We study the dielectric function of ${\mathrm{CuBr}}_x{\mathrm{I}}_{1-x}$ thin film alloys using spectroscopic ellipsometry in the spectral range between 0.7 eV and 6.4 eV, in combination with first-principles calculations based on density functional theory. Through the comparison of theory and experiment, we attribute features in the dielectric function to electronic transitions at specific $\mathbf{k}$-points in the Brillouin zone. The observed band gap bowing as a function of alloy composition is discussed in terms of different physical and chemical contributions. The band splitting at the top of the valence band due to spin-orbit coupling is found to decrease with increasing Br-concentration, from a value of 660 meV for CuI to 150 meV for CuBr. This result can be understood considering the contribution of copper $d$ orbitals to the valence band maximum as a function of the alloy composition.

A promising route for manufacturing emergent metamaterials is the use of ultrafast laser pulses in complex alloy processing. Laser-induced structural transformations in Zr-based alloys for crystalline and glassy states are investigated here. The ultrafast thermomechanical response is compared between relevant stable crystalline structures ${\mathrm{B}}_2\text{−}{\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50}$, $\mathrm{C}{11}_b\text{−}{\mathrm{Cu}}_{33.3}{\mathrm{Zr}}_{66.7}$ and the amorphous structures $\alpha \text{−}{\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50}$, $\alpha \text{−}{\mathrm{Cu}}_{33.3}{\mathrm{Zr}}_{66.7}$. The subsurface modification resulting from ultrafast laser irradiation is investigated by a hybrid simulation to capture the phenomenon occurring at a picosecond time scale. This combines a two temperature model and molecular dynamics approaches to simulate laser matter interaction at the mesoscale. Our results indicate that the involved structural dynamics strongly depend on the initial atomic composition and phase structure. In particular, a martensite phase transition is unveiled for ${\mathrm{B}}_2$ crystalline alloy, and defects are induced in the irradiated $\mathrm{C}{11}_b$ phase, whereas the amorphous state of photoexcited metallic glasses remains remarkably preserved.

Discovering relationships between materials' microstructures and mechanical properties is a key goal of materials science. Here, we outline a strategy exploiting Bayesian optimization to efficiently search the multidimensional space of microstructures, defined here by the size distribution of precipitates (fixed impurities or inclusions acting as obstacles for dislocation motion) within a simple two-dimensional discrete dislocation dynamics model. The aim is to design a microstructure optimizing a given mechanical property, e.g., maximizing the expected value of shear stress for a given strain. The problem of finding the optimal discretized shape for a distribution involves a norm constraint, and we find that sampling the space of possible solutions should be done in a specific way in order to avoid convergence problems. To this end, we propose a general mathematical approach that can be used to generate trial solutions uniformly at random while enforcing an Euclidean norm constraint. Both equality and inequality constraints are considered. A simple technique can then be used to convert between Euclidean and other Lebesgue $p$-norm (the 1-norm in particular) constrained representations. Considering different dislocation-precipitate interaction potentials, we demonstrate the convergence of the algorithm to the optimal solution and discuss its possible extensions to the more complex and realistic case of three-dimensional dislocation systems where also the optimization of precipitate shapes could be considered.

Electromagnons, which are coupled polar and magnetic excitations in magnetoelectric materials, are of large interest for electronic and computing technological devices. Using molecular dynamics simulations based on an ab initio effective Hamiltonian, we predict that the frequency of several electromagnons can be tuned by the application of electric fields in the model multiferroic ${\mathrm{BiFeO}}_3$, with this frequency either increasing or decreasing depending on the selected electromagnon. In particular, we show that the frequency of electromagnons localized at ferroelectric domain walls can be tuned over a 200 GHz range by realistic dc electric fields. We interpret the realized frequency increase (decrease) by local hardening (softening) of the associated polar phonons which couples to the applied electric field. The increase versus decrease in the electromagnon frequency is further found to be correlated with the real-space localization of such phonons.

The mechanism of austenite-martensite interface migration is a key component in understanding the phase transformations in shape memory alloys. It is also intimately tied to their observed hysteresis. Molecular dynamics simulations offer a unique capability to study phase transformations in detail, however, their associated timescales prevent the observation of interface formation via nucleation and growth near the transformation temperature. To address this challenge, we present a simulation methodology in which steady-state austenite-martensite interfaces are allowed to form close to equilibrium. The resulting structures contain well-defined interfaces which can be perturbed from equilibrium to study their migration. In NiTi specifically, the austenite-martensite interfaces are semicoherent, made up of terrace planes separated by structural disconnections. The disconnections advance via kink pairs and provide an atomic-scale mechanism for interface migration. The methodology and results presented here provide a foundation toward further leveraging molecular dynamics simulations to better understand how the atomic-scale structure of austenite-martensite interfaces impacts macroscopic properties such as hysteresis in shape memory alloys.

