Strain engineering in transition metal dichalcogenides is an important means to manipulate these materials' electronic and optical properties. Recently, it has been shown that ${\mathrm{WSe}}_2$ monolayers grown on fused silica substrates using chemical vapor deposition can retain residual strain due to thermal expansion mismatch. Moreover, it was demonstrated that this strain can be released using a solvent-evaporation mediated decoupling method. A continuum theory to explain these observations is introduced and its predictions analyzed. The theory is used to establish that it is plausible that bonds much weaker than typical covalent bonds are sufficient to stabilize strains in the range of those experimentally observed. It is shown that the presence of the solvent modifies the equilibrium in-plane displacement of the film, while its lifting is negligible. Under the proper conditions, this displacement leads to an increase in the decoupling force, thereby initiating the strain relief process. The theory clarifies the role of the liquid surface tension in the relaxation process, and it identifies the relationships between droplet wetting behavior and initial strain state that will lead to solute-evaporation mediated decoupling.

We report experimental values for the flexoelectric responses of ${\mathrm{SrTiO}}_3, {\mathrm{KTaO}}_3, {\mathrm{TiO}}_2$, and ${\mathrm{YAlO}}_3$ single crystals using a three-point bending approach. We find all samples possess a linear flexoelectric response with effective short-circuit flexoelectric coefficients $\sim |1–10|\mathrm{nC}/\mathrm{m}$. Flexocoupling voltages computed from these measured effective flexoelectric coefficients are found to significantly vary across the investigated materials and refute the previous suggestions that they should be $\sim \left|1–10\right|$ V. Importantly, we find that low dielectric constant materials can have large flexocoupling voltages exceeding nominal expectations.

Radiation-induced segregation of alloying elements to crystallographic defects is commonly observed in irradiated austenitic stainless steels. The interaction between solutes and radiation-induced defects changes the physical distribution of solutes and thus affects the formation and growth of defects. The change of the microstructure consequently affects the mechanical properties of the material. A qualitative and quantitative understanding of the interaction between solutes and defects is desirable to better predict the service lifetime of nuclear materials. We used atom probe tomography to measure the distribution of solutes at dislocation loops in 304L stainless steel, irradiated with 2 MeV protons up to 1.5 displacements per atom at 373 and 633 K. No segregation at dislocation loops was found in samples irradiated at 373 K, whereas Ni and Si enrichment and Cr depletion were detected at dislocation loops irradiated at 633 K. The experimentally observed perfect and faulted dislocation loops in vacancy and interstitial types were reproduced by molecular dynamics (MD). A hybrid MD/Monte Carlo method was used to predict the redistribution of alloying atoms at all possible types of dislocation loops in face-centered cubic Fe-Cr-Ni alloys at the same irradiated temperatures (373 and 633 K). The simulations show that, at both temperatures, Cr clusters were formed and distributed randomly, and Ni atoms enriched or depleted at interstitial or vacancy dislocation loops, respectively. The change of solute concentration reaches the highest at the edge of the loop. Ni profiles exhibit characteristic behavior in terms of the stress field of the loops: tension inside of vacancy loops showing depletion of Ni atoms compared with compression inside of interstitial loops showing enrichment of Ni atoms. In addition, the stress field is reduced after solute redistribution. The absence of alloying segregation observed in experiments at a lower temperature (373 K) is explained by a rate theory model: Low-temperature irradiation requires significantly longer irradiation time to see the same amount of segregation as at high temperatures because of the extremely low diffusion of vacancies at low temperatures.

Employing the density functional theory with local density approximation, we show that the fully hydrogenated monolayer-hexagonal boron nitride (${\mathrm{H}}_2\mathrm{BN}$) has a direct band gap of 2.96 eV in the blue-light region, while the pristine $h$-BN has a wider indirect band gap of 4.78 eV. The hole-doped ${\mathrm{H}}_2\mathrm{BN}$ is stable at low carrier density ($n$) but becomes dynamically unstable at higher $n$. We predict that it is a phonon-mediated superconductor with a transition temperature ($T_c$) which can reach $\sim 31$ K at $n$ of $1.5\times {10}^{14}$ holes ${\mathrm{cm}}^{-2}$ near the lattice instability. The $T_c$ could be enhanced up to $\sim 82$ K by applying a biaxial tensile strain at 6% along with doping at $n$ of $3.4\times {10}^{14}$ holes ${\mathrm{cm}}^{-2}$ close to a new lattice instability.

