We present the development of machine-learning interatomic potentials for uranium dioxide ${\mathrm{UO}}_2$. Density functional theory calculations with a Hubbard $U$ correction were leveraged to construct a training set of atomic configurations. This training set was designed to capture elastic and plastic deformations, as well as point and extended defects, and it was enriched through an active learning procedure. New configurations were added to the training database using a multiobjective criterion based on predicted uncertainties on energy and forces (obtained using a committee of models) and relative distances between new configurations in descriptor space. Two machine-learning potentials were developed based on physically sound pairwise potentials, which include the Coulombic interaction: a neural network potential and a SNAP potential. These potentials were optimized to minimize the root mean square error on the training database. Subsequently, the SNAP potential was used to compute the stacking fault energy surface in multiple directions, and the stabilized configurations were employed for subsequent DFT minimizations. The final DFT stacking fault energy surfaces of ${\mathrm{UO}}_2$ are presented, and the associated configurations are included in the training database for a new optimization. Finally, the results obtained from both machine-learned potentials were compared to standard semiempirical ones, demonstrating their excellent predictive capabilities for solid properties. These properties include defect formation energies, $\gamma$ surface, elastic properties, and phonon dispersion curves up to the Breidig transition temperature.

The FeRh compound has been known for a long time as an itinerant magnet with a peculiar first-order antiferromagnetic (AFM)-to-ferromagnetic (FM) transition near room temperature. Although a lot of work has been done, the origin of the physical properties associated with the AFM ↔ FM transition is still an ongoing debate and needs deeper investigation using good single crystals. Here, we report on the single-crystal growth of FeRh from the AuPb flux and confirm it by x-ray crystallographic methods such as Laue diffraction, four-circle diffractometer measurements, and electron probe microanalyzer elemental analysis. The temperature dependence of magnetization in our single crystals below the AFM ↔ FM transition shows an anomalous cascadelike multiple transition behavior, which is obviously very different from the reported results previously. It is only such high-quality single crystals grown here that will pave the way for a comprehensive understanding of the longstanding issues of the FeRh compound.

We report a comprehensive temperature-dependent investigation of the 1:2 ordered triple perovskite system ${\mathrm{Sr}}_3\mathrm{Ca}{\mathrm{Ru}}_2{\mathrm{O}}_9$. It crystallizes in the monoclinic structure with space group $P{2}_1/c$, consisting of corner-sharing $\mathrm{Ca}{\mathrm{O}}_6$ and $\mathrm{Ru}{\mathrm{O}}_6$ octahedra. Using dc magnetization and neutron diffraction measurements, we show that this system undergoes successive magnetic transitions at ∼190 and ∼160 K. From the analysis of the temperature-dependent neutron diffraction and dielectric data, we demonstrate two distinguishing features of the ${\mathrm{Sr}}_3\mathrm{Ca}{\mathrm{Ru}}_2{\mathrm{O}}_9$ system: (i) magnetoelastic coupling associated with the two magnetic transitions, as revealed by a change in the unit cell volume, direct Ru-Ru distance, and bond angles and (ii) magnetodielectric coupling, as revealed by the anomalies across the two magnetic transitions. The present results would add significantly to the current understanding of the triple perovskites with incipient spin-orbit coupling.

The effect of ion irradiation on MnAs single crystalline thin films is studied. The role of elastic collisions between ions and atoms of the material is singled out as the main process responsible for modifying the properties of the material. Thermal hysteresis suppression, and the loss of sharpness of the magnetostructural phase transition, are studied as a function of different irradiation conditions. While the latter is shown to be associated with ion-induced disorder at the scale of the transition correlation length, the former is related to the coupling between disorder and the large-scale elastic field associated with the phase coexistence pattern.

