Machine learning techniques have been recently applied in predicting deformation in amorphous materials. In this study, we extract structural features around liquid film vertices from images of flowing 2D foam and apply a multilayer perceptron to predict local yielding. We evaluate their importance in the description of the T1 events and show that a high level of predictability may be achieved using well-chosen combinations of features as the prediction data. The most relevant features are extracted by performing the predictions separately for isolated sets of features, and these findings are verified using principal component analysis. Using this approach, we determine which properties of the images are most important with regard to the physics of the processes. Our findings indicate that film lengths and angles between the liquid films joining at the vertex are the most important features that predict the local yield events. These two features describe 83% of the yield events. As an application, we extract the statistics of event waiting times from the experiment.

Coupling between ferroelastic domains and magnetism (CFM) is usually observed in ferromagnetic phases where an easily magnetized direction can change with the movement of ferroelastic domains. Organic-inorganic hybrids (OIHs) are expected to enhance the coupling due to their low elastic modules. However, it is challenging to find CFM above room temperature in OIHs since the magnetic orderings are generally at low temperatures. The occurrence of CFM without magnetic ordering remains a question. Here, we report an interesting OIH: $\mathrm{CsCu}{\left(\mathrm{HCOO}\right)}_2\mathrm{Cl}$, where canted antiferromagnetic and ferroelastic ordering occur at 32 and 331 K, respectively. Induced by Cu dimers, $\mathrm{CsCu}{\left(\mathrm{HCOO}\right)}_2\mathrm{Cl}$ shows large magnetic anisotropy at room temperature, the strength of which is twice that below 32 K. It is an example of an OIH showing that ferroelastic domains can couple with magnetism above room temperature. Moreover, our findings present a way to realize room-temperature CFM in OIHs and enhance our understanding of CFM.

Crystallographically complex compounds often possess peculiar physical properties, the evolution of which can be tracked by changing one of the constituent elements at a time. We report the discovery and synthesis of isotypic $R_4{\mathrm{Be}}_{33}{\mathrm{Pt}}_{16}$ (R = Y, La–Nd, Sm–Lu) compounds, which crystallize with the noncentrosymmetric cubic space group $I\overline{4}3d$. The lattice parameters vary from $a=13.6682\left(4\right)\AA$ for R = La to $a=13.4366\left(3\right)\AA$ for R = Lu. $R_4{\mathrm{Be}}_{33}{\mathrm{Pt}}_{16}$ phases exhibit a wide range of ground states. R = Y, La, and Lu analogs display superconductivity. Their calculated electronic structures show nonzero density of states at the Fermi level, with the value of the Sommerfeld coefficient consistent with those obtained experimentally. The rest of the $R_4{\mathrm{Be}}_{33}{\mathrm{Pt}}_{16}$ compounds exhibit magnetic ground states with ordering temperatures ranging from $T_{\text{mag}}=0.4$ K (R = Yb) to $T_{\text{mag}}=40$ K (R = Pr). The diversity of physical properties of $R_4{\mathrm{Be}}_{33}{\mathrm{Pt}}_{16}$ compounds can likely be attributed to the nature of the individual rare-earth elements, structural noncentrosymmetry, the large number of atoms per unit cell (212), as well as the complex multicenter interactions within the Be-Pt framework.

We employ atomic-scale computational frameworks to study the surface crystallization in AuSi films and droplets as a function of composition, temperature, and size. Above the melting point, the surfaces of both thin films and droplets undergo a first-order transition from a two-dimensional (2D) ${\mathrm{Au}}_2\mathrm{Si}$ crystalline phase to a laterally disordered yet stratified layer. The thin film surfaces exhibit an effective surface tension that increases with temperature and decreases with Si concentration, while for droplets in the size range 10–30 nm, the bulk Laplace pressure alters the surface segregation as it occurs with respect to a strained bulk. Above the transition, the size effect on the surface tension due to the stratified surface layer is small, while the crystalline surface layer below the transition is strained and composed of 2D crystallites separated by extended grain boundary scars that lead to large fluctuations in its energetics. As a specific application, all-atom simulations of AuSi droplets on Si(111) substrate subject to Si surface flux show that the supersaturation dependent surface tension destabilizes the contact line via formation of a precursor wetting film on the solid-vapor interface and has ramifications for size selection during droplet-catalyzed routes for nanowire growth. Our study sheds light on the interplay between stability and energetics of surfaces in this unique class of binary alloys and offers pathways for exploiting their surface structure for varied applications such as catalytic nanocrystal growth, dealloying, and polymer crystallization.

