{Pb}}^{2+}$, indicating a relationship between Weyl points and Fermi energy as suggested in previously reported Ca doped SRO system. Evidenced from longitudinal resistivity and Hall effect, ferromagnetic phase is weakened rapidly and vanishes at $x\approx 0.6$, which is in good agreement with bulk samples.

The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming ${\mathrm{O}}_2+{\mathrm{H}}_2$ reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in ${\mathrm{H}}_2$ produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the ${\mathrm{O}}_2+{\mathrm{H}}_2$ reaction involve a cyclic transformation between 3D-NiO and 2D-NiO.

Previously published data, not ours, on the coarsening of solid β-Sn particles in a liquid Pb-Sn matrix of near-eutectic composition are reanalyzed within the framework of the trans-interface-diffusion-controlled (TIDC) theory of coarsening. The data were obtained under conditions of microgravity from specimens heat-treated at 458 K and containing four equilibrium volume fractions $f_e$ equaling 0.10, 0.15, 0.20, and 0.30. We show that the rate constants $k\left(f_e\right)$ in the traditional coarsening equation ${\langle r\rangle }^3\approx k\left(f_e\right)t$ for the kinetics of growth of the average particle radius $\langle r\rangle$ are nearly independent of $f_e$, in disagreement with numerous theories wherein coarsening is controlled by diffusion in the host matrix phase. Atom transport in TIDC coarsening is instead controlled by slow diffusion through the diffuse interface, of width δ, separating the dispersed particles from the matrix; the kinetics of this process is independent of $f_e$. Atomistic simulations were performed to estimate the properties of the solid-liquid (S-L) interface at 458 K, 2 K above the Pb-Sn eutectic temperature. The S-L interfaces normal to (001) and (010) of tetragonal β-Sn were examined and found to have nearly identical properties, including interface widths of ∼1.7 nm. In conjunction with the diffusivities in solid β-Sn and liquid hypereutectic Pb-Sn at 458 K, we estimate that TIDC coarsening should prevail for solid Sn particles <∼1700 μm in radius, far exceeding the maximum radius of ∼100 μm measured experimentally. The TIDC theory also predicts that the kinetics of growth obeys the equation ${\langle r\rangle }^n\propto t$. The temporal exponent $n$ was evaluated to be ∼2.5, as ascertained by analyzing data on the particle size distributions (PSDs; histograms) for the alloys with $f_e=0.15$, 0.20, and 0.30. The histograms were converted to experimental cumulative distribution functions (ECDFs) and analyzed using the Kolmogorov-Smirnov (K-S) test applied to the theoretical CDFs predicted by the TIDC theory. We report the first successful application of the K-S test to experimental PSDs concomitant with particle coarsening. From every aspect of the experimental data amenable to analysis, we conclude that the coarsening behavior of solid Sn particles in liquid hypereutectic Pb-Sn alloys is fully consistent with the predictions of the TIDC theory of coarsening.

WTi is used as an adhesive layer in integrated circuit devices. The temperature dependent mechanical properties of WTi are still largely unexplored. In this paper we investigate WTi solid solutions with density functional theory calculations to determine the temperature and concentration dependent behavior of volume and coefficient of thermal expansion. The coefficient of thermal expansion is analyzed in terms of the bulk modulus, heat capacity, and Grüneisen parameter. Furthermore, we gain insight into the bonding of the system via investigation of the electronic structure, phonon density of states, and analysis of the formation energy. Low Ti concentrations lead to strong W-Ti bonding, as manifested in additional high frequency peaks in the phonon density of states. As a consequence, deviations from Vegard's law are found at low Ti concentrations, with a minimum of the lattice constant at about 15 at. % Ti. The CTE as a function of Ti concentration shows a negative trend at low temperatures and Ti concentrations, which is related to a strong decrease of heat capacity. Finally we show that the Debye-Grüneisen model yields results for WTi comparable to the quasiharmonic approach at a fraction of the computational cost.

Spin-transition materials can exhibit thermal hysteresis due to cooperative elastic interactions in between active molecular sites. It results from the coupling of the nonsymmetry-breaking high spin fraction order parameter to the lattice volume strain. However, a symmetry-breaking order parameter responsible for crystallographic phase transition, like a ferroelastic distortion, can also couple to the volume strain. Here, we use the Landau theory to study the elastic coupling between a spin-crossover instability and a discontinuous ferroelastic distortion and the different phase transition lines. Below the triple point, the first-order line involves simultaneous ferroelastic distortion and spin transition. Above, the purely first-order ferroelastic distortion and the spin crossover occur sequentially. Our model, related to the coupling and decoupling of the ferroelastic phase transition and spin crossover, explains exotic behaviors reported experimentally in the literature for diverse spin-crossover systems, plastic crystals, or Prussian blue analogues. The unsymmetric hysteresis loops and the stepwise evolution of the spin conversion or volume strain under pressure result from different sequences of phase transitions. The model shows that the ferroelastic phase transition is the driving force of the cooperative spin transition hysteresis in this case.

