In principle, the ductile-to-brittle transition (DBT) of metallic glasses may be dominated by the competition between shearing and cracking at different stress states. In response to this issue, we propose a new and concise criterion for DBT, named as the shearing-cracking (S-C) criterion, which can accurately predict the critical transition behaviors in metallic glasses induced by variations in both stress state (extrinsic factor) and Poisson's ratio (intrinsic factor). Furthermore, the S-C criterion provides a reasonable explanation for the peculiar phenomenon in metallic glasses that there are rare experimental cases of tensile fracture angle within the range of ${60}^{\circ }$ to ${90}^{\circ }$. Importantly, the S-C criterion offers theoretical guidance for toughness enhancement in metallic glasses from both intrinsic and extrinsic factors, thus facilitating the advancement of engineering applications involving metallic glasses.

The microstructural evolution at and near preexisting grain boundaries (GBs) and dislocations in materials under high radiation doses is still poorly understood. In this work, we use the creation relaxation algorithm (CRA) developed for atomistic modeling of high-dose irradiation in bulk materials to probe the athermal limit of saturation of GB and dislocation core regions under irradiation in fcc Ni. We find that, upon continuously subjecting a single dislocation or GB to Frenkel pair creation in the athermal limit, a local steady-state disordered defect structure is reached with excess properties that fluctuate around constant values. Case studies are given for a straight screw dislocation which elongates into a helix under irradiation and several types of low- and high-angle GBs, which exhibit coupled responses such as absorption of extrinsic dislocations, roughening, and migration. A positive correlation is found between the initial GB energy and the local steady-state GB energy under irradiation across a wide variety of GB types. Metastable GB structures with similar density in the defect core region but different initial configurations are found to converge to the same limiting structure under CRA. The mechanical responses of pristine and irradiated dislocations and GB structures are compared under an applied shear stress. Irradiated screw and edge dislocations are found to exhibit a hardening response, migrating at larger flow stresses than their pristine counterparts. Mobile GBs are found to exhibit softening or hardening responses depending on GB character. Although some GBs recover their initial pristine structures upon migration outside of the radiation zone, many GBs sustain different flow stresses corresponding to altered mobile core structures.

Lead-based metal halide perovskites (MHPs) have wide-ranging applications as solar cells, field-effect transistors, diodes, and photodetectors. However, their poor stability and concerns about toxicity have enabled lead-free tin-based MHPs to emerge as a promising alternative. We utilize molecular dynamics (MD) simulations to investigate the anisotropic mechanical behavior of single-crystal cubic $\mathrm{CsSn}{\mathrm{I}}_3$, a promising lead-free MHP, under uniaxial tension. Among the three investigated crystal orientations, [111] is found to be the strongest and to exhibit the highest ultimate strain while [100] is the weakest. While shear strain localization and amorphization precede fracture along [100], fracture directly follows strain localization along [110] and [111]. We also investigated the influence of a crystal defect, in the form of an embedded rectangular crack, on the anisotropic mechanical behavior of cubic $\mathrm{CsSn}{\mathrm{I}}_3$. The presence of crystal defects is found to substantially reduce the anisotropy in mechanical properties, with very similar crack growth behavior and almost identical stress-strain response noted along starkly different crystal orientations of loading. Finally, the ultimate strengths and ultimate strains of cubic $\mathrm{CsSn}{\mathrm{I}}_3$ determined here are comparable to or higher than those of cubic $\mathrm{CsPb}{\mathrm{I}}_3$ and $\mathrm{MAPb}{\mathrm{I}}_3$ determined in prior MD-based investigations. Thus, our study supports the applicability of cubic $\mathrm{CsSn}{\mathrm{I}}_3$ as a lead-free alternative to commonly used cubic MHPs, while the sensitivity to crystal defects revealed here underlines the importance of defect control for obtaining robust devices with reliable properties.

