We demonstrate the agreement between first-principles calculations and experimental measurements of size effects in thermal transport in SiGe alloys without fitting parameters. Transient thermal grating (TTG) is used to measure the effect of the grating period on the temperature decay. The virtual crystal approximation under the density-functional-theory framework combined with impurity scattering is used to determine the phonon properties for the exact alloy composition of the measured samples. With these properties, classical size effects are calculated for the experimental geometry of reflection mode TTG using the recently developed variational solution to the phonon Boltzmann transport equation, which is verified against established Monte Carlo simulations. We find agreement between theoretical predictions and experimental measurements in the reduction of thermal conductivity (as much as fourfold of the bulk value) across grating periods spanning one order of magnitude. This paper provides a framework for the study of size effects in thermal transport in opaque materials.

Heterojunctions are the backbone of established semiconductor technology. The highly desirable reliable creation of dielectrically defined heterojunctions in transition metal dichalcogenide monolayers (TMD-MLs) requires an in-depth understanding of dielectric screening effects induced by the ML's environment. Here we report on the modulations of excitonic transitions in TMD-MLs through the effect of dielectric environments including low-$k$ and high-$k$ dielectric materials. We present absolute tuning ranges as large as 37 meV for the optical band gaps of ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$ MLs and relative tuning ranges on the order of 15% for the binding energies of charged excitons. Additionally, we measure relative changes of 30% in the energy splittings of exciton Rydberg states of ${\mathrm{WSe}}_2$. The findings enable us to estimate changes in the exciton binding energies and the electronic band gaps of the studied materials.

We present a three-dimensional study on the anisotropy of the charge density wave (CDW) in $1T-\mathrm{TiS}{\mathrm{e}}_2$, by means of in situ atomically resolved electron microscopy at cryogenic temperatures in both reciprocal and real spaces. Using coherent nanoelectron diffraction, we observed short-range coherence of the in-plane CDW component while the long-range coherence of out-of-plane CDW component remains intact. An in-plane CDW coherence length of $\sim 10\mathrm{nm}$ and an out-of-plane CDW coherence length of 17.5 nm, as a lower bound, were determined. The electron modulation was observed using electron energy-loss spectroscopy and verified by an orbital-projected density of states. Our integrated approach reveals anisotropic CDW domains at the nanoscale, and illustrates electron modulation-induced symmetry breaking of a two-dimensional material in three dimensions, offering an opportunity to study the effect of reduced dimensionality in strongly correlated systems.

A ferromagnetic insulator in contact with a superconductor is known to induce an exchange splitting of the singularity in the Bardeen-Cooper-Schrieffer (BCS) density of states (DoS). The magnitude of the splitting is proportional to the exchange field that penetrates into the superconductor to a depth comparable with the superconducting coherence length and which ranges in magnitude from a few to a few tens of tesla. We study this magnetic proximity effect in EuS/Al bilayers and show that the domain structure of the EuS affects the positions and the line shapes of the exchange-split BCS peaks. Remarkably, a clear exchange splitting is observed even in the unmagnetized state of the EuS layer, suggesting that the domain size of the EuS is comparable with the superconducting coherence length. Upon magnetizing the EuS layer, the splitting increases while the peaks change shape. Conductance measurements as a function of bias voltage at the lowest temperatures allowed us to relate the line shape of the split BCS DoS to the characteristic domain structure in the ultrathin EuS layer. These results pave the way to engineering triplet superconducting correlations at domain walls in EuS/Al bilayers. Furthermore, the hard gap and large splitting observed in our tunneling spectroscopy measurements make EuS/Al an excellent candidate for substituting strong magnetic fields in experiments studying Majorana bound states.