By determining the lattice parameters as a function of temperature of the hexagonal van der Waals ferromagnet ${\mathrm{Fe}}_{2.92\left(1\right)}{\mathrm{Ge}}_{1.02\left(3\right)}{\mathrm{Te}}_2$ we obtain the temperature dependence of the spontaneous in-plane magnetostriction in the ferromagnetic and the linear thermal expansion coefficients in the paramagnetic state. The spontaneous magnetostriction is clearly seen in the temperature dependence of the in-plane lattice parameter $a\left(T\right)$, but less well pronounced perpendicular to the planes along $c$. Below $T_{\mathrm{C}}$ the spontaneous magnetostriction follows a power law $M{\left(T\right)}^{3/2}$ of the magnetization and leads to an expansion of the hexagonal layers. Extrapolating to $T\to$ 0 K we obtain a spontaneous in-plane saturation magnetostriction of ${\lambda }_{\mathrm{sp},a}(T\to 0)\approx -220\times {10}^{-6}$. In the paramagnetic state the linear thermal expansion coefficients amount to $13.9\left(1\right)\times {10}^{-6}{\mathrm{K}}^{-1}$ and to $23.2\left(2\right)\times {10}^{-6}{\mathrm{K}}^{-1}$ for the in-plane and out-of-plane direction, respectively, indicating a linear volume thermal expansion coefficient of $50.8\left(4\right)\times {10}^{-6}{\mathrm{K}}^{-1}$ which we use to estimate the volume thermal expansion contribution to the heat capacity determined at constant pressure. A Sommerfeld-type linear term in the low-temperature heat capacities can be quantitatively ascribed to two-dimensional (2D) ferromagnetic magnon excitations.

Topological Weyl semimetals represent a novel class of nontrivial materials, where band crossings with linear dispersions take place at generic momenta across reciprocal space. These crossings give rise to low-energy properties akin to those of Weyl fermions, and are responsible for several exotic phenomena. Up to this day, only a handful of Weyl semimetals have been discovered, and the search for new ones remains a very active area. The main challenge on the computational side arises from the fact that many of the tools used to identify the topological class of a material do not provide a complete picture in the case of Weyl semimetals. In this work, we propose an alternative and inexpensive, procedure to screen for possible Weyl fermions, starting from the analysis of the band structure along high-symmetry directions in the absence of spin-orbit coupling. We test the method by running a high-throughput screening on a set of 5455 inorganic bulk materials and identify 49 possible candidates for topological properties. A further analysis, carried out by identifying and characterizing the crossings in the Brillouin zone, shows us that three of these candidates are Weyl semimetals. Interestingly, while these three materials underwent other high-throughput screenings, none had revealed their topological behavior before.

The correlation of thin films structure and orientation of small organic molecules with the physical properties like charge carrier transport is not only important for basic understanding but also crucial for various application as functional devices. Small organic semiconductor molecules assemble in different configuration in thin films resulting in exhibiting a wide variety of optoelectronic properties. Here, we have investigated the structure of thin films of three metal phthalocyanines (MPcs) having a metal ion as Cu, Co, and Ni using a combination of x-ray scattering techniques and microscopic images. The x-ray results reveal that the molecules assemble in sticklike nanograins of well-ordered structure on Si surface. These nanograins packed differently for three different metal ions and CuPc growth are more of a columnar type whereas NiPc form much smoother films. The $I\text{−}V$measurements reveal higher conductivity for the thinner films, both under optical illumination and dark condition, in contrary to the normal convention. This observation was explained in terms of the transport of carriers through the region of trapped charge states in the ordered phase of the nanograins and the porous region between the grains resulting in different threshold voltages for differently ordered films.