We investigate the structural, electronic, and optical properties of oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in crystalline $\alpha , \beta$, and $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ using density functional theory (DFT) calculations with the PBE0-TC-LRC functional. Our results reveal that the charge transition levels (CTLs) associated with ${\mathrm{V}}_{\mathrm{O}}$ exhibit significant variations depending on the crystal phase and the coordination environment of surrounding atoms. In particular, ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ surrounded by tetrahedral Ga atoms ($T\text{-Ga}$) exhibit deeper CTLs compared to those surrounded by octahedral Ga atoms ($O\text{-Ga}$). We also observe distinct atomic relaxations, with larger displacements of $T\text{-Ga}$ atoms compared to $O\text{-Ga}$ atoms in the vicinity of ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$. Using linear-response time-dependent DFT, we investigate the optical transitions of ${\mathrm{V}}_{\mathrm{O}}$ and identify two distinct types of transitions: defect state to conduction band state and valence band to defect state. These results can be used to better understand the optical properties of ${\mathrm{V}}_{\mathrm{O}}$ defects in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ films.

Next-generation nanoelectronic, energy, and quantum technologies require increasingly stringent thermal, optical, mechanical, and electrical properties of component materials, often surpassing the limits of widely used materials such as silicon. Diamond, an ultrawide bandgap semiconductor, is a promising material for these applications because of its very high stiffness, thermal conductivity, and electron mobility. However, incorporating diamond into devices that require high-quality metal-diamond interfaces is challenging. In this work, we use a suite of electron microscopy measurements to reveal an ultrathin amorphous carbon layer that emerges at metal-diamond interfaces after electron beam lithography. Using extreme ultraviolet scatterometry, we nondestructively determine lower bounds on the layer's Young's modulus and thermal conductivity, which at $>230\mathrm{GPa}$ and $>1.1$ W/($\mathrm{m}\mathrm{K}$) are indicative of a diamondlike form of amorphous carbon with high ${sp}^3$ bonding. However, extreme ultraviolet coherent diffractive imaging reflectometry and energy-dispersive x-ray spectroscopy measurements indicate a low and likely inhomogeneous density in the range of $1–2{\mathrm{g}/\mathrm{cm}}^3$. The low density of such a stiff and conductive layer could indicate that it contains nanometer-scale voids or atomic-scale vacancies. The appearance of this unusual layer illustrates the nanofabrication challenges for diamond and highlights the need for better techniques to characterize surfaces and interfaces in nanoscale devices.

In a recent class of phase field crystal (PFC) models, the density order parameter is coupled to powers of its mean field. This effectively introduces a phenomenology of higher-order direct correlation functions acting on long wavelengths, which is required for modeling solid-liquid-vapor systems. The present work generalizes these models by incorporating, into a single-field theory, higher-order direct correlations, systematically constructed in reciprocal space to operate across long and short wavelengths. The correlation kernels introduced are also readily adaptable to describe distinct crystal structures. We examine the three-phase equilibrium properties and phase diagrams of the proposed model, and reproduce parts of the aluminum phase diagram as an example of its versatile parametrization. We assess the dynamics of the model, showing that it allows robust control of the interface energy between the vapor and condensed phases (liquid and solid). We also examine the dynamics of solid-vapor interfaces over a wide range of parameters and find that dynamical artifacts reported in previous PFC models do not occur in the present formalism. Additionally, we demonstrate the capacity of the proposed formalism for computing complex microstructures and defects such as dislocations, grain boundaries, and voids in solid-liquid-vapor systems, all of which are expected to be crucial for investigating rapid solidification processes.

Interactions between polarons and oxygen vacancies in oxides, which cause them to modify one another's physical properties, are highly important for applications such as photovoltaics and ferroelectrics. While the difficulty in modeling polarons using density functional theory (DFT) calculations has been alleviated by the recent development of various techniques, including, e.g., the Hubbard-U parameter and finite-size corrections, the underlying physics of polaron interactions with defects remains unknown. Here, we demonstrate that the polaron-vacancy complexes in $\mathrm{PbTi}{\mathrm{O}}_3$ have a preferred orbital configuration, different from the orbital configuration of the bulk polaron, by exploring multiple nearby local minima using $\mathrm{DFT}+\mathrm{U}$. To address the issue of polaron property dependence on the Hubbard-U value, we determine the U value via enforcement of piecewise linearity, and we employ finite-size corrections. Three local minima with different electronic configurations are found by varying the initial conditions: (i) a polaron trapped in a $\mathrm{Ti}\text{−}3d_{eg}$ orbital on the first-nearest-neighbor Ti-ion of the oxygen vacancy ($eg$ complex), (ii) a polaron trapped in a $\mathrm{Ti}\text{−}3d_{t2g}$ orbital at the same position ($t2g$ complex), and (iii) electrons delocalized across several nearby sites and both spin channels, resulting in a semilocalized state. We find that the $eg$ complex is the most energetically favorable state, revealing a change in the orbital of the polaron when trapped by an oxygen vacancy, since the bulk polaron is found to be in a $t2g$ orbital. Furthermore, we demonstrate that great care must be taken to find the correct physical picture with $\mathrm{DFT}+\mathrm{U}$, since a small change in the initial conditions results in finding different local minima.