Dislocation motion and interactions determine mechanical properties in body-centered cubic (bcc) metallic materials. However, studying mechanisms for the screw dislocation interaction is fundamentally challenging since many underlying processes involve mesotimescales and atomistic resolution, currently inaccessible by either experimental techniques or continuum theoretical methods. In this paper, we develop a computational capability based on self-evolving atomistic kinetic Monte Carlo (SEAKMC) to sample the critical events and saddle point energies related to screw dislocations and their junctions. The method is first validated by calculating the stress dependence of Peierls barriers and formation energies of kink pairs and cross-slip kink pairs on a single screw dislocation in bcc iron. Then the method is applied to a binary junction of a pair of intersecting screw dislocations, the structure of which is crucial for low-temperature plastic deformation. We identify three important mechanisms: coplanar cross-slipping, jog-pinning, and a previously unknown unzipping mechanism during the evolution of the binary junction. The mechanisms are then further validated using classical molecular dynamics simulations. The computational capability developed in this paper provides an effective tool to evaluate screw dislocation related thermally activated events in complex stress conditions. The mechanisms discovered in this paper provide critical insights into temperature dependence of the anomalous slip, a breakdown of the Schmidt law, during the plastic deformation in bcc iron and can be generalized to other bcc metals.

Ferroelectric domain wall motion is fundamental to the switching properties of ferroelectric devices and is influenced by a wide range of factors including spatial disorder within the material and thermal noise. We build a Landau-Ginzburg-Devonshire (LGD) model of ${180}^{\circ }$ ferroelectric domain wall motion that explicitly takes into account the presence of both spatial disorder and thermal noise. We demonstrate both creep flow and linear flow regimes of the domain wall dynamics by solving the LGD equations in a Galilean frame moving with the wall velocity $v$. Thermal noise plays a key role in the wall depinning process at small fields $E$. We study the scaling of the velocity $v$ with the applied DC electric field $E$ and show that noise and spatial disorder strongly affect domain wall velocities. We also show that domain walls develop a local, metastable paraelectric region and widen significantly in the presence of thermal noise in materials with “multiwell” potentials, representative of ferroelectrics at temperatures $T$ just below a first-order phase transition (Curie) temperature $T_c$. These calculations point to the potential of noise and disorder to become control factors for the switching properties of ferroelectric materials, for example for advancement of microelectronic applications.

${\mathrm{CoFe}}_2{\mathrm{O}}_4$ and ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ are well-known insulating and ferrimagnetic spinel ferrites with high Curie temperatures, an important characteristic for electronic and spintronic applications. We used first-principles calculations to investigate how their electronic and magnetic properties can be altered or tuned by the presence of structural point defects. We considered successively the effects of cation distribution in the spinel lattice for stoichiometric compounds and of atom substitutions or vacancies. Our calculations demonstrate that a deviation from the perfectly inverse distribution of cations increases the magnetization and decreases the width of the band gap at the Fermi level. In contrast to cation vacancies, oxygen vacancies are not expected to strongly affect the magnetization. We show that ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ crystals with an excess of Ni cations can display a spin-polarized hole conductivity. We finally calculated the formation energy of the different defects and we give details on their gap states.

Caloric effects allow for temperature control through adiabatic application of external fields and are actively explored for solid-state refrigeration. The common wisdom is that the application of ultrahigh fields enhances the effects, thus providing a route to their practical applications. Using the ferroelectric relaxor (Ba, Ca)(Ti, Zr)${\mathrm{O}}_3$, we demonstrate that in ferroics, which are the prime candidates for such application, this is not true in general and that caloric effects can be enhanced through the reduction of the applied field. The explanation of such a counterintuitive response is in the dependence of the electrocaloric effect on the effective poling field that can be regarded as a “hidden” variable of the caloric effects.

