We devise a scheme for simulating diffusion of interstitial solutes in metals when the diffusivity is modified by the presence of defects. The method is quite general and may be used to model the effects of more than one defect in combination. In addition, it is amenable to being described by a simple analytic model fitted to a small number of parameters that can be determined by computer experiments. Using classical interatomic potentials and molecular dynamics, we calculate the diffusivity of carbon in iron and study the effects of a substitutional alloying element and an edge dislocation. Some of our results are validated against published experimental findings and previous simulations, and we are able to apply our theoretical model to rationalize our results and to show how, once the model is fitted to the computer experiments, it may be used in a predictive manner.

Iridates have attracted much interest in the last decade for their novel magnetism emerging in the limit of strong spin-orbit coupling and possible unconventional superconductivity. A standard for growing iridate single crystals has been the flux method using platinum crucibles. Here, we show that this widely used method compromises the sample quality by inclusion of platinum impurities. We find that ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ single crystals grown in iridium crucibles show remarkable differences from those grown in platinum crucibles in their sample characterizations using Raman spectroscopy, resistivity, magnetization, optical third harmonic generation, resonant x-ray diffraction, and resonant inelastic x-ray scattering measurements. In particular, we show that several peaks of sizable intensities disappear in the Raman spectra of samples free of platinum impurities, and a significantly larger activation energy is extracted from the resistivity data compared to previously reported values. Furthermore, we find no evidence of the previously reported glide-symmetry-breaking structural distortions and confirm the $I{4}_1/acd$ space group of the lattice symmetry. Although the platinum impurities are not apparent in the magnetic properties and thus went unnoticed in the stoichiometric insulating phase for a long time, their effects can be much more detrimental to transport properties in chemically doped compounds. Therefore, our result suggests using growth methods that avoid platinum impurities for an investigation of the intrinsic physical properties of iridates, and possible superconducting phases.

In this paper, we experimentally demonstrate two-state fluctuations in a metal-insulator-semiconductor device formed out of a metallic atomic force microscopy tip, vacuum gap, and multilayer ${\mathrm{MoSe}}_2$ sample. We show that noise in this device is intrinsically bias dependent due to the bias-dependent surface potential and does not require that the frequency or magnitude of individual dopant fluctuations are themselves bias dependent. Finally, we measure spatial nonhomogeneities in band bending (charge reorganization) timescales.

A first-principles computational study is presented of various phases of ${\mathrm{LiAlO}}_2$. The relative total energies and equations of state of the $\alpha$, $\beta$, and $\gamma$ phases are determined after structural relaxation of each phase. The $\beta$ and $\gamma$ tetrahedral phases are found to be very close in energy and lattice volume with the $\gamma$ phase having the lowest energy. The octahedral $\alpha$ phase is a high-pressure phase and the transition pressure from the $\gamma$ and $\beta$ phases to $\alpha$ is determined to be about 1 GPa. The electronic band structures of each phase at their own equilibrium volume are determined using the quasiparticle self-consistent (QS) $GW$ method as well as using the $0.8\mathrm{\Sigma }$ approach in which the $\mathrm{QS}GW$ self-energy is reduced by a factor of 0.8 to correct for the underscreening of $W$ in $\mathrm{QS}GW$. The effective masses of the band edges and the nature of the band gaps are presented. The lowest energy $\gamma$ phase is found to have a pseudodirect gap of 7.69 eV. The gap is direct at $\mathrm{\Gamma }$ but corresponds to a dipole forbidden transition. The imaginary part of the dielectric function and the absorption coefficient are calculated in the long-wavelength limit and the random phase approximation, without local field or electron-hole interaction effects for each phase, and their anisotropies are discussed. Si doping on the Al site is investigated as a possible $n$-type dopant in $\gamma \text{−}{\mathrm{LiAlO}}_2$ using a 128-atom supercell corresponding to 3.125% Si on the Al sublattice in the generalized gradient approximation and a smaller 16-atom cell with 25% Si in the $\mathrm{QS}GW$ approximation. The Si is found to significantly perturb the conduction band and lower the gap but a clearly separated deep donor defect level is not found. However, the donor binding energy is still expected to be relatively deep, on the order of a few tenths eV in the hydrogenic effective mass approximation.