Epitaxial films of multiferroic $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ (BFO) with different thicknesses are grown on $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ buffered $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ (001) substrates using the pulsed laser deposition technique. The room-temperature polarization-voltage (P-V) hysteresis loop, leakage current and average domain size of films with varying thicknesses have been investigated. The frequency dependence of the coercive voltage and variation of the coercive voltage as a function of film thickness are ascertained to examine the validity of the Ishibashi-Orihara model and the semi-empirical Kay-Dunn scaling relationship, respectively. The results demonstrate that at room temperature the BFO films indeed follow the Ishibashi-Orihara (I-O) model and Kay-Dunn scaling law. The dimensionality of the domains acquired by piezoresponse force microscopy imaging further validates that found from the I-O model scaling exponent and is consistent with Kittel's law regarding dependence of domain width on film thickness. These combined results elucidate the fundamental properties that govern the switching kinetics of BFO and are relevant for BFO-based multiferroic materials in next generation logic and memory devices operating over a wide range of frequencies.

In this paper, a temperature-induced valence-state transition is studied in a narrow composition range $0.2⩽x⩽$ 0.375 of ${\mathrm{Ba}}_{2-x}{\mathrm{Sr}}_x{\mathrm{TbIrO}}_6$ by means of x-ray and neutron powder diffraction, resonant inelastic x-ray scattering, magnetic susceptibility, electrical resistivity, and specific heat measurements. The valence-state transition involves an electron transfer between Tb and Ir leading to the valence-state change between ${\mathrm{Tb}}^{3+}/{\mathrm{Ir}}^{5+}$ and ${\mathrm{Tb}}^{4+}/{\mathrm{Ir}}^{4+}$ phases. This first-order transition has a dramatic effect on the lattice, transport properties, and the long-range magnetic order at low temperatures for both Tb and Ir ions. ${\mathrm{Ir}}^{5+}$ ion has an electronic configuration of $5d^4$ ($J_{\mathrm{eff}}=0$), which is expected to be nonmagnetic. In contrast, ${\mathrm{Ir}}^{4+}$ ion with a configuration of $5d^5(J_{\mathrm{eff}}$ = 1/2) favors a long-range magnetic order. For $x=0.1$ with ${\mathrm{Tb}}^{3+}/{\mathrm{Ir}}^{5+}$ configuration to the lowest temperature (2 K) investigated in this paper, a spin-glass behavior is observed around 5 K indicating ${\mathrm{Ir}}^{5+}$ ($J_{\mathrm{eff}}=0$) ions act as a spacer reducing the magnetic interactions between ${\mathrm{Tb}}^{3+}$ ions. For $x=0.5$ with ${\mathrm{Tb}}^{4+}/{\mathrm{Ir}}^{4+}$ configuration below the highest temperature 400 K of this paper, a long-range antiferromagnetic order at $T_{\mathrm{N}}$ = 40 K is observed highlighting the importance of ${\mathrm{Ir}}^{4+}$ ($J_{\mathrm{eff}}$ = 1/2) ions in promoting the long-range magnetic order of both Tb and Ir ions. For 0.2 $⩽x⩽$ 0.375, a temperature-induced valence-state transition from high-temperature ${\mathrm{Tb}}^{3+}/{\mathrm{Ir}}^{5+}$ phase to low-temperature ${\mathrm{Tb}}^{4+}/{\mathrm{Ir}}^{4+}$ phase occurs in the temperature range 180 K $⩽T⩽$ 325 K and the transition temperature increases with $x$. The compositional dependence demonstrates the ability to tune the the valence state for a critical region of $x$ that leads to a concurrent change in magnetism and structure. This tuning ability could be employed with suitable strain in thin films to act as a switch as the magnetism is manipulated.

Metal halide perovskites exhibit a rich crystal structure, with multiple phases as well as ferroelastic domains, which is crucial for the optical and electrical properties. The average crystal phase-transition temperatures can be shifted by size, strain, or defects, but it is not clear whether such differences can also appear locally within a single crystal. The experimental study of domain dynamics within nanocrystals is challenging and requires a method capable of probing crystal lattice variations with both high spatial and temporal resolution. Here, we show that in situ full-field diffraction x-ray microscopy can be used to image domains in a single crystal ${\mathrm{CsPbBr}}_3$ nanoplatelet as the temperature traverses the orthorhombic to tetragonal phase transition, at 150 nm spatial resolution and 6 s time resolution. The images reveal sudden domain pattern changes faster than the temporal resolution. Surprisingly, we observe substantial local variations during heating, with domain changes occurring at different temperatures within the single crystal. The nanoplatelet exhibits a high-temperature domain pattern completely different from the low-temperature one, but both patterns are reproducible, and we reversibly switch between them in multiple cycles. These results demonstrate that single ${\mathrm{CsPbBr}}_3$ crystals can exhibit substantial local variation of their basic crystal properties.