This work presents the first-principles calculations of the lattice thermal conductivity degradation due to point defects in thorium dioxide using an iterative solution of the Peierls-Boltzmann transport equation. We have used the nonperturbative Green's function methodology to compute the phonon-point defect scattering rates that consider the local distortion around the point defect, including the mass difference changes, interatomic force constants, and structural relaxation near the point defects. The point defects considered in this work include the vacancy of thorium (${\mathrm{V}}_{\mathrm{Th}}$) and oxygen (${\mathrm{V}}_O$), substitutions of helium (${\mathrm{He}}_{\mathrm{Th}}$), krypton (${\mathrm{Kr}}_{\mathrm{Th}}$), zirconium (${\mathrm{Zr}}_{\mathrm{Th}}$), iodine (${\mathrm{I}}_{\mathrm{Th}}$), and xenon (${\mathrm{Xe}}_{\mathrm{Th}}$) in the thorium site, and the three different configurations of the Schottky defects. The results of the phonon-defect scattering rate reveal that among all the considered intrinsic defects, the thorium vacancy and helium substitution in the thorium site scatter the phonon most due to the substantial changes in the force constant and structural distortions. The scattering of phonons due to the substitutional defects unveils that the zirconium atom scatters phonons the least, followed by xenon, iodine, krypton, and helium. This is contrary to the intuition that the scattering strength follows ${\mathrm{He}}_{\mathrm{Th}}>{\mathrm{Kr}}_{\mathrm{Th}}>{\mathrm{Zr}}_{\mathrm{Th}}>{\mathrm{I}}_{\mathrm{Th}}>{\mathrm{Xe}}_{\mathrm{Th}}$ based on the mass difference. This striking difference in the zirconium phonon scattering is due to the local chemical environment changes. Zirconium is an electropositive element with valency similar to thorium and, therefore, can bond with the oxygen atoms, thus creating less force constant variance compared to iodine, an electronegative element, and the noble gases helium, xenon, and krypton. These results can serve as a benchmark for analytical models and help the engineering-scale modeling effort for nuclear design.

${\mathrm{SrTiO}}_3$ is one of the most studied transition metal oxides. Recently, a breakthrough was achieved with the fabrication of freestanding ${\mathrm{SrTiO}}_3$ ultrathin films down to the monolayer limit. However, the many-body effects on the quasiparticle and optical properties of monolayer ${\mathrm{SrTiO}}_3$ remain unexplored. Using state-of-the-art many-body perturbation theory in the GW approximation combined with the Bethe-Salpeter equation, we study the quasiparticle band structure, optical, and excitonic properties of monolayer ${\mathrm{SrTiO}}_3$. We show that quasiparticle corrections significantly alter the band-structure topology; however, the widely used diagonal ${\mathrm{G}}_0{\mathrm{W}}_0$ approach yields unphysical band dispersions. The correct band dispersions are restored only by taking into account the off-diagonal elements of the self-energy. The optical properties are studied both in the optical limit and for finite momenta by computing the electron energy loss spectra. We find that the imaginary part of two-dimensional polarizability at the long wavelength limit is dominated by three strongly bound excitonic peaks and the direct optical gap is associated to a bright exciton state with a large binding energy of 0.93 eV. We discuss the character of the excitonic peaks via the contributing interband transitions and reveal that the lowest bound excitonic state becomes dark for finite momenta along $\mathrm{\Gamma }-M$, while the other two excitonic peaks disperse to higher energies and eventually merge for momenta close to $M$.

Surprisingly little is known about the microscopic processes that govern ferroelectric switching in orthorhombic ferroelectrics. To study these processes, we combine ab initio-based molecular dynamics simulations and data science on the prototypical material ${\mathrm{BaTiO}}_3$. We reveal two different field regimes: For moderate field strengths, the switching is dominated by domain wall motion, while a fast bulklike switching can be induced for large fields. Switching in both field regimes follows a multistep process via polarization directions perpendicular to the applied field. In the former case, the moving wall is of Bloch character and hosts dipole vortices due to nucleation, growth, and crossing of two-dimensional ${90}^{\circ }$ domains. In the second case, the local polarization shows a continuous correlated rotation via an intermediate tetragonal multidomain state.