There are a multitude of applications in which structural materials would be desired to be nondestructively evaluated, while in a component, for plasticity and failure characteristics. In this way, safety and resilience features can be significantly improved. Nevertheless, while failure can be visible through cracks, plasticity is commonly invisible and highly microstructure-dependent. Here, we show that an equation-free method based on principal component analysis is capable of detecting yielding and tertiary creep onset, directly from strain fields that are obtained by digital image correlation, applicable on components, continuously and nondestructively. We demonstrate the applicability of the method to yielding of Ni-based Haynes 230 metal alloy polycrystalline samples, which are also characterized through electron microscopy and benchmarked using continuum polycrystalline plasticity modeling. Also, we successfully apply this method to yielding during uniaxial tension of Hastelloy X polycrystalline samples, and also to the onset of tertiary creep in quasibrittle fiber composites under uniaxial tension. We conclude that there are key features in the spatiotemporal fluctuations of local strain fields that can be used to infer mechanical properties.

In two-dimensional (2D) materials, Coulomb interactions induce large binding energies of excitons, which are detrimental to the charge separations and hence crucial to photovoltaic performance. Here, we elucidate that this drawback can be well restrained in 2D materials with an intrinsic polarization field. Based on first-principles calculations combined with nonadiabatic molecular dynamics, we propose an embodied example of a ${\mathrm{LiAlTe}}_2$ quadruple layer. Due to the inherent electrical polarization field in 2D ${\mathrm{LiAlTe}}_2$, the electron and hole wave functions are separated into opposite atomic layers similar to the case of heterostructures, reducing the undesired Coulomb interactions and contributing to the small binding energy of excitons. Through modeling and recording the excited-state dynamics, we reveal that 2D ${\mathrm{LiAlTe}}_2$ harbors an ultralong lifetime of excitons of about 1.69 ns, a recombination time that is similar to that found in type-II van der Waals heterostructures and superior to hitherto known $2\mathrm{D}$ photovoltaic components. Furthermore, the effect of strain on the electron-hole recombination is studied quantitatively. Our findings not only provide a compelling candidate for applications in wearable and flexible thin-film solar cells, but also suggest one of the possible ways to design the thin-film solar cells.

By means of a ternary chemical potential phase diagram and phonon spectrum calculations, we propose that $M{\mathrm{Tl}}_4{\mathrm{Te}}_3$ ($M$ = Cd, Hg), which are derivatives of ${\mathrm{Tl}}_5{\mathrm{Te}}_3$, are thermodynamically and dynamically stable in the body-centered tetragonal crystal structure with $I4/mcm$ symmetry. Our electronic structure calculations confirm that a robust $s-p$ band inversion occurs near the Fermi level in $M{\mathrm{Tl}}_4{\mathrm{Te}}_3$, and a topological band gap of $\sim 0.13$ eV in ${\mathrm{CdTl}}_4{\mathrm{Te}}_3$ is induced by the spin-orbit coupling. These results suggest that $M{\mathrm{Tl}}_4{\mathrm{Te}}_3$ are large band gap three-dimensional strong topological insulators that are stable and synthesizable in experiment and could be used to design efficient spin torque equipment and spin devices.

Low-dimensional ferroelectricity and polar materials have attracted considerable attention for their fascinating physics and potential applications. Based on first-principles calculations, here we investigate the stacking modes and polar properties of a typical series of quasi-one-dimensional ferroelectrics: double-chain oxyhalides $\mathrm{NbO}{X}_3$ ($X=\mathrm{Cl}$, Br, and I). The geometry of their double-chains allows both the interchain/intrachain permutation. Thus, different stacking modes of double-chains lead to a variety of ferroelectric and antiferroelectric phases in both the tetragonal and the monoclinic crystals. The proximate energies of these phases may lead to multiphase coexistence in real materials, as well as the hydrostatic pressure driving structural phase transition. Their spontaneous polarizations and piezoelectricity of the ferroelectric phases are prominent, comparable to commercially used ferroelectric ${\mathrm{BaTiO}}_3$ and piezoelectric ZnO, respectively. Our paper demonstrates that the van der Waals $\mathrm{NbO}{X}_3$ are promising materials for exploring quasi-one-dimensional ferroelectricity and antiferroelectricity.