We consider electronic correlation effects and their impact on magnetic properties of tetragonally distorted chemically ordered FeCo alloys ($\mathrm{L}{1}_0$ structure) being a promising candidate for rare-earth-free permanent magnets. We employ a state-of-the-art method combining density functional and dynamical mean-field theory. According to our results, the predicted Curie temperature reduces with increase of lattice parameter ratio $c/a$ and reaches nearly 850 K at $c/a=1.22$. For all considered $c/a$ from 1 to $\sqrt{2}$, we find well-localized magnetic moments on Fe sites, which are formed due to strong correlations originating from Hund's coupling. At the same time, magnetism of Co sites is more itinerant with a much less lifetime of local magnetic moments. However, these short-lived local moments are also formed due to Hund's exchange. Electronic states at Fe sites are characterized by a nonquasiparticle form of self-energies, while the ones for Co sites are found to have a Fermi-liquid-like shape with quasiparticle mass enhancement factor $m^{*}/m\sim 1.4$, corresponding to moderately correlated metal. The strong electron correlations on Fe sites leading to Hund's metal behavior can be explained by peculiarities of the density of states, which has pronounced peaks near the Fermi level, while weaker many-body effects on Co sites can be caused by stronger deviation from half-filling of their $3d$ states. The obtained momentum dependence of magnetic susceptibility suggests that the ferromagnetic ordering is the most favorable one except for the near vicinity of the fcc structure and the magnetic exchange is expected to be of RKKY type.

We use atomistic modeling to show that externally applied shear strain causes the lowest energy self-interstitial atom (SIA) structure in copper (Cu) to change from a ⟨100⟩-type dumbbell to a ⟨110⟩-type dumbbell. Concurrently, SIA migration switches from the three-dimensional (3D) random walk characteristic of ⟨100⟩-type dumbbells to a 1D mechanism analogous to that of crowdion SIAs. Furthermore, the relative energies of these two dumbbell structures as a function of strain are well predicted using elastic dipole tensors computed at zero strain, indicating that examination of these tensors may be used to assess the likelihood of strain-induced SIA structure transitions in other materials. Changes in lowest energy SIA structures and associated migration mechanisms stand to impact predictions of SIA behavior in irradiated solids.

The new class of Janus two-dimensional (2D) transition metal dichalcogenides with two different interfaces is currently gaining increasing attention due to the possibility to access properties different from the typical 2D materials. Here, we show that in-plane anisotropy of a 2D atomic crystal, like ${\mathrm{ReS}}_2$ or ${\mathrm{ReSe}}_2$, allows the formation of a large number of inequivalent Janus monolayers. We use first-principles calculations to investigate the structural stability of 29 distinct $\mathrm{Re}{X}_{2-x}{Y}_x$ ($X,Y\in \left\{\mathrm{S},\mathrm{Se}\right\}$) structures, which can be obtained by selective exchange of exposed chalcogens in a $\mathrm{Re}{X}_2$ monolayer. We also examine the electronic properties and work function of the most stable Janus monolayers and show that a large number of inequivalent structures provides a way to engineer spin-orbit splitting of the electronic bands. We find that the breaking of inversion symmetry leads to sizable spin splittings and spontaneous dipole moments that are larger than those in other Janus dichalcogenides. Moreover, our calculations suggest that the work function of the Janus monolayers can be tuned by varying the content of the substituting chalcogen. Our work demonstrates that in-plane anisotropy provides additional flexibility in sublayer engineering of 2D atomic crystals.

We report ferroelectric (FE) order associated with a linear magnetoelectric consequence in ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$. The ferroelectric order around $\sim$72 K ($T_{FE}$), which is much above the antiferromagnetic ordering ($T_N$) at 26 K, is confirmed by the pyroelectric as well as bias-electric field measurement. A reasonable value of the electrocaloric effect is observed around $T_{FE}$. The magnetic entropy change ($\mathrm{\Delta }{S}_M$) starts to emerge at the onset of FE order and exhibits a maximum around 35 K, which is also much higher than $T_N$, pointing towards a dominant short-range ordering above $T_N$. A structural transition is proposed to a polar $P{6}_3$ structure at $T_{FE}$ from a high-temperature $P{6}_{3}22$ space group, which accounts for the emergence of polar order. The distortion of the ${\mathrm{CoO}}_6$ octahedral as well as layered honeycomb structure around $T_{FE}$ and $T_N$ has been discussed to correlate significance of the magnetic frustration, the occurrence of FE order, multicaloric effect, and feasible Kitaev spin liquid state. The multifunctional properties of the compound have drawn special attention and are of fundamental interest.