The crystal structure, vibrational spectra, and magnetic structure of quasi-two-dimensional layered van der Waals material ${\mathrm{MnPS}}_3$ were studied using x-ray diffraction and Raman spectroscopy at high pressures up to 28 GPa, and neutron diffraction up to 3.6 GPa, respectively. A structural phase transition between two monoclinic modifications of the same $C2/m$ symmetry was observed, evolving gradually in the pressure range of about 1–6 GPa. The transition is accompanied by abrupt shortening of lattice parameters, significant reduction of the monoclinic distortion, and anomalies in the pressure behavior of several Raman-mode frequencies. No more structural phase transitions were revealed in the studied pressure range. The antiferromagnetic (AFM) state with a propagation vector $k$ = (0, 0, 0) remains stable in ambient pressure and high-pressure structural phases of ${\mathrm{MnPS}}_3$ at least up to 3.6 GPa. The Néel temperature increases noticeably with a pressure coefficient of $d{T}_{\mathrm{N}}/dP=6.7$ K/GPa, leading to modification of the dominant first-neighbor magnetic interaction exchange parameter with a relevant coefficient ${dJ}_1/dP\approx -0.6$ meV/GPa. This observation is in contrast to the pressure behavior of ${\mathrm{FePS}}_3$, demonstrating modification of the AFM state from 2D-like to 3D-like at the similar pressure-induced structural phase transition. The different pressure response of the magnetic states of ${\mathrm{MnPS}}_3$ and ${\mathrm{FePS}}_3$ is analyzed in terms of competing in-plane and interplane magnetic interactions.

Ferrimagnetic ${\mathrm{SrFe}}_{12}{\mathrm{O}}_{19}$ (SrM) is a mother phase for ferrite permanent magnets, which are commonly used due to its low cost compared to that of Nd-based permanent magnets and broader uses in permanent magnet markets. Recently, its electronic ground state has been debated. It has been proposed that the SrM is in a quantum paraelectric state in its single-crystal form, while polycrystalline SrM is a ferroelectric state. In this work, strain can stabilize ferroelectricity at room temperature, while keeping its ferrimagnetism. The strained SrM shows not only clear magnetic hysteresis but also $\sim 4.4\mathrm{\mu }\mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ as remnant polarization. From high-temperature thermal annealing, its crystallinity and ferroelectricity are even strengthened. Those are visualized by significant reduction in full width at half maximum of the rocking curve and $\sim 7.9\mathrm{\mu }\mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ in remnant polarization The results indicate this functionality can be discovered from old and well-known materials.

A Monte Carlo model is used to investigate electron-hole (e-h) generation created by incident gamma-ray radiation in ${\mathrm{LaB}}_3, {\mathrm{CeBr}}_3$, and ${\mathrm{Cs}}_2{\mathrm{LiLaBr}}_6$ (CLLB) scintillators. Our approach follows the detailed energy loss mechanisms and describes the microscopic structure of ionization tracks in order to address differences in the scintillation properties of these three materials. The mean energy required to create an e-h pair, $W$, theoretical light yield, and the spatial distribution of e-h pairs are determined. We found that $W$ approaches constant and similar values at high incident energies for ${\mathrm{LaB}}_3$ and CLLB, suggesting that these materials should have similar light yields. However, the experimental light yield of CLLB is almost half that of ${\mathrm{LaBr}}_3$. Unlike for ${\mathrm{LaBr}}_3, W$ of ${\mathrm{CeBr}}_3$ increases with increasing energy excitation and shows a nonlinear behavior in e-h creation, potentially explaining slightly lower light yield and worse energy resolution of ${\mathrm{CeBr}}_3$. Furthermore, we observed that the spatial distributions of electron-hole pairs in ${\mathrm{LaBr}}_3$ and ${\mathrm{CeBr}}_3$ are very similar, while the number of high-density e-h domains in CLLB is greater in comparison. This discrepancy could explain the lower light yield of CLLB. The thermalization model of e-h pairs showed that the longitudinal optical phonon energy has a profound effect on the thermalization time and distance of e-h pairs, leading to a much higher density of excitation in ${\mathrm{LaBr}}_3$ but a more diffuse one in CLLB. This effect leads to steeper gradients in ${\mathrm{LaBr}}_3$, resulting in varying density effects and worse proportionality, while CLLB suffers from more uniform but more pronounced quenching. The fraction of nonradiatively recombined electrons in ${\mathrm{LaBr}}_3$ and CLLB was estimated to be 30% and 45%, respectively. These results correlate well with experimental observations of the scintillation properties of these materials. The approach can be used to predict the expected properties of new materials and support further development of existing materials.