With the integration of 5G in day-to-day devices and the foreseeable 6G revolution, demand for advanced radio frequency (RF) microelectromechanical systems (MEMS) is growing. Aluminum scandium nitride (AlScN) has emerged as the material of choice for many of those applications due to its superior piezoelectric and electromechanical properties compared to aluminum nitride (AlN). However, synthesizing high-quality, textured AlScN thin films is challenging. Alloying of Sc in AlN induces structural frustration leading to strain, defects, disoriented grains, and disrupted crystal symmetry during growth. Higher deposition temperatures, while improving crystalline quality, risk undesirable phase precipitation and limit industrial sustainability. In addition, future MEMS technologies also demand conformal and textured coatings over diverse topographies. Addressing these challenges collectively requires new and innovative synthesis approaches. In this study, we investigate the feasibility of ionized physical vapor deposition to deposit highly oriented AlScN films with minimal defects at lower temperatures. To this end, we employ combinations of different deposition approaches, such as metal-ion synchronized (MIS) high-power impulse magnetron sputtering (HiPIMS). Leveraging the high ionization rates of HiPIMS and optimally timed substrate bias potentials, we selectively bombard the growing film with Al and/or Sc ions to enhance the adatom mobility at low temperatures while simultaneously providing the ability to tune stress and coat complex structures conformally. The nonequilibrium solubility of Sc in wurtzite AlN under different conditions is investigated using a combinatorial deposition approach. Promising candidates with $\sim 20\%$ Sc composition are isolated and characterized for crystallinity and residual stress. Disoriented grains, a significant issue in growing AlScN films, are observed through atomic force microscopy and found to be completely removed by substrate rotation and application of substrate biasing. The measured piezoelectric response of the films with approximately 20% Sc concentration ranges from 6.3 to 8.8 $\mathrm{pm}/\mathrm{V}$, in line with density functional theory predictions and experimentally reported values for films deposited in a production tool with coplanar geometry. At the same time, MIS-HiPIMS-deposited films offer unique properties and flexibility to tune their stress state and structural properties, thus presenting exciting opportunities for the fabrication of advanced RF filters and next-generation MEMS devices.

Mechanical stresses and strains in the microstructure of cathode materials evolving during charge/discharge cycles can reduce the long-term stability of intercalation type alkali-metal-ion batteries. In this context, crystalline compounds exhibiting zero-strain (ZS) behavior are of particular interest. Near-ZS sodiation was experimentally measured in the tetragonal tungsten bronze (TTB) type compound ${\mathrm{Na}}_x{\mathrm{FeF}}_3$. Using a first-principles method based on density functional theory, we investigate the potential of iron-based fluoride compounds with tungsten bronze (TB) structures as ZS cathode materials. Simulations were conducted to study the intercalation of the alkali metal ions ${\mathrm{Li}}^{+}, {\mathrm{Na}}^{+}$, and ${\mathrm{K}}^{+}$ into the TTB and two related TB structures of the cubic perovskite and hexagonal types. We describe compensating local volume effects that can explain the experimentally measured low volume change of ${\mathrm{Na}}_x{\mathrm{FeF}}_3$. We discuss the structural and chemical prerequisites of the host lattice for a ZS insertion mechanism for alkali ions in TB structures and present a qualitative descriptor to predict the local volume change, which provides a way for faster screening and discovery of novel ZS battery materials.