We study the effect of characteristic size variation on the phonon thermal transport in crystalline GeTe for a wide range of temperatures using the first-principles density-functional method coupled with the kinetic collective model approach. The characteristic size dependence of phonon thermal transport reveals an intriguing collective phonon transport regime, located in between the ballistic and the diffusive transport regimes. Therefore, systematic investigations have been carried out to describe the signatures of phonon hydrodynamics via the competitive effects between grain size and temperature. A characteristic nonlocal length associated with phonon hydrodynamics and a heat wave propagation length has been extracted. The connections between phonon hydrodynamics and these length scales are discussed in terms of the Knudsen number. Further, the scaling relation of thermal conductivity as a function of characteristic size in the intermediate size range emerges as a crucial indicator of the strength of the hydrodynamic behavior. A ratio concerning normal and resistive scattering rates has been employed to understand these different scaling relations, which seems to control the strength and prominent visibility of the collective phonon transport in GeTe. This systematic investigation emphasizes the importance of the competitive effects between temperature and characteristic size on phonon hydrodynamics in GeTe, which can lead to a better understanding of the generic behavior and consequences of the phonon hydrodynamics and its controlling parameters in low-thermal conductivity materials.

We report the competing effects of disorder pinning and thermal roughening on ferroelectric domain walls as a function of temperature in epitaxial $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ thin films deposited on (001) $\mathrm{SrTi}{\mathrm{O}}_3$ substrates buffered by three types of correlated oxide electrodes, ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$, $\mathrm{LaNi}{\mathrm{O}}_3$, and $\mathrm{SrIr}{\mathrm{O}}_3$. Piezoresponse force microscopy studies show that the 50-nm $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ films are uniformly polarized in the as-grown states, with the patterned domain structures persisting above 700 °C. For all three types of films, the domain wall roughness is dominated by two-dimensional (2D) random bond disorder at room temperature, and transitions to 1D thermal roughening upon heating. The roughness exponent $\zeta$ increases progressively from 0.3 to 0.5 within a temperature window that depends on the bottom conducting oxide type, from which we extracted the distribution of disorder pinning energy. We discuss the possible origins of the disorder pinning and the effect of the correlated oxide electrodes on the energy landscape of DW motion.

The synthesis, structural, and magnetic characterization of the series ${\mathrm{Ba}}_{1-x}{\mathrm{La}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4+\delta }$ ($0\le x\le 0.5$) with the ${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$-type structure are reported. We previously found that the $x=0.2$ member exhibits the very rare anisotropic spin-glass behavior with only the $c$-axis spin component freezing below $T_{\mathrm{g}}$. Here we show that each member of the ${\mathrm{Ba}}_{1-x}{\mathrm{La}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4+\delta }$ ($0\le x\le 0.4$) series exhibits the same spin-glass behavior. Moreover, $T_{\mathrm{g}}$ varies with $x$, reaching a maximum of 26.4(4) K for $x=0.20$ compared with 19.2(2) K for $x=0$. The spin-glass behavior was confirmed by both dc and ac magnetic susceptibility measurements. No long-range magnetic order was found down to 2 K. All series members adopt the $I4/mmm$ space group and subtle structural transformations occur with increasing La content. The unit cell volume contracts for $0.0<x<0.3$ and expands for $0.3<x\le 0.5$. Similar behavior is seen for the equatorial Mn-O bonds in the Mn-O octahedron while the axial Mn-O distances increase to $x\sim 0.3$ but remain unchanged for higher $x$. X-ray absorption near-edge structure analysis revealed that the oxidation state of Mn in the ${\mathrm{Ba}}_{1-x}{\mathrm{La}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4+\delta }$ samples varies with $x$: For $x\le 0.2$ Mn is in the +3.0(1) oxidation state only, while for $x>0.2$ a mixed +2/+3 oxidation state was found. Therefore, for the charge balance, samples with $x\le 0.2$ contain excess oxygen as an interstitial species, while for $x>0.2$, the structure cannot sustain any more interstitial oxygen and the La excess is accommodated through the reduction of some ${\mathrm{Mn}}^{3+}$ to ${\mathrm{Mn}}^{2+}$. Based on the oxidation state of Mn, the possible origins of the spin-glass magnetism in the ${\mathrm{Ba}}_{1-x}{\mathrm{La}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4+\delta }$ series are discussed.

In this paper, the full magnetization process demonstrated by the series of ferrimagnetic intermetallic compounds ${\left(\mathrm{Nd},\mathrm{Ho}\right)}_2{\mathrm{Fe}}_{14}\mathrm{B}$ and ${\mathrm{Ho}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ and their hydrides with the maximum possible hydrogen content (for the given crystal structure type) is studied theoretically and experimentally using megagauss magnetic fields. We observe field-induced phase transitions from the initial ferrimagnetic to the forced-ferromagnetic state in magnetic fields up to 130 T and describe the magnetization process analytically. We find a drastic decrease of the critical transition fields in the hydrogenated compounds. This is due to extremely strong, nearly twofold reduction of the $R$-Fe intersublattice exchange interaction because of the combined substitution and hydrogenation effects. A comparative analysis of the magnetization behavior for the system ${\mathrm{Ho}}_2{\mathrm{Fe}}_{17}\text{−}\mathrm{H}$ is also performed.