Vibrational thermal properties of $\mathrm{Cu}{\mathrm{Zn}}_2\mathrm{In}{\mathrm{Te}}_4$, $\mathrm{Ag}{\mathrm{Zn}}_2\mathrm{In}{\mathrm{Te}}_4$, and ${\mathrm{Cu}}_2\mathrm{CdSn}{\mathrm{Te}}_4$, derived from binary II-VI zinc-blendes, are reported based on first-principles calculations. While the chalcogenide atoms in these materials have the same lattice positions, the cation atom arrangements vary, resulting in different crystal symmetries and subsequent properties. The compositional differences have important effects on the vibrational thermal characteristics of the studied materials, which demonstrate that low-frequency optical phonons hybridize with acoustic phonons and lead to enhanced phonon-phonon scattering and low lattice thermal conductivities. The phonon density of states, mode Grüneisen parameters, and phonon scattering rates are also calculated, enabling deeper insight into the microscopic thermal conduction processes in these materials. Compositional variations drive differences among the three materials considered here; nonetheless, their structural similarities and generally low thermal conductivities (0.5–4 W/m K at room temperature) suggest that other similar II-VI zinc-blende derived materials will also exhibit similarly low values, as also corroborated by experimental data. This, combined with the versatility in designing a variety of motifs on the overall structure, makes quaternary chalcogenides interesting for thermal management and energy conversion applications that require low thermal conductivity.

We report the synthesis and initial characterization of ${\mathrm{Ba}}_6{\mathrm{Y}}_2{\mathrm{Rh}}_2{\mathrm{Ti}}_2{\mathrm{O}}_{17-\delta }$, a previously unreported material, to the best of our knowledge, with a hexagonal symmetry structure. Face-sharing $\mathrm{Rh}{\mathrm{O}}_6$ octahedra form triangular planes of ${\mathrm{Rh}}_2{\mathrm{O}}_9$ dimers that are widely separated in the perpendicular direction. The material displays a small effective magnetic moment, due to the Rh ions present, and a negative Curie-Weiss temperature. The charge transport and optical band gaps are very similar, near 0.16 eV. A large upturn in the heat capacity at temperatures below 1 K, suppressed by applied magnetic fields larger than ${\mu }_0\mathrm{H}=2\mathrm{T}$, is observed. A large T-linear term in the specific heat ($\gamma =166\mathrm{mJ}/{\mathrm{mol}\mathrm{f}.\mathrm{u}\mathrm{K}}^2$) is seen, although the material is insulating at low temperatures. These results suggest the possibility of a spin liquid ground state in this material.

Transition metal carbide is a class of materials characterized by high hardness, high melting points, and low chemical reactivity. It is widely used in industrial applications involving exposure to elevated temperatures, aggressive media, and heavy loads, and is thus of technological and industrial importance. In this paper the high-temperature thermodynamic properties of tungsten carbide, WC, is studied. At most temperatures below melting, WC assumes a hexagonal structure with essentially no vacancies ($\delta$-WC). Only at very high temperatures (around 3000 K), WC crystallizes in the cubic rocksalt structure ($\gamma$-WC), which is more common for the transition metal carbides and in the case for WC can contain up to 40% carbon vacancies. At lower temperatures, $\gamma$-WC can, however, form as thin interfacial structures or nanoparticles. Hence, the thermodynamic properties of both $\gamma$-WC and $\delta$-WC are of relevance. Here, we conduct a first-principles density-functional theory based computational investigation of the $\gamma$-WC and $\delta$-WC phases, which requires modeling of high carbon vacancy concentrations at high temperatures. The configurational degrees of freedom are modeled with an alloy cluster expansion and sampled through Monte Carlo simulations. To account for the dynamic instability of the cubic $\gamma$-WC phase at low temperatures, the vibrational degrees of freedom are treated using effective harmonic models constructed from $ab$ initio molecular dynamics simulations. Finally, we obtain a part of the W-C phase diagram in reasonably quantitative agreement with experimental data.