Due to its phenomenal thermoelectric properties, SnSe has received increased interest, triggering systematic studies of both electronic and vibrational properties and the associated coupling. Recent experimental work claims that orthorhombic SnSe sustains a one-dimensional large polaron with a dimension of about 2 nm. In search of its theoretical signature, we first establish the level of precision that can be reached in describing the electronic structure of SnSe by means of ab initio density functional and many-body perturbation theories. As the characterization of band extrema by means of effective masses plays a crucial role in determining polaron properties, we signal the existence of a broad variation of such quantity among the various ab initio methodologies employed and in the available experimental data. The impact of electron-phonon coupling is then analyzed by employing the recently developed generalized Fröhlich model as well as the nonadiabatic Allen-Heine-Cardona formalism, and their relative accuracy is rationalized. We found that, although the vast majority of band extrema in SnSe cannot sustain a large one-dimensional polaron with a radius as small as 2 nm, there is one case in which another type of polaron emerges, indeed one-dimensional, but with an unusual oscillating electronic density of an approximate real space period of $\sim 3$ nm that evokes a stack of disks. Such type of polaron is obtained from two theoretical treatments: a fixed Gaussian ansatz for the polaron wave function and a variational approach, both within the Fröhlich formalism. We hypothesize that such cylindrical polaron might be found in other materials with extended, shallow double-well band extrema.

The effects of inserting chargeable α-quaterthiophene ($\alpha 4T$) crystallites in polystyrene (PS) multilayers used as a transistor gate and capacitor dielectric were investigated. X-ray diffraction, scanning electron microscopy with energy-dispersive x-ray spectroscopy, and confocal microscopy indicated the formation of $\alpha 4T$ crystallites in the PS matrix. A modified saturation-regime current-voltage relationship was used to estimate organic field-effect transistor (OFET) threshold voltage $V_{\mathrm{TH}}$ shifting, and in turn the quantities of stored charge that were observed as a result of dielectric charging. The crystallites increased the maximum charge-storage capacity as well as the charge-retention capability of the dielectrics. Kelvin probe force microscopy showed that charges were localized near the $\alpha 4T$ crystallites upon charging. Trilayer experiments validated the charge-retention improvement of $\alpha 4T$ crystallite-embedded PS dielectrics. The crystallites also improved breakdown characteristics in PS used as a capacitor dielectric, suggesting their application to storage capacitors in addition to OFET logic.

A simple stochastic model of solute drag by moving grain boundaries (GBs) is presented. Using a small number of parameters, the model describes solute interactions with GBs and captures nonlinear GB dynamics, solute saturation in the segregation atmosphere, and the breakaway from the atmosphere. The model is solved by kinetic Monte Carlo (KMC) simulations with time-dependent transition barriers. The non-Markovian nature of the KMC process is discussed. In this paper (which is Paper I of this work), the model is applied to planar GBs driven by an external force. The model reproduces all basic features of the solute drag effect, including the maximum of the drag force at a critical GB velocity. The force-velocity functions obtained depart from the scaling predicted by the classical models by Cahn and Lücke-Stüwe, which are based on more restrictive assumptions. The paper sets the stage for an accompanying paper [Paper II, Phys. Rev. Mater. 7, 063404 (2023)] in which the GB will be treated as a two-dimensional solid-on-solid interface.

In Part I of this work, we proposed a stochastic model describing solute interactions with stationary and moving grain boundaries (GBs) and applied it to planar GBs in 1D systems. The model reproduces nonlinear GB dynamics, solute saturation in the segregation atmosphere, and all basic features of the solute drag effect. Part II of this work extends the model to 2D GBs represented by solid-on-solid interfaces. The model predicts a GB roughening transition in stationary GBs and reversible dynamic roughening in moving GBs. The impacts of the GB roughening on GB migration mechanisms, GB mobility, and the solute drag are studied in detail. The threshold effect in GB dynamics is explained by the dynamic roughening transition, which is amplified in the presence of solute segregation. The simulation results are compared with the classical models by Cahn and Lücke-Stüwe and previous computer simulations.