It was experimentally reported that light illumination leads to reduced deformation stresses in some III-V compound semiconductors such as GaP. This phenomenon is known as the negative photoplastic effect, which is expected to originate from interactions between photoexcited carriers and glide dislocations. To clarify its physical origin at the atomic and electronic levels, density-functional-theory calculations were performed for Shockley ${30}^{\circ }$ partial dislocations in GaP. In the absence of excess carriers, both Ga and P cores of the partial dislocations were found to have reconstructed structures that are energetically most stable. This can be understood by the fact that dangling-bond-like states at undercoordinated atoms of the dislocation cores are removed by core reconstruction. In the presence of excess carriers that would be formed by light illumination, the reconstructed Ga and P cores were able to trap excess holes and electrons, respectively, and were subsequently transformed to unreconstructed structures. It was also found that the unreconstructed structures due to excess carriers tend to have smaller potential barrier heights for dislocation glide, as compared to the pristine reconstructed structures without any excess carriers. This is in good agreement with the increased dislocation mobility in GaP under external light illumination that has been experimentally reported.

Accurate predictions of material properties within the chemical space of transition metal fluorides (TMFs), using computational frameworks such as density functional theory (DFT), is important for advancing several technological applications. The state-of-the-art semilocal exchange-correlation functionals within DFT include the strongly constrained and appropriately normed (SCAN) and the restored regularized SCAN (${\mathrm{r}}^2\mathrm{SCAN}$), both of which are meta generalized gradient approximation (metaGGA) functionals. Given their semilocal nature, both SCAN and ${\mathrm{r}}^2\mathrm{SCAN}$ are susceptible to self-interaction errors (SIEs) while modeling highly correlated $d$ electrons of transition metals. Hence, in this work, we evaluate the accuracy of both SCAN and ${\mathrm{r}}^2\mathrm{SCAN}$ functionals in estimating several properties of TMFs, including redox enthalpies, lattice geometries, on-site magnetic moments, and band gaps. Specifically, we consider binary fluorides of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. We observe both SCAN and ${\mathrm{r}}^2\mathrm{SCAN}$ exhibit poor accuracy in estimating fluorination enthalpies among TMFs, which can be primarily attributed to SIEs among the $d$ electrons, given both functionals bind ${\mathrm{F}}_2$ accurately. Thus, we derive optimal Hubbard $U$ corrections for both functionals based on experimental fluorination (or oxidation) enthalpies within binary TMFs. Note that our attempts at using the linear response theory to derive $U$ corrections yielded unphysical values for V, Fe, and Ni fluorides. While adding the fluorination-enthalpy-derived $U$ corrections to the metaGGA functionals does not significantly affect the lattice volumes and on-site magnetic moments (and in turn, the accuracy of these property estimations versus experiments), it does cause a significant increase in calculated band gaps. Note that the $U$-corrected band gaps in several fluorides deviate to a lesser extent from band gaps calculated with a hybrid functional compared to the non-$U$-corrected functionals. Also, we calculated the average Na intercalation voltage in Mn, Fe, Co, and Ni fluorides, and stabilities of Na-V-F, Na-Cr-F, Na-Mn-F, and Na-Fe-F ternary compounds as transferability checks of our optimal $U$ values. Overall, we do recommend the incorporation of the Hubbard $U$ correction to improve predictions of redox enthalpies in other TMFs. Finally, our study should advance the accuracy of DFT-based screening studies to unearth novel TMFs, which can be used in various applications, including energy storage, catalysis, and magnetic devices.