First-principles methods have recently established themselves in the field of photocathode research to provide a microscopic, quantum-mechanical characterization of relevant materials for electron sources. While most of the existing studies are focused on bulk crystals, knowledge of surface properties is essential to assess the photoemission performance of the samples. In the framework of density-functional theory, we investigate the stability and the electronic properties of surface slabs of ${\mathrm{CsK}}_2\mathrm{Sb}$, an emerging semiconducting photocathode material for particle accelerators. Considering surfaces with Miller indices 0 and 1, and accounting for all possible terminations, we find that, at the interface with vacuum, the atomic layers may rearrange considerably to minimize the electrostatic repulsion between neighboring alkali species. From the analysis of the surface energy as a function of the chemical potential, we find a striking preference for surfaces oriented along the (111) direction. Yet, at large and intermediate concentrations of Cs and K, respectively, (100) and (110) slabs are energetically most favorable. The considered surfaces exhibit either semiconducting or metallic character. Of the former kind is the most stable (111) slab, which has a band gap of about 1.3 eV, in excellent agreement with experimental values for ${\mathrm{CsK}}_2\mathrm{Sb}$ samples. Metallic surfaces have a lower work function, on the order of 2.5 eV, in line with the emission threshold measured for ${\mathrm{CsK}}_2\mathrm{Sb}$ photocathodes. All in all, these results contribute to the fundamental understanding of the microscopic properties of ${\mathrm{CsK}}_2\mathrm{Sb}$ in particular and of multi-alkali antimonides in general, and they represent a useful complement to the ongoing experimental efforts in the characterization of this emerging class of photocathode materials.

SiGe alloys, widely used in various technological applications, are typically interfaced with a thermally grown oxide layer that is composed of ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}{\mathrm{O}}_2$, a composite material which is also used for technological applications in its own right. Point defects in this oxide layer influence the electronic and structural properties, which can detrimentally affect the desired application. In this paper, we use ab initio calculations to investigate the canonical oxygen vacancy in systems of varying compositions of ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}{\mathrm{O}}_2$. We find that the electronic structures and geometries of the vacancies remain qualitatively similar to their well-known analogs in ${\mathrm{SiO}}_2$ and ${\mathrm{GeO}}_2$ regardless of the composition and similar to previous results in the literature on Ge-doped ${\mathrm{SiO}}_2$. They show a wide distribution of formation energies and one-electron levels across the various concentrations of ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}{\mathrm{O}}_2$. However, our results show that the factor defining their quantitative behavior is not the concentration, rather it is the chemistry of the atoms around the vacancy, each combination of which has its own distribution of properties. The resulting charge transition levels similarly cover a wide range of the band gap. These results aid the understanding of reliability issues in technological applications.

Realizing the potential of plasmonic hot carrier harvesting for energy conversion and photodetection requires new materials that resolve the bottleneck of extracting carriers prior to energy relaxation within the metal. Using first-principles calculations of optical response and carrier transport properties, we show that directional conductors with Fermi velocities restricted predominantly to one or two directions present significant advantages for efficient hot carrier harvesting. We show that the optical response of filmlike conductors ${\mathrm{PtCoO}}_2$ and ${\mathrm{Cr}}_2\mathrm{Al}\mathrm{C}$ resembles that of two-dimensional (2D) metals, while that of wirelike conductors CoSn and $\mathrm{Y}{\mathrm{Co}}_3{\mathrm{B}}_2$ resembles that of 1D metals, which can lead to high mode confinement and efficient light collection in small dimensions, while still working with 3D materials with high carrier densities. Carrier lifetimes and transport distances in these materials, especially in ${\mathrm{PtCoO}}_2$ and CoSn, are competitive with noble metals. Most importantly, we predict that the carrier injection efficiency from all of these materials into semiconductors can exceed 10% due to the small component of carrier momentum parallel to the metal surface, substantially improving upon the typical injection efficiency of less than 0.1% from noble metals into semiconductors.

We studied the superconducting properties of the one-unit cell FeSe grown on high-doped ${\mathrm{SrTiO}}_3$(001) (STO) substrate by in situ transport and angle-resolved photoemission spectroscopy (ARPES) measurements. By comparing with previous data taken for the low-doped STO substrate [Phys. Rev. Lett. 124, 227002 (2020)], it was revealed that the onset of superconductivity occurs at $\sim 40$ K irrespective of the STO doping level. Furthermore, from the temperature dependence of the critical current, we were able to deduce the superconducting gap that is relevant in the transport phenomena, which gradually increases by cooling the sample down from 40 K. In contrast, the coherent peak emerges at 60 K that likely corresponds to the Cooper pair formation, while the leading edge shift near the Fermi level is clearly observed at 40 K in the ARPES spectra. We speculate that the STO surface presumably acts as an effective “disorder” in this system and the Cooper pairs are preformed well above the onset of the resistance drop.