Hydrogen is present during the growth of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ using chemical vapor deposition techniques. A detailed understanding of hydrogen-related surface reconstructions is therefore essential for controlling the material properties. We use density functional theory to explore the adsorption of hydrogen, gallium, and oxygen adatoms on the ${\mathrm{Ga}}_2{\mathrm{O}}_3$(010) and ${\mathrm{Ga}}_2{\mathrm{O}}_3$(110) surfaces and generate a surface phase diagram, which shows surface reconstructions as a function of Ga and H chemical potentials. We find that the reconstructions on (110) and (010) surfaces are similar, due to the similarity in bonding. In the absence of hydrogen we find that the ideal unreconstructed surface is low in energy but that reconstructions with Ga and O adatoms can be favorable under more Ga-rich conditions. We question whether such “bare” surfaces can be experimentally observed, since hydrogen-related reconstructions are favored even at very low hydrogen pressures (consistent with residual gas pressures in ultrahigh-vacuum systems). Under more H-rich conditions, multiple hydrogen-containing reconstructions are found, with H adsorption being more stable under O-rich conditions. We find that the electron counting rule is valuable for assessing the stability of surface reconstructions. Knowledge of surface reconstructions and of the stability of hydrogen on the surface will help tailor growth conditions to achieve optimal layer quality.

The first sharp diffraction peak (FSDP) in the reciprocal-space structure factor $S\left(Q\right)$ of glasses has been associated with their medium-range order (MRO) structure, but the real-space origin remains debated. While some progress has been made in the case of silicate and borate glasses, the MRO structure of phosphate glasses has not been studied in detail. Here, we apply persistent homology (PH), a topological data analysis method, to extract the MRO features and deconvolute the FSDP of zinc phosphate glasses. To this end, the oxygen, phosphorus, and zinc atoms in the atomic configurations of the glasses are regarded as vertices weighted by initial atom radii for PH computation before decomposing their contributions to the FSDP. To determine the vertex weights, we vary the oxygen (O) radius systematically and set the radii of zinc (Zn) and phosphorus (P) atoms based on the positions of the first peak in the O-Zn and O-P partial radial distribution functions. These configurations with varying atom radii are used as inputs for PH computation, allowing us to assess the contributions of the different ring structures to the MRO. In turn, this comparison between the computed and measured $S\left(Q\right)$ gives rise to an optimized oxygen radius for the best agreement. The optimized vertex weight (oxygen radius) is found to have a physical meaning based on the covalent and ionic bonding characters. Finally, using the optimized atom radii, we are able to decompose the hierarchical structural contributions to the FSDP.

The ferroelectric material usually exhibits temperature-dependent spontaneous polarization, known as pyroelectricity, which can be used to directly convert thermal energy to electricity from ambient low-grade waste heat. When utilizing the structural phase transformations of the material, the conversion capability can be magnified, consequently the device performance can be strongly boosted by orders of magnitude. However, common ferroelectric oxides suffer mechanical fatigue and functional degradation over cyclic phase transformations, hindering widespread applications of the energy conversion device. In this paper, we investigate the mechanical and functional reversibility of the material by lattice tuning and grain coarsening. We discover the lead-free compound $\mathrm{Ba}\left({\mathrm{Ce}}_{0.005}{\mathrm{Zr}}_{0.005}\right){\mathrm{Ti}}_{0.99}{\mathrm{O}}_3\text{−}0.10\left({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}\right){\mathrm{TiO}}_3 \left({\mathrm{BaCeZrTiO}}_3\text{−}0.10{\mathrm{BaCaTiO}}_3\right)$ satisfying the compatibility condition among all present phases by its lattice parameters, making the phase transformations highly reversible. We demonstrated that the energy conversion device with equiaxial coarse grains exhibits exceptional fatigue resistance, with stable pyroelectric current output at $4\mu \mathrm{A}/{\mathrm{cm}}^2$ over 3000 energy conversion cycles. Our work opens another way to fabricate high-performance materials that advances the pyroelectric energy conversion for practical applications in engineering.