We report on measurements and analysis of ionoluminescence properties of pure polystyrene films and polystyrene films doped with four types of fluorophores in low kinetic energies (50–350 keV) of ion irradiation. We have developed a theoretical model to understand the experimentally observed ionoluminescence behaviors in terms of scintillation yield from individual ion tracks, photophysical energy transfer mechanisms, and irradiation-induced defects. A comparison of the model and experimental results suggests that singlet up-conversion resulting from triplet-triplet annihilation processes may be responsible for enhanced singlet emission of the fluorophores at high ion beam flux densities. Energy transfer from the polystyrene matrix to the fluorophore molecules has been identified as an effective pathway to increasing the fluorescence efficiency in the doped scintillator films.

Ion implantation of Mn combined with pulsed laser melting is employed to obtain two representative compounds of dilute ferromagnetic semiconductors (DFSs): $\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{M}{\mathrm{n}}_x\mathrm{As}$ and $\mathrm{I}{\mathrm{n}}_{1-x}\mathrm{M}{\mathrm{n}}_x\mathrm{As}$. In contrast to films deposited by the widely used molecular beam epitaxy, neither Mn interstitials nor As antisites are present in samples prepared by the method employed here. Under these conditions the influence of localization on the hole-mediated ferromagnetism is examined in two DFSs with a differing strength of p-d coupling. On the insulating side of the transition, ferromagnetic signatures persist to higher temperatures in $\mathrm{I}{\mathrm{n}}_{1-x}\mathrm{M}{\mathrm{n}}_x\mathrm{As}$ compared to $\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{M}{\mathrm{n}}_x\mathrm{As}$ with the same Mn concentration $x$. This substantiates theoretical suggestions that stronger p-d coupling results in an enhanced contribution to localization, which reduces hole-mediated ferromagnetism. Furthermore, the findings support strongly the heterogeneous model of electronic states at the localization boundary and point to the crucial role of weakly localized holes in mediating efficient spin-spin interactions even on the insulator side of the metal-insulator transition.

We have developed an automatic machine learning potential (MLP) construction scheme, the self-learning and adaptive database (SLAD). The sample structures for training are collected by the molecular dynamics simulations using the MLP itself with the aid of the spilling factor for simultaneous error estimation. The utility of the SLAD is demonstrated by applying it for the solid-state ionic conductor, ${\mathrm{Li}}_2{\mathrm{B}}_{12}{\mathrm{H}}_{12}$. Starting from the low-temperature $\alpha$-phase structure, the MLP is successfully developed, for which the total number of density functional calculations required is only 84. The constructed MLP reproduces well the phase transition to the high-temperature $\beta$ phase accompanied by an abrupt lattice expansion, promotion of Li diffusion, and orientational disordering of ${\mathrm{B}}_{12}{\mathrm{H}}_{12}$ complexes. The SLAD allows us the efficient construction of MLPs for complicated systems.

Defects at the $\text{Si}\left(001\right)/a\text{−}{\mathrm{SiO}}_2$ interface were simulated using a combination of classical force field molecular dynamics and first-principles total-energy minimization techniques. The second generation charge-optimized many-body potential COMB10 was used to generate $a\text{−}{\mathrm{SiO}}_2$ which was placed on an ideally terminated Si(001) surface and briefly annealed before being relaxed using density functional theory. Si dimers form at the Si interface on relaxation unless the Si surface has been oxidized. $P_{b1}$ defects form when Si dimers do not bond to O atoms in the $a\text{−}{\mathrm{SiO}}_2$ layer. Mismatch in Si atom pairings at the dimerized surface leads to ${\mathrm{Si}}_2\mathrm{O}\equiv \mathrm{Si}·$ defects at the interface which may be the structure of a defect denoted $S$ in electron-spin resonance (ESR) studies. Relatively few $P_{b0}$ defects with a magnetic moment form at the Si interface. $a\text{−}{\mathrm{SiO}}_2$ generated by heating and quenching a system with periodic boundary conditions is free of $E^{\prime }$ defects. Afterward, this structure is placed on the Si surface and allowed to relax. The main type of $E^{\prime }$ defect which forms is a forward oriented $E_{\alpha }^{\prime }$ defect. ESR hyperfine parameters for each defect category are calculated and compared to experimental data. Defect densities of states are calculated and compared to experiment.