Reducing the number of candidate structures is crucial to improve the efficiency of global optimization. Herein, we demonstrate that the generalized Hamiltonian can be described by the atom classification model (ACM) based on symmetry, generating competent candidates for the first-principles calculations to determine ground states of alloy directly. The candidates can be obtained in advance through solving the convex hull step by step because the correlation functions of the ACM can be divided into various subspace according to the defined index $l$. As an important inference, this index can be converted to the number of Wyckoff positions, revealing the dominant effect of geometry symmetry on structural stability. Taking Ni-Pt, Ag-Pd, Os-Ru, Ir-Ru, and Mo-Ru as examples, we not only identify the stable structures in previous theoretical and experimental results, but also predict a dozen of configurations with lower formation energies, such as ${\mathrm{Ag}}_{0.5}{\mathrm{Pd}}_{0.5}$ ($Fd\text{−}3m$), ${\mathrm{Os}}_{0.5}{\mathrm{Ru}}_{0.5}$ ($Pnma$), ${\mathrm{Ir}}_{1/3}{\mathrm{Ru}}_{2/3}$ ($P{6}_3/mmc$), and ${\mathrm{Mo}}_{0.25}{\mathrm{Ru}}_{0.75}$ ($Cmcm$).

We present first-principles density functional theory calculations of atomic properties relevant to the material strength of an austenitic (face-centered-cubic) steel alloy, ${\mathrm{Fe}}_{0.70}{\mathrm{Ni}}_{0.12}{\mathrm{Cr}}_{0.18}$. While alloys in this class appear nonmagnetic macroscopically because their local magnetic moments are disordered, significant deviation from experiment can result when calculating properties using nonmagnetic calculations or magnetically ordered calculations. To overcome these difficulties, in this paper we employ a magnetic sampling method with spin-polarized density functional theory to approximate magnetically disordered states by averaging over several static configurations. Using this approach, we calculate elastic and point defect properties, which are then used to evaluate the dominant solid-solution strengthening contribution to the yield strength. Moreover, since the local composition can interact with strain fields, whether externally applied or induced by dislocations, diffusion can give rise to local changes in mechanical properties. We therefore calculate contributions to the vacancy-mediated diffusivity from the formation and migration terms in the activation energy, and the harmonic-transition-state-theory prefactor which depends on the vibrational states of the initial and transition states. While the prefactor is very sensitive to noise and therefore only loosely constrained by our calculations, the activation energies and yield stress values obtained from paramagnetic calculations are in good agreement with available experimental results.

The dependence of ab initio many-body perturbation theory within the $GW$ approximation on the eigensystem used in calculating quasiparticle corrections limits this method's predictive power. Here, we investigate the accuracy of the recently developed Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional as a generalized Kohn-Sham starting point for single-shot $GW$ ($G_0{W}_0$) calculations for a range of semiconductors and insulators. Comparison to calculations based on well-established functionals, namely, PBE, PBE0, and HSE, as well as to self-consistent $GW$ schemes and to experiment, shows that band gaps computed via $G_0{W}_0$@WOT-SRSH have a level of precision and accuracy that is comparable to that of more advanced methods such as quasiparticle self-consistent $GW$ and eigenvalue self-consistent $GW$. We also find that $G_0{W}_0$@WOT-SRSH improves the description of states deeper in the valence band manifold. Finally, we show that $G_0{W}_0$@WOT-SRSH significantly reduces the sensitivity of computed band gaps to ambiguities in the underlying WOT-SRSH tuning procedure.