A previously unreported layered triangular lattice polymorph of $\mathrm{Ti}{\mathrm{I}}_3$ is described, synthesized under 6 GPa of applied pressure at $900{}^{\circ }\mathrm{C}$, but stable at atmospheric pressure. This air-sensitive material has a $\mathrm{Cd}{\mathrm{I}}_2\text{-type}$ layered structure [ $P-3m1$ (no. 164), $a=4.012\AA$ and $c=6.641\AA$ at 120 K, $Z=1$ of ${\mathrm{Ti}}_{0.667}{\mathrm{I}}_2$ ] with an in-plane triangular lattice, related to that of $\mathrm{Ti}{\mathrm{I}}_4$ (${\mathrm{Ti}}_{0.5}{\mathrm{I}}_2$). Although the $\mathrm{Ti}{\mathrm{I}}_3$ formula is consistent with expectations for a layered honeycomb lattice of spin $1/2$ Ti(III), there is disorder in the crystal structure, and the complex magnetic behavior observed is inconsistent with a simple local moment picture of spin $1/2$ per Ti(III). Magnetic susceptibility and heat capacity measurements suggest that the material undergoes several low-temperature phase transitions.

We present an analysis of temperature-dependent atomic short-range ordering and phase stability of the face-centered cubic CrCoNi medium-entropy alloy employing a combination of ab initio calculations and on-lattice machine learning interatomic potentials. Temperature-dependent properties are studied with canonical Monte Carlo simulations. At around 975 K a phase transition into an ordered $\mathrm{Cr}{(\mathrm{Ni},\mathrm{Co})}_2$ phase (${\mathrm{MoPt}}_2$-type) is found. This hitherto not reported state has an ordering energy twice as large than the ordered structures previously suggested. We show that magnetism is not responsible for the observed chemical ordering.

Theoretical approaches to study thermal properties of magnetic materials typically require accurate models of magnetic interactions in order to define the entropy. Here we introduce a complementary approach for examining thermal properties in magnetic systems where an accepted model for such interactions does not exist. In place of a specific model for magnetic interactions, the approach integrates measurements of temperature dependent magnetization of the studied material into a first principles computational scheme. The approach calculates system pressure from thermally disordered microstates that properly incorporate vibrational and spin subsystems at each temperature as well as the coupling between these subsystems. We apply the approach to calculate phonon modes and to investigate the anomalously low thermal expansion of the classical Invar alloy ${\mathrm{Fe}}_{0.65}{\mathrm{Ni}}_{0.35}$. The calculated phonon dispersions for Invar are in excellent agreement with measured data. The Invar thermal expansion is shown to remain small between 50 K and room temperature, consistent with the experimentally observed low thermal expansion value in this same temperature range. This anomalously small thermal expansion is directly connected to a small positive contribution from lattice thermal disorder that is nearly canceled by a large negative magnetic disorder contribution. By contrast, calculations for bcc Fe show a much larger thermal expansion, consistent with experiment, which is dominated by a large contribution from lattice thermal disorder that is reduced only slightly by a small negative contribution from that of magnetism. These findings give insight into the unusual nature of magnetism and spin-lattice coupling in Invar and Fe. In addition, they give promising preliminary support to the presented methodology as a complementary way to investigate thermal properties of magnetic materials. The success achieved on Invar and Fe motivates future testing of the approach on other magnetic materials.

Microscopic understanding of magnetization switching via domain nucleation and/or domain-wall propagation is fundamental knowledge for developing magnetic and spintronic devices. Here, we explore the underlying mechanism of the large coercivity of the magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ thin films, which is roughly ten times larger than that of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ bulk single crystal, by measuring Hall resistance in constricted wire devices. The discretized steplike variations appear in the hysteresis loops of the Hall resistance in $0.6\mu \mathrm{m}$ wide and narrower devices, indicating that the size of the reversed magnetic domain is comparable to the active area of the Hall devices. By counting the number of discrete features, the average diameter of the reversed magnetic domain is estimated to be 80 nm. Individually, the diameter of the reversed domain nucleus is evaluated to be roughly 2 nm. Considering the difference in the diameters of the reversed magnetic domain and the reversed domain nucleus, we ascribed the large coercivity of the ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ thin films to a large nucleation field owing to the uniform crystallinity within grains and strong domain-wall pinning at grain boundaries specific to the thin films. With the large nucleation field in the films, an engineering of the domain-wall pinning sites is a promising approach to control the nucleation, manipulation, and detection of the single domain wall in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ thin-film devices.