We study the modification of the magnetocrystalline anisotropy (MCA) of Co slabs induced by several different conjugated molecular overlayers, i.e., benzene, cyclooctatetraene, naphthalene, pyrene, and coronene. We perform first-principles calculations based on density functional theory and the magnetic force theorem. Our results indicate that molecular adsorption tends to favor a perpendicular MCA at surfaces. A detailed analysis of various atom-resolved quantities, accompanied by an elementary model, demonstrates that the underlying physical mechanism is related to the metal-molecule interfacial hybridization and, in particular, to the chemical bonding between the molecular C $p_z$ and the out-of-plane Co $d_{z^2}$ orbitals. This effect can be estimated from the orbital magnetic moment of the surface Co atoms, a microscopic observable accessible to both theory and experiments. As such, we suggest a way to directly assess the MCA modifications at molecule-decorated surfaces, overcoming the limitations of experimental studies that rely on fits of magnetization hysteresis loops. Finally, we also study the interface between Co and both ${\mathrm{C}}_{60}$ and ${\mathrm{Alq}}_3$, two molecules that find widespread use in organic spintronics. We show that the modification of the surface Co MCA is similar on adsorption of these two molecules, thereby confirming the results of recent experiments.

The cubic perovskite strontium titanate ${\mathrm{SrTiO}}_3$ (STO) is one of the most studied, polarizable transition metal oxides. When excess charge is introduced into this material through doping or atomic defects, STO tends to host polarons: quasiparticles formed by excess charge carriers coupled to the crystal phonon field. Their presence alters the properties of this material and is a key for many applications. Considering that polarons form preferentially on or near surfaces, we study small polaron formation at the ${\mathrm{TiO}}_2$ termination of the STO(001) surface via density functional theory calculations. We model several supercell slabs of Nb-doped and undoped STO(001) surfaces with increasing size, also considering the recently observed as-cleaved ${\mathrm{TiO}}_2$-terminated surface hosting Sr adatoms. Our findings suggest that small surface polarons become less stable at low concentrations of Nb doping, in analogy to polarons localized in the bulk. Further, we inspect the stability of different polaron configurations with respect to Nb and Sr impurities and discuss their spectroscopic properties.

The introduction of obstacles (e.g., precipitates) for controlling dislocation motion in molecular structures is a prevalent method for designing the mechanical strength of metals. Owing to the nanoscale size of the dislocation core ($\le 1$ nm), atomic modeling is required to investigate the interactions between the dislocation and obstacles. However, conventional empirical potentials are not adequately accurate in contrast to calculations based on density functional theory (DFT). Therefore, the atomic-level details of the interactions between the dislocations and obstacles remain unclarified. To this end, in this paper, we applied an artificial neural network (ANN) framework to construct an atomic potential by leveraging the high accuracy of DFT. Using the constructed ANN potential, we investigated the dynamic interaction between the (${\mathrm{a}}_0$/2)$\langle 111\rangle \left\{110\right\}$ edge dislocation and obstacles in body-centered cubic (bcc) iron. When the dislocation crossed the void, an ultrasmooth and symmetric half-loop was observed for the bowing-out dislocation. Except for the screw dislocation, the Peierls stress of all the dislocations predicted using the ANN was <100 MPa. More importantly, the results confirmed the formation of an Orowan loop in the interaction between a rigid sphere and dislocation. Furthermore, we discovered a phenomenon in which the Orowan loop disintegrated into two small loops during its interaction with the rigid sphere and dislocation.

In this paper we present a model system built out of artificially layered materials, allowing us to understand the interrelation of magnetic phases with the metallic-insulating phase at long length scales, and enabling new strategies for the design and control of materials in devices. The artificial model system consists of superlattices made of ${\mathrm{SmNiO}}_3$ and ${\mathrm{NdNiO}}_3$ layers, – two members of the fascinating rare earth nickelate family, having different metal-to-insulator and magnetic transition temperatures. By combining two complementary techniques—resonant elastic x-ray scattering and muon spin relaxation—we show how the magnetic order evolves, in this complex multicomponent system, as a function of temperature and superlattice periodicity. We demonstrate that the length scale of the coupling between the antiferromagnetic and paramagnetic phases is longer than that of the electronic metal-insulator phase transition—despite being subsidiary to it. This can be explained via a Landau theory—where the bulk magnetic energy plus a gradient cost between magnetic and nonmagnetic phases is considered. These results provide a clear understanding of the coupling of magnetic transitions in systems sharing identical order parameters.