The rare-earth nickelates possess a diverse set of collective phenomena including metal-to-insulator transitions, magnetic phase transitions, and upon chemical reduction, superconductivity. Here, we demonstrate epitaxial stabilization of layered nickelates in the Ruddlesden-Popper form ${\mathrm{Nd}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ using molecular beam epitaxy. By optimizing the stoichiometry of the parent perovskite ${\mathrm{NdNiO}}_3$, we can reproducibly synthesize the $n=1-5$ member compounds. X-ray absorption spectroscopy at the O $K$ and Ni $L$ edges indicate systematic changes in both the nickel-oxygen hybridization level and nominal nickel filling from $3d^8$ to $3d^7$ as we move across the series from $n=1$ to $\infty$. The $n=3-5$ compounds exhibit weakly hysteretic metal-to-insulator transitions with transition temperatures that depress with increasing order toward ${\mathrm{NdNiO}}_3$ $(n=\infty )$.

We report a tetragonal polymorph of ${\mathrm{BaFe}}_2{\mathrm{S}}_2\mathrm{O}$ synthesized under high pressures. This $\beta \text{−}{\mathrm{BaFe}}_2{\mathrm{S}}_2\mathrm{O}$ phase is structurally characterized by two-dimensional ${\mathrm{Fe}}_2\mathrm{O}$ square nets that are sandwiched by sulfur atomic layers. The electrical, magnetic, and thermodynamic measurements indicate that it is an antiferromagnetic insulator with a Néel temperature of 121 K. A successive magnetic transition at $\sim 40\mathrm{K}$ is observed, possibly associated with spin canting due to Dzyaloshinskii-Moriya interactions. Density functional theory based calculations reveal Mott localization driven by the on-site electron-electron Coulomb repulsion. The calculations also suggest a noncollinear ${\mathrm{Fe}}^{2+}$-spin structure with two-$\mathbf{k}$ [${\mathbf{k}}_{\mathbf{1}}=(1/2,0,1/2)$ and ${\mathbf{k}}_{\mathbf{2}}=(0,1/2,1/2)$] propagation vectors, primarily due to the magnetocrystalline anisotropy as well as the next-nearest-neighbor superexchange interactions mediated by oxygen and sulfur anions, respectively.

The tuning of the structural and electronic properties of two-dimensional semiconductor monolayers is highly desirable for designing van der Waals heterostructures, which can be employed for several optoelectronic applications. Here, we report a theoretical investigation based on the combination of spin-polarized density functional theory calculations and alloying structures generated by the special quasirandom structure method to investigate the energetic stability and band gap engineering of the compounds ${\mathrm{Mo}}_x{\mathrm{Cr}}_{1-x}{\mathrm{Se}}_2$ and ${\mathrm{W}}_x{\mathrm{Cr}}_{1-x}{\mathrm{Se}}_2$ as a function of the Cr composition for $x=0$ up to 1. We found that even a small concentration of Cr already flattens the low-energy electronic bands and decreases the fundamental electronic band gap. Due the lattice mismatch of the compounds ${\mathrm{CrSe}}_2$ and Mo(W)${\mathrm{Se}}_2$, the renormalization of the electronic properties is nonlinear as a function of the Cr composition. We found bowing parameters for the work function and band gap that change in magnitude from 0.066 to $1.178\mathrm{e}\mathrm{V}$, respectively. From our analyses, Cr alloying decreases the band gap of these monolayers in the direction of the maximum performance band gap predicted by the Shockley-Queisser limit for photovoltaic applications. Band alignment analysis reveals that stacks of Mo(W)${}_x{\mathrm{Cr}}_{1-x}{\mathrm{Se}}_2$ monolayers with particular compositions $x$ can form type II heterojunctions with a high solar harvesting efficiency.

Phase separation—and conversely, the propensity for solid-solution formation—in half-Heusler ($XYZ$) and Heusler ($X{Y}_{2}Z$) compounds is suggested from first-principles electronic-structure-based modeling to be strongly linked to the electronic behavior of the end-members. Alloying between distinct pairs of half-Heusler and Heusler compounds is possible at accessible processing temperatures when the two end-members are either isoelectronic or metallic. The formation of a band gap in semiconducting half-Heusler compounds is associated with significant stabilization. Attempts to create solid solutions with a semiconducting half-Heusler compound would lead to phase separation across the tie line because of the energy penalty associated with filling states in the gap. The alloying between two Heusler compounds, however, is expected even when the electronic behaviors of the end-members differ—clearly demonstrating the distinction between the underlying bonding within half-Heuslers and Heuslers: Half-Heuslers are well-defined intermetallics, whereas Heuslers tend to behave in a manner more in line with conventional alloys. The simple proxy related to electronic structure developed here differentiates Heusler and half-Heusler compositions that truly alloy from those that phase separate, aiding in the pursuit of reliable first-principles materials discovery.