Strain in materials changes their electronic structure, and the strain response realizes rich material properties and devices. Superconductivity under hydrostatic pressure and epitaxial strain suggests significant response to an external variable strain in a single sample, but this has not yet been demonstrated because the strain is usually a fixed parameter after sample fabrication. ${\left(\mathrm{La},\mathrm{Sr}\right)}_2\mathrm{Cu}{\mathrm{O}}_4$ films were fabricated on flexible metal substrates, and bending strain was applied to them to observe the critical temperature $\left(T_{\mathrm{c}}\right)$ and resistivity variation induced by strain. The compressive bending strain of −0.005 increased the $T_{\mathrm{c}}$ from 23.4 to 27.3 K. The magnitude of the $T_{\mathrm{c}}$ change by the bending strain is independent of the doping level and initial epitaxial strain. Furthermore, the irreversibility temperature was also improved by the compressive bending, and reasonable $T_{\mathrm{c}}$ variation with respect to the reversible strain was observed. $A$b initio density functional calculation for the mother compound $\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4$ clarified that the low-energy electronic structures are sensitive to the bending strain. While the carriers (holes) are preferentially injected into the in-plane orbitals of the $\mathrm{Cu}{\mathrm{O}}_2$ plane under the compressive strain, the tensile strain leads to the carrier injection into the perpendicular orbitals which is unfavorable to the superconductivity. The strain-sensitive high-$T_{\mathrm{c}}$ superconductor under the external strain highlights a new aspect for cuprate superconductors, which opens monitoring of the stress situation in the cryogenic systems such as superconducting magnet and liquid hydrogen container.

Spin splitting, or removal of spin degeneracy in the electronic energy band/level, is often a measure of spin-orbit coupling strength and a way to manipulate spin degrees of freedom. We use first-principles simulations to predict giant spin splitting in methylhydrazinium lead halide (${\mathrm{MHyPbX}}_3, \mathrm{MHy}={\mathrm{CH}}_3{\mathrm{NH}}_2{\mathrm{NH}}_2, X$ = Br and Cl) hybrid organic-inorganic perovskites. The values can reach up to 408.0 meV at zero Kelvin and 281.6 meV at room temperature. The origin of the effect is traced to the large distortion of ${\mathrm{PbX}}_3$ framework, driven primarily by Pb ions in the ferroelectric ${\mathrm{\Gamma }}^{3-}$ mode. The Pb displacements consist of a combination of polar and antipolar arrangements and result in up to 39.2 meV/atom enhancement of the spin-orbit coupling energy in the polar phase of the materials. The spin-orbit coupling gives origin to persistent spin textures in ${\mathrm{MHyPbX}}_3$, which are desirable for applications in spintronics and quantum computing. Our findings reveal an additional functionality for hybrid organic-inorganic perovskite and open a way for the design of more materials with giant spin splitting.

We elucidated the core structure of screw dislocations in ordered B2 FeCo using a recent magnetic bond-order potential (BOP) [Egorov et al., Phys. Rev. Mater. 7, 044403 (2023)]. We corroborated that dislocations in B2 FeCo exist in pairs separated by antiphase boundaries. The equilibrium separation is about 50 Å, which demands large-scale atomistic simulations—inaccessible for density functional theory but attainable with BOP. We performed atomistic simulations of these separated dislocations with BOP and predicted that they reside in degenerate core structures. Additionally, dislocations induce changes in the local electronic structure and magnetic moments.

We use photo-electron paramagnetic resonance (EPR) measurements and first-principles calculations to identify and explain the properties of carbon in AlN. We present clear evidence for carbon substitution on the nitrogen site (${\mathrm{C}}_{\mathrm{N}}$). We also clarify the origin of a widely observed EPR spectra in AlN that, although often attributed to a deep donor defect, we demonstrate is surprisingly due to ${\mathrm{C}}_{\mathrm{N}}$. Finally, we show the presence of ${\mathrm{C}}_{\mathrm{N}}$ is consistent with the absorption spectra at 4.7 eV observed in AlN.

The AlCrTiV high entropy alloy undergoes an order-disorder transition from body-centered cubic (Strukturbericht A2) at high temperatures to the CsCl structure (B2) at intermediate temperatures. We model this transition using first principles Monte Carlo/molecular dynamics simulations. Simulation results yield the temperature-dependent energy, entropy, heat capacity, occupancy fluctuations, and diffraction patterns. The contribution of chemical disorder to the entropy is calculated on the basis of point and pair cluster frequencies. The simulated structures exhibit compensated ferrimagnetism, and the Fermi level lies in a pseudogap. Sensitivity of structure and magnetism to the exchange-correlation functional is discussed, and neutron diffraction experiments are proposed to help resolve the true chemical order.