We present a systematic investigation of the role and importance of excitonic effects on the optical properties of transitions metal oxide perovskites. A representative set of 14 compounds has been selected, including $3d$ (${\mathrm{SrTiO}}_3$, ${\mathrm{LaScO}}_3$, ${\mathrm{LaTiO}}_3$, ${\mathrm{LaVO}}_3$, ${\mathrm{LaCrO}}_3$, ${\mathrm{LaMnO}}_3$, ${\mathrm{LaFeO}}_3$, and ${\mathrm{SrMnO}}_3$), $4d$ (${\mathrm{SrZrO}}_3$, ${\mathrm{SrTcO}}_3$, and ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$) and $5d$ (${\mathrm{SrHfO}}_3$, ${\mathrm{KTaO}}_3$, and ${\mathrm{NaOsO}}_3$) perovskites, covering a band gap ranging from 0.1 eV to 6.1 eV and exhibiting different electronic, structural, and magnetic properties. Optical conductivities and optical transitions including electron-hole interactions are calculated through the solution of the Bethe-Salpeter equation (BSE) with quasiparticle energies evaluated by the single-shot $G_0{W}_0$ approximation. The exciton binding energies are computed by means of a model BSE, carefully benchmarked against the full-BSE method, in order to obtain well-converged results in terms of $k$-point sampling. The predicted results are compared with available measured data, with an overall satisfactory agreement between theory and experiment.

Dielectric and antiferroelectric materials are particularly promising for high-power energy storage applications. However, relatively low energy density greatly hinders their usage in storage technologies. Here, we report first-principles-based calculations predicting that epitaxial and initially nonpolar AlN/ScN superlattices can achieve an ultrahigh energy density of up to $200\mathrm{J}/{\mathrm{cm}}^{\text{3}}$, accompanied by an ideal efficiency of 100%. We also show that high energy density requires that the system be neither too close nor too far from a ferroelectric phase transition under zero electric field. A phenomenological model is further proposed to rationalize such striking features.

${\mathrm{Cu}}_2\mathrm{S}$ in its high-temperature phases is a promising candidate for thermoelectric materials with the combination of a solid S lattice and liquidlike Cu atoms in the context of the so-called phonon-liquid electron-crystal (PLEC) mechanism. The atomic dynamics associated with the high Cu mobility is a critical component in our understanding of the low thermal conductivity in these materials. Among various possible PLEC compounds that have been studied experimentally, hexagonal $\beta -{\mathrm{Cu}}_2\mathrm{S}$ is a unique case that exhibits anisotropic diffusion channels for Cu atoms in the presence of crystalline S layers. To unveil the diffusion mechanism for such liquidlike Cu atoms, we present first-principles molecular dynamics simulations over 50 ps for hexagonal ${\mathrm{Cu}}_2\mathrm{S}$ at 450 K. Quantitative analysis of the atomic radial distributions, mean-square displacements, velocity autocorrelations, and atomic trajectories is reported and confirms the liquid-solid hybrid features. Our simulations reveal the preference of threefold triangular sites by Cu atoms in the S lattice. The triangular sites in the interlayer region do not bind Cu atoms strongly, allowing them to diffuse in the horizontal direction between S layers like a mobile liquid with a calculated diffusion coefficient $\sim 5\times {10}^{-6}{\mathrm{cm}}^2$/s. The corresponding atomic trajectories have a wide spread and cannot be described by the Chudley-Elliott jump diffusion model. In contrast, Cu atoms are more strongly confined at the triangular sites within the S planes, and Cu diffusion takes place only when the atom hops out of the S layer and enters the interlayer region. This yields a 50% smaller diffusion coefficient in the vertical direction. The anisotropic diffusion channels for liquidlike Cu atoms in hexagonal $\beta -{\mathrm{Cu}}_2\mathrm{S}$ may provide an additional degree of freedom in designing promising systems for future energy applications.

The high solubility of oxygen in Ti, Zr, and Hf makes it difficult to stabilize protective oxide scales on their surfaces as the subsurface regions can serve as boundless sinks that continuously dissolve oxygen. Alloying elements are crucial to reduce the oxygen solubility and diffusivity within early transition metals. Past studies have shown that most substitutional alloying additions to titanium repel interstitial oxygen. Here we use first-principles calculations to show that this repulsion is short ranged and explore several factors that are likely responsible for the repulsive interaction. Calculations of Bader charges suggest the existence of short-range Coulomb interactions due to the accumulation of charge on the substitutional solute and interstitial oxygen that is drawn from the Ti host. Misfit strains due to differences in the atomic radii of the solutes and Ti are also found to play a role. We identify a unique hybridization phenomenon between dissolved substitutional elements and interstitial oxygen within hcp Ti that leads to a repulsive interaction at short distances, similar to that between closed-shell atoms, which is especially pronounced for Al and Si solutes.