We introduce the concept of transport waves by showing that the linearized Boltzmann transport equation admits excitations in the form of waves that have well-defined dispersion relations and decay times. Crucially, these waves do not represent single-particle excitations, but are collective excitations of the equilibrium distribution functions. We study in detail the case of thermal transport, where relaxons are found in the long-wavelength limit, and second sound is reinterpreted as the excitation of one or several temperature waves at finite frequencies. Graphene is studied numerically, finding decay times of the order of microseconds. The derivation, obtained by a spectral representation of the Boltzmann equation, holds in principle for any crystal or semiclassical transport theory and is particularly relevant when transport takes place in the hydrodynamic regime.

Controlling the composition of ternary III-V semiconductor nanowires is of high technological importance and the current theoretical understanding is so far limited. We derive a model for the kinetically limited composition of metal-particle-seeded, ternary nanowires. The model is based on the diffusion controlled growth rate of supercritical nuclei. Applying this model to gold-seeded and self-seeded growth of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ we are able to explain the experimentally observed features related to nanowire compositions, including the attainability of compositions within the miscibility gap. By directly comparing with experiments we find that 2% arsenic in the alloy particle during self-seeded growth of InGaAs nanowires is a realistic assumption.

We study late-stage solidification and the associated formation of defects in alloy materials using a novel model based on the phase-field-crystal technique. It is shown that our model successfully captures several important physical phenomena that occur in the late stages of solidification, including solidification shrinkage, liquid cavitation and microsegregation, all in a single framework. By examining the interplay of solidification shrinkage and solute segregation, this model reveals that the formation of gas pore defects at the late stage of solidification can lead to nucleation of second phase solid particles due to solute enrichment in the eutectic liquid driven by gas-phase nucleation and growth. We also predict a modification of the Gulliver-Scheil equation in the presence of gas pockets in confined liquid pools.

Thermoelectric properties of a semiconducting silicide, $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$, were investigated by first-principles calculations. The calculation results revealed that $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ has a relatively low lattice thermal conductivity of around $1.2\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 800 K. The Seebeck coefficients and the electrical conductivities of $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ were evaluated by using the Boltzmann transport equation with an energy-dependent relaxation time under the assumption of electron scattering by acoustic phonons. The Seebeck coefficient of $n$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ along the $x$ axis is larger than that along the $z$ axis, while the Seebeck coefficient of $p$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ along the $x$ axis is smaller than that along the $z$ axis. The electrical conductivity of $p$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ is higher than that of $n$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ owing to the smaller effective mass of holes, which results in the higher power factor of $p$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$. Maximum $ZT$ (a dimensionless figure of merit) of single-crystalline $p$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ is higher than that of $n$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$, reaching 0.9 at 800 K. Grain-size effects on the lattice thermal conductivities and power factors were also investigated. Reducing lattice thermal conductivities overcomes the decrease of electrical conductivities and thereby enhances $ZT$, taking maximum of 1.0 for $n$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ and 1.5 for $p$-type $\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_4$ when the grain size is 10 nm.

We report a high-pressure study of tetragonal scheelite-type $\mathrm{CaMo}{\mathrm{O}}_4$ up to 29 GPa. In order to characterize its high-pressure behavior, we have combined Raman and optical-absorption measurements with density functional theory calculations. We have found evidence of a pressure-induced phase transition near 15 GPa. Experiments and calculations agree in assigning the high-pressure phase to a monoclinic fergusonite-type structure. The reported results are consistent with previous powder x-ray-diffraction experiments, but are in contradiction with the conclusions obtained from earlier Raman measurements, which support the existence of more than one phase transition in the pressure range covered by our studies. The observed scheelite-fergusonite transition induces significant changes in the electronic band gap and phonon spectrum of $\mathrm{CaMo}{\mathrm{O}}_4$. We have determined the pressure evolution of the band gap for the low- and high-pressure phases as well as the frequencies and pressure dependencies of the Raman-active and infrared-active modes. In addition, based on calculations of the phonon dispersion of the scheelite phase, carried out at a pressure higher than the transition pressure, we propose a possible mechanism for the reported phase transition. Furthermore, from the calculations we determined the pressure dependence of the unit-cell parameters and atomic positions of the different phases and their room-temperature equations of state. These results are compared with previous experiments showing a very good agreement. Finally, information on bond compressibility is reported and correlated with the macroscopic compressibility of $\mathrm{CaMo}{\mathrm{O}}_4$. The reported results are of interest for the many technological applications of this oxide.