We report on the analysis of stress-induced structural changes and the formation of an omega $\left(\omega \right)$ phase in polycrystalline Nb thin films deposited on Si by high-power impulse magnetron sputtering (HiPIMS) for superconducting qubits using x-ray diffraction (XRD), transmission electron microscopy (TEM), and density-functional theory (DFT). XRD analysis indicates that internal stresses in the Nb thin films lead to the formation of $\left\{112\right\}\langle 111\rangle$ deformation twins and TEM analysis shows that $\omega$ phases nucleate at some of the twin boundaries in the Nb thin films. The size of $\omega$ phases ranges from 10 to 100 nm, which is comparable to the coherence length of Nb $(\approx 40 \mathrm{nm})$, and $\approx 1\%$ volume fraction of Nb grains exhibit this $\omega$ phase. The details of the formation mechanism and superconducting properties of the $\omega$ phases are investigated by DFT and potential roles of the $\omega$ phase in Nb as a source of decoherence in superconducting qubits are also discussed.

A detailed neutron diffraction study of a stoichiometric ${\mathrm{Fe}}_3\mathrm{Ge}$ alloy with different initial states in a wide temperature range (up to 1000 K) made it possible to accurately reveal the features of magnetic and structural phase transformations in it. The use of complementary x-ray diffraction and analysis of neutron diffraction patterns by the Rietveld method allowed reliably separating the nuclear and magnetic contributions to the intensity of the diffraction peaks and performing refinement of the ${\mathrm{Fe}}_3\mathrm{Ge}$ magnetic state characteristics. Both main phases appearing in ${\mathrm{Fe}}_3\mathrm{Ge}$ are ferromagnetic with $T_{\mathrm{C}}=629$ K (hexagonal, $D{0}_{19}$) and $T_{\mathrm{C}}=714$ K (cubic, $L{1}_2$). In $D{0}_{19}$, the presence of a spin-flip transition ($T_{\mathrm{sf}}=385$ K) was confirmed and the temperature dependence of the components of magnetic moment along the hexagonal axis and in the basal plane was obtained. It was shown that the transformation between ordered $L{1}_2$ and $D{0}_{19}$ structural states, predicted by the equilibrium-phase diagram, includes three steps: two diffusional stages ($L{1}_2\to A1, A3\to D{0}_{19}$) and one displacive stage ($A1\to A3$). The obtained structural data suggest that in Fe-Ge, as well as in Fe-Ga alloy, direct transitions between ordered phases are impossible. They should include a transition between disordered states.

In this study we present an implementation of a coupled multiscale quantum mechanics/molecular mechanics approach well suited for studying compositionally rich extended defects. Our focus is on interfacial phenomena of ${\alpha }_2/\gamma$ phase boundaries in intermetallic TiAl alloys. We prove that our implementation is capable of accurately reproducing site-preference energies of solutes reported by previous density functional theory studies. To properly study segregation phenomena, we developed a formalism for segregation energies in systems with two sublattices (Ti and Al). Our model provides predictions consistent with atom probe tomography measurements reported in literature for a large number of solute atoms.

Polarization-dependent photocurrent spectra are measured on a (001) $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ Schottky photodetector, where the linear polarization of light is rotated within the ab plane. Three spectral peaks at 4.92, 5.15, and 5.44 eV are observed that vary in intensity with the optical polarization direction. The peak transition energies are consistent with excitons previously reported in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ due to interband transitions modified by the valence-band $p$-orbital anisotropy and the electron-hole Coulombic attraction. The measured polarization dependence of the photocurrent matches our predictions based on electromagnetic simulations of anisotropic absorption using the complex dielectric function tensor extracted from previous ellipsometry studies. These results illustrate the dominance of excitonic absorption and photocurrent in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ both below and above the band gap, demonstrate a combined theoretical/experimental understanding of anisotropic photocarrier generation, and validate previous atomistic band-structure calculations in this low-symmetry ultrawide-band-gap semiconductor.

We address here the description of the different phases of ${\mathrm{U}}_4{\mathrm{O}}_9$ with ab initio density functional theory and molecular dynamics empirical potential calculations. These phases are built from the $\mathrm{U}{\mathrm{O}}_2$ fluorite by the addition of oxygen interstitials which assemble in clusters named cuboctahedra. Due to the unit cell size and complexity, their simulation represents a formidable task for numerical approaches. With $\mathrm{DFT}+U$, we study in detail two potential structures for $\alpha -{\mathrm{U}}_4{\mathrm{O}}_9$, however, these are just two representatives among the numerous coexisting structures we found. The role of the different valence uraniums (${\mathrm{U}}^{4+}, {\mathrm{U}}^{5+}$, and possibly ${\mathrm{U}}^{6+}$) is highlighted thanks to our quantum-mechanical approach. Temperature effects are then appraised with empirical potentials to describe the evolution of the ${\mathrm{U}}_4{\mathrm{O}}_9$ structure with temperature. We observe a continuous symmetrization of the structure with increasing temperature. Indeed, below 800 K, it gradually turns cubic and the cuboctahedral oxygen clusters tend to symmetrize. Beyond 800 K, the cuboctahedra start to disappear up to 1400 K, where none can be found in our simulated structures.