A full energy range primary radiation damage model is presented here. It is based on the athermal recombination corrected displacements per atom (arc-dpa) model but includes a proper treatment of the near threshold conditions for metallic materials. Both ab initio (AIMD) and classical molecular dynamics (MD) simulations are used here for various metals with body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) structures to validate the model. For bcc and hcp metals, the simulation results fit very well with the model. For fcc metals, although there are slight deviations between the model and direct simulation results, it is still a clear improvement on the arc-dpa model. The deviations are due to qualitative differences in the threshold energy surfaces of fcc metals with respect to bcc and hcp metals according to our classical MD simulations. We introduce the minimum threshold displacement energy (TDE) as a term in our damage model. We calculated minimum TDEs for various metal materials using AIMD. In general, the calculated minimum TDEs are in very good agreement with experimental results. Moreover, we noticed a discrepancy in the literature for fcc Ni and estimated the average TDE of Ni using both classical MD and AIMD. It was found that the average TDE of Ni should be ∼70 eV based on simulation and experimental data, not the commonly used literature value of 40 eV. The most significant implications of introducing this full energy range damage model will be for estimating the effect of weak particle-matter interactions, such as for γ- and electron-radiation-induced damage.

We investigate the polymeric adsorption of formic acid (HCOOH) on the Cu(111) surface by means of density functional theory. We present structural models for the polymeric form of HCOOH on Cu(111) and characterize their stability, electronic, and vibrational properties. Based on the energetics and dynamics, as well as simulated scanning tunneling microscopy and atomic force microscopy images, we propose that the $\alpha$ polymorph is likely to be formed on the Cu(111) surface and can explain the experimental findings. We also study the initial step of the catalytic dehydrogenation of HCOOH and find that the O-H bond dissociation at the edge of the polymer is facilitated, rather than that forming hydrogen bonding, agreeing well with the experiments.

We present a combined angle-resolved photoemission spectroscopy and low-energy electron diffraction (LEED) study of the prominent transition metal dichalcogenide ${\mathrm{IrTe}}_2$ upon potassium (K) deposition on its surface. Pristine ${\mathrm{IrTe}}_2$ undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Supported by density functional theory calculations, we first show that the K atoms dope the topmost ${\mathrm{IrTe}}_2$ layer with electrons, therefore strongly decreasing the work function and shifting only the electronic surface states towards higher binding energy. We then follow the evolution of its electronic structure as a function of temperature across the charge-ordered phase transitions and observe that their critical temperatures are unchanged for K coverages of 0.13 and 0.21 monolayer. Using LEED we also confirm that the periodicity of the related stripe phases is unaffected by the K doping. We surmise that the charge-ordered phase transitions of ${\mathrm{IrTe}}_2$ are robust against electron surface doping, because of its metallic nature at all temperatures, and due to the importance of structural effects in stabilizing charge order in ${\mathrm{IrTe}}_2$.

The sequence of deformation bursts during plastic deformation exhibits scale-free features. In addition to the burst or avalanche sizes and the rate of avalanches the process is characterized by correlations in the series which become manifest in the resulting shape of the stress-strain curve. We analyze such features of plastic deformation with two-dimensional and three-dimensional simulations of discrete dislocation dynamics models and we show that only with severe plastic deformation the ensuing memory effects become negligible. The role of past deformation history and dislocation pinning by disorder are studied. In general, the correlations have the effect of reducing the scatter of the individual stress-strain curves around the mean one.

Two-dimensional (2D) magnetic crystals are ideal platforms for the employment of simple physical models in the exploration of magnetism in a 2D limit. Instead of examining 2D van der Waals materials, the focus of our study is on adatoms that carry intrinsic magnetic moments and are assembled into 2D arrays at a suitable surface. We applied density functional theory (DFT) to investigate Fe nanostructures formed on a borophene sheet deposited at Ag(111) surface and identified stable Fe-based 2D magnets formed either on top of the borophene or at the interface between the borophene and Ag(111) surface. The structures are composed of close-packed Fe wires, featuring ferromagnetism within the chain and the interchain antiferromagnetic coupling. Exchange- and single-ion anisotropy constants extracted from DFT calculations are used to describe these systems with the classical Ising and Heisenberg models. The corresponding Monte Carlo simulations revealed finite temperature magnetic ordering, with the estimates of critical temperatures of 105 and 30 K derived from the anisotropic Heisenberg model, for the Fe-based magnets grown above and under borophene, respectively.