Polycrystalline and single-crystal samples of the insulating Shastry-Sutherland compound ${\mathrm{Er}}_2{\mathrm{Be}}_2{\mathrm{SiO}}_7$ were synthesized via a solid-state reaction and the floating zone method, respectively. The crystal structure, Er single-ion anisotropy, zero-field magnetic ground state, and magnetic phase diagrams along high-symmetry crystallographic directions were investigated with bulk measurement techniques, x-ray and neutron diffraction, and neutron spectroscopy. We establish that ${\mathrm{Er}}_2{\mathrm{Be}}_2{\mathrm{SiO}}_7$ crystallizes in a tetragonal space group with planes of orthogonal Er dimers and a strong preference for the Er moments to lie in the local plane perpendicular to each dimer bond. We also find that this system has a noncollinear ordered ground state in zero field with a transition temperature of 0.841 K consisting of antiferromagnetic dimers and in-plane moments. Finally, we mapped out the $H\text{−}T$ phase diagrams for ${\mathrm{Er}}_2{\mathrm{Be}}_2{\mathrm{SiO}}_7$ along the directions $H\parallel$ [001], [100], and [110]. While an increasing in-plane field simply induces a phase transition to a field-polarized phase, we identify three metamagnetic transitions in the $H\parallel$ [001] case. Single-crystal neutron diffraction results reveal that the $H\parallel$ [001] phase diagram can be explained predominantly by the expected field-induced behavior of classical, anisotropic moments, although the microscopic origin of one phase requires further investigation.

Magnetic tunnel junctions (MTJs) that are comprised of epitaxially grown complex oxides offer a versatile platform to control the symmetry of tunneling states and tailor magnetic anisotropy useful for practical applications. This work employs thin films of ${\mathrm{SrTiO}}_3$ as an insulating barrier deposited between two ferromagnetic ${\mathrm{SrRuO}}_3$ electrodes to form fully epitaxial MTJs and demonstrate these functionalities. Transport measurements demonstrate large tunneling magnetoresistance (TMR), significantly exceeding previously found values of TMR in MTJs based on ${\mathrm{SrRuO}}_3$ electrodes. These results are explained by perpendicular magnetic anisotropy of ${\mathrm{SrRuO}}_3$ and matching (mismatching) between symmetry and spin across the ${\mathrm{SrTiO}}_3/{\mathrm{SrRuO}}_3$ (001) interface for the parallel (antiparallel) MTJ magnetization state, supported by density functional (DFT) calculations. The angular variation of TMR indicates that the ${\mathrm{SrRuO}}_3$ electrodes contain multiple magnetic domains, allowing the devices to exhibit at least three stable resistance states.

We report a layered transition-metal-ordered oxypnictide ${\mathrm{Sr}}_2{\mathrm{CrCoAsO}}_3$. The new material was synthesized by solid-state reactions under vacuum. It has an intergrowth structure with a perovskite-like ${\mathrm{Sr}}_3{\mathrm{Cr}}_2{\mathrm{O}}_6$ unit and ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ block stacking coherently along the crystallographic $c$ axis. The measurements of electrical resistivity, magnetic susceptibility, and specific heat indicate metallic conductivity from the CoAs layers and short-range antiferromagnetic ordering in the ${\mathrm{CrO}}_2$ planes. No itinerant-electron ferromagnetism expected in CoAs layers is observed. This result, combined with the first-principles calculations and the previous reports of other CoAs-layer-based materials, suggests that the $\mathrm{Co}-\mathrm{Co}$ bond length plays a crucial role in the emergence of itinerant ferromagnetism.

We report a systematic study of the thickness effect on the superconductivity of overdoped $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{C}{\mathrm{e}}_x\mathrm{Cu}{\mathrm{O}}_{4-\delta }$ films. As the film thickness decreases, a recurrence of superconductivity is observed in the $x=0.19$ film, while this doping level is commonly expected to show Fermi liquid behavior. The superconducting critical temperatures exhibit a domelike shape with varying thickness. The magnetic susceptibility measurements suggest that the presence of superconductivity in thin films should be filamentary. The combined effect of stress and oxygen in the thickness-gradient films is proposed to comprehend the experimental observations. Our findings provide new perspectives on superconductivity in other systems, such as the hotly debated nickelate superconductors.