Neutron irradiation analysis, inductively coupled plasma mass spectrometry (ICPMS), and cathodoluminescence (CL) spectroscopy are used to investigate the influence of transmuted Ge incorporation on the luminescence properties of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ single crystals. Calculations based on ${\mathrm{Ga}}_2{\mathrm{O}}_3$-neutron interaction reveal temporal variations of both Ge and Zn concentrations as a function of time during and after neutron irradiation. To produce a concentration of $5\times {10}^{18}\mathrm{Ge}\mathrm{donors}/\mathrm{c}{\mathrm{m}}^3$ from the neutron transmutation of Ga, the $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ crystal was irradiated for 27 h, which was accompanied by the incorporation of ${10}^{16}\mathrm{Zn}\mathrm{acceptors}/\mathrm{c}{\mathrm{m}}^3$. These calculated dopant concentrations are confirmed by ICPMS. The $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ crystals exhibit a UV band at 3.40 eV due to self-trapped holes (STHs) and two blue donor-acceptor pair (DAP) peaks at 3.14 eV (BL1) and 2.92 eV (BL2). In addition to the neutron-induced incorporation of substitutional Ge donors and Zn acceptors on Ga sites, Ga vacancies ($V_{\mathrm{Ga}}$) were created by high-energy neutrons in the flux, which strongly enhanced the BL1 peak. The $V_{\mathrm{Ga}}$ acceptors compensate the neutron-induced Ge donors, making the ${\mathrm{Ga}}_2{\mathrm{O}}_3$ crystal highly resistive. Concurrent temperature-resolved CL measurements of the $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ before and after neutron irradiation reveal a twofold increase in both the STH and BL1 peaks. This result suggests that STHs are preferentially localized at an O site adjacent to $V_{\mathrm{Ga}}$, as theoretically predicted by Kananen et al. [Appl. Phys. Lett. 110, 202104 (2017).]. Analysis of the ${\mathrm{Ga}}_2{\mathrm{O}}_3$ CL temperature dependence reveals that the UV and BL1 bands after the neutron irradiation exhibit an equivalent activation energy of $100\pm 10\mathrm{meV}$ due to the presence of a neutron-induced defect that acts as an efficient competitive nonradiative recombination channel. The results also provide evidence that the BL1 and BL2 bands arise from different DAP pairs.

We report a detailed structural characterization of single and double layers of InAs and InAlAs quantum dots (QDs) and their wetting layers (WLs) by atomic force microscopy (AFM) and cross-sectional scanning tunneling microscopy (X-STM). The X-STM analysis with atomic resolution showed that the InAlAs WL consists of two distinct layers: a bottom part where all the Al atoms of the InAlAs alloy settled, and a top part containing exclusively In and Ga atoms. The QDs formed from the InAlAs layer contains no Al atoms at all and lie on top of the Al-rich WL. In the double layers of QDs, the InAlAs QDs were used as a seed to influence the nucleation of the InAs QDs grown on top. A gradual decrease in the density of the top InAs QDs was observed in the AFM images with increasing thickness of the GaAs spacer. The X-STM images showed that both QDs layers were completely intermixed for a 2-nm-thick spacer, while effective strain-induced stacking of both types of QDs was observed for a GaAs spacer thickness of 4 nm. However, both QD layers were completely decoupled for a GaAs spacer thickness of 8 nm and could thus be treated as individual layers.

Machine learning surrogate models employing atomic environment descriptors have found wide applicability in materials science. In our previous work, this approach yielded accurate and transferable predictions of the vibrational formation entropy of point defects for $\mathcal{O}\left(N\right)$ computational cost. The present study investigates the limits of data driven surrogate models in accuracy and applicability for vibrational properties. We propose an improvement of the accuracy by extending the fitting capacity of the model by increasing the dimension of the descriptor space. This is achieved by using a nonlinear relation between descriptors—target observables and when it is possible by including physical relevant information of the underlying energy landscape. The nonlinear extension is used to learn the formation entropy of defects with or without applied strain while including physical information, such as the minimum-saddle point sequences employed for the migration of point defects, is a key ingredient of transition state theory rate approximations. We find excellent predictive power after augmenting the dimensionality of the descriptor space, as demonstrated on large defect databases in $\alpha$-iron and amorphous silicon based on semiempirical force fields. The current linear surrogate models are used to investigate the correlation between migration entropy and energy. Our approaches reproduce the Meyer-Neldel compensation law observed from direct calculations in amorphous Si systems. Moreover, the same abstract descriptor space representation for entropy and energy is then used for the statistical correlation analysis. For linear surrogate models, we show that the energy-entropy statistical correlations can be reinterpreted in descriptor space. This provides a simple statistical criterion for the marginal interpretation of the compensation law. More generally, the present work shows how linear surrogate models can accelerate high-throughput workflows, aid the construction of mesoscale material models, and provide new avenues for correlation analysis.