Flat-band materials (FBMs) serve as a platform for a variety of exotic properties and applications, such as strongly correlated states, topological states, and superconductivity. However, reported FBMs highly rely on materials engineering, such as Moiré lattices. Here, we demonstrate the acceleration of intrinsic FBM discovery using explainable statistical learning within a periodic table representation (PTR). Our model achieves validation accuracies of 0.81–0.97 for $X_{2}YZ$ full-Heusler alloys across three different databases and 0.84 for $\mathrm{AB}{\mathrm{C}}_3$ perovskites. Our interpretable model and statistical analysis reveal several important valence electron-related features for FBMs, supported by atomic orbital hybridization theory. We further discuss various physical properties and applications strongly associated with flat bands, including topology, thermal conductivity, electron-phonon coupling, and superconductivity. Finally, we predict 25 high-potential, previously unreported flat-band Heusler alloys using the PTR model, validated by first-principles calculations.

Nanocolumnar tungsten surfaces were investigated with numerical techniques regarding their potential to reduce sputtering by ion bombardment. We simulated a large number of different configurations with the SPRAY sputtering code to identify an optimal geometry. For the test case of 2 keV ${\mathrm{Ar}}^{+}$ irradiation, a specific configuration was found which led to a significant sputter yield reduction for all studied ion incidence angles. For example, reductions by $\sim 80$% in comparison to the sputter yield values for a flat tungsten surface were observed during irradiation along the surface normal. These properties appear beneficial for application on first wall materials in nuclear fusion devices. The optimized surface was further investigated with the molecular dynamics code PARCAS and the binary-collision-approximation code SDTrimSP-3D, which supported the choice of this configuration. Furthermore, dynamic erosion simulations using SDTrimSP-3D were conducted, which predict a relatively long-lasting persistence of the sputter yield reduction if the structure is exposed to higher fluences.

The internal structure of metals is defined by a complex network of grain boundaries (GBs) and their intersections—triple junctions (TJs). Heat or stress forces GBs to migrate, thus altering many physical properties of the material, and TJs play a key role in this migration as they control GB edges. The GB-TJ interaction strongly depends on the local crystallography, so it can only be properly described at the atomic scale. Although recent direct atomic-scale imaging of GB migration provides some insight into their dynamic behavior, the nanoscale details of the coupled motion of TJs and GBs in a natural nanocrystalline system remain elusive. Here, using in situ transmission electron microscopy (TEM) imaging, we describe the dynamics of the GB-TJ system in a nanocrystalline Co film with mixed fcc and hcp crystal phases during thermal annealing at both the mesoscale and the atomic scale. Our observations reveal that GB evolution proceeds through destabilization and displacement of TJs followed by GB relaxation within the new TJ geometry, which modifies the dihedral angles. This process is mediated by atomic-scale disconnection dynamics, where TJs serve as their sources and sinks. These findings advance our understanding of grain evolution in metal nanostructures, which is crucial for engineering their properties.

In this work we provide a first-principles description of the electronic and optical properties of bilayers ${\mathrm{C}}_3\mathrm{N}$, with different stacking motifs AB, $\mathrm{AB}\prime$, and $\mathrm{AA}\prime$. Starting from quasiparticle electronic band structures, we solve the Bethe-Salpeter equation (BSE) to access the excitonic properties of these bilayers. For all stacking sequences, we see strong optical absorption at energies lower than but close to that of the monolayer. Most relevant, we predict a strong quenching of the low-energy optical absorption, with negligible oscillator strengths of low-lying bound excitons. This is a unique phenomenology that does not arise in the monolayer case, nor in other common homobilayers. We explain these findings in terms of the small interband dipole matrix elements associated to the valence-conduction transitions involved in these excitons, and we discuss them in view of the different stacking motifs.