Linking the experimentally accessible macroscopic properties of thermoplastic polymers to their microscopic static and dynamic properties is a key requirement for targeted material design. Classical molecular dynamics simulations enable us to study the structural and dynamic behavior of molecules on microscopic scales, and statistical physics provides a framework for relating these properties to the macroscopic properties. We take a first step toward creating an automated workflow for the theoretical prediction of thermoplastic material properties by developing an expeditious method for parameterizing a simple yet surprisingly powerful coarse-grained bisphenol-A polycarbonate model which goes beyond previous coarse-grained models and successfully reproduces the thermal expansion behavior, the glass transition temperature as a function of the molecular weight, and several elastic properties.

$\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSn}{\mathrm{S}}_4$ (CZTS) is a promising photovoltaic absorber material with earth-abundant and nontoxic elements. However, the detrimental native defects and secondary phases of CZTS will largely reduce the energy-conversion efficiencies. To understand the origin of these problems during the growth of CZTS, we investigated the kinetic processes on CZTS $\left(\overline{1}\overline{1}\overline{2}\right)$ surface, using first-principles calculations. A surface Zn atom was found to occupy the Cu site near the surface easily due to a low reaction barrier, which may lead to a high $\mathrm{Z}{\mathrm{n}}_{\mathrm{Cu}}$ concentration and a Zn-rich secondary phase. These $n$-type defects may create deep electron traps near the interface and become detrimental to device performance. To reduce the population of $\mathrm{Z}{\mathrm{n}}_{\mathrm{Cu}}$ and the Zn-rich secondary phase, we propose to use K as a surfactant to alter surface kinetic processes. Improvements on crystal quality and device performance based on this surfactant are consistent with early experimental observations.

We propose that supported gold clusters on MgO thin film can potentially serve as an efficient photocatalyst for water splitting. The catalytic activity of the gold cluster is enhanced by excess electrons occupying its quantum well states (QWSs) and can be controlled by varying the oxide thickness, introducing defects/doping in the substrate, and modulating the plasmonic response of the Au cluster. We find that the bonding between the water molecule and certain QWSs can significantly reduce the water splitting energy barrier in its ground state. More importantly, the water splitting is nearly spontaneous when the QWS is photoexcited. First-principles real-time electron dynamics simulations reveal that the excited QWS in the supported gold cluster has a long lifetime on the scale of picoseconds. Generation of activated hydrogen atoms is predicted to occur spontaneously following photoexcitation, and the yield of ${\mathrm{H}}_2$ gas is maintained by enriching hydrogen concentration without poisoning the catalyst. These results illustrate promising routes for promoting photocatalysis via engineering the energy levels of supported metal clusters.

Charge transport in a network of only semiconducting single-walled carbon nanotubes is modeled as a random-resistor network of tube-tube junctions. Solving Kirchhoff's current law with a numerical solver and taking into account the one-dimensional density of states of the nanotubes enables the evaluation of carrier density dependent charge transport properties such as network mobility, local power dissipation, and current distribution. The model allows us to simulate and investigate mixed networks that contain semiconducting nanotubes with different diameters, and thus different band gaps and conduction band edge energies. The obtained results are in good agreement with available experimental data.