First-principles density functional theory computations are used to predict Rashba effects cofunctional with ferroelectricity in a recently synthesized lead-free hybrid organic-inorganic perovskite ${\mathrm{MPSnBr}}_3$ (MP=methylphosphonium, ${\left[{\mathrm{CH}}_3{\mathrm{PH}}_3\right]}^{+}$). The ground state of the material is polar monoclinic with calculated spontaneous polarization of $3.01\mu \mathrm{C}/{\mathrm{cm}}^2$. It exhibits near band edges' spin splitting of up to 3.3 meV and Rashba coefficient up to 0.62 eV Å. The spin textures have different topology in the conduction and valence band, which originates from the difference in the spin-momentum coupling strengths. They occur in two orthogonal planes of the Brillouin zone and are coupled to the direction of spontaneous polarization. These features persist at a finite temperature of 293 K. At 333 K, that is, above monoclinic to orthorhombic phase transition, spontaneous polarization is reduced to $0.15\mu \mathrm{C}/{\mathrm{cm}}^2$, while the maximum spin splitting and Rashba coefficient reduce slightly to the values of 2.9 meV and 0.41 eV Å, respectively. The spin textures remain coupled with the polarization direction. We investigate the dependence of the aforementioned properties on the choice of computational methodology to extend the first-principles predictions to include finite temperature effects. We find that the predictions are sensitive to the methodology. Our study reveals the potential of ${\mathrm{MPSnBr}}_3$ for low-temperature applications in spintronics and quantum computing.

Hematite is a promising material for photoelectrochemical water splitting applications and understanding the complete mechanism of water oxidation reaction close to its surfaces is of great importance. Herein, we present a theoretical investigation of the electronic states of nonstoichiometric hematite(0001) films in the presence of absorbed H on the surfaces, as well as Ti doping. The calculations, performed at the $\text{DFT}+U$ level, indicate that iron can appear in different oxidation states, from ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Fe}}^{5+}$ in the vicinity of impurities or surfaces. The electron and hole distributions in hematite slabs could control the bending and local state of valence and conduction bands. The projected density of states in different atomic positions shows an upward band bending close to the surfaces, which varies with the number of absorbed H or with Ti doping. In the dark, the width of the depletion layer is computed as less than 12 Å for neutral slabs, while adding excess electrons expands it to involve more atomic layers, depending on the number of absorbed H on the surface. In the neutral slabs in the presence of H, we obtain an upward band bending of 0.18, 0.42, and 0.24 eV by increasing the number of H from one to three on each side, respectively. The evolution of bands in neutral slabs, with/without Ti doping, occurs in a few layers in nearby the surface and also Fe with an oxidation state rather than ${\mathrm{Fe}}^{3+}$. According to our results, doping Ti creates a polaron and locally affects the band structure. In the charged surface with excess electrons, a metallic behavior is observed on the surface and a large band bending in the middle layers. This work shows that a substantial upward bending of the bands in hematite can exist even at neutral surfaces in contact with a vacuum.

We present a systematic study of the structure, electronic, and magnetic properties of a new branch of intermetallic compounds, $R{\mathrm{V}}_6{\mathrm{Sn}}_6(R=\mathrm{Tb}\text{−}\mathrm{Tm})$ by using x-ray diffraction, magnetic susceptibility, magnetization, electrical transport, and heat-capacity measurements. These compounds feature a combination of a nonmagnetic vanadium kagome sublattice and a magnetic rare-earth triangular sublattice that supports various spin anisotropies based on different $R$ ions. We find magnetic orders for the $R$ = Tb, Dy, and Ho compounds at 4.4, 3, and 2.5 K, respectively, while no ordering is detected down to 0.4 K for the $R$ = Er and Tm compounds with easy-plane anisotropies. Electronically, we found no superconductivity or charge ordering transition down to 0.4 K for any member of this family, while all compounds exhibit multiband transport properties.

Single crystals of a Shastry-Sutherland magnetic semiconductor, ${\mathrm{BaNd}}_2{\mathrm{ZnS}}_5$, were synthesized through a high-temperature solution growth technique. Physical properties were characterized by powder and single crystal x-ray diffraction, temperature- and field-dependent magnetization, and temperature-dependent specific heat measurements. ${\mathrm{BaNd}}_2{\mathrm{ZnS}}_5$ orders antiferromagnetically at 2.9 K, with magnetic moments primarily aligned within the $ab$-plane. Magnetic isothermal measurements show metamagnetic transitions at $\sim 15\mathrm{kOe}$ for the [110] direction and $\sim 21\mathrm{kOe}$ for the [100] direction. Estimated magnetic entropy suggests a double ground state for each neodymium ion.

${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ ternary alloys are emerging ultrawide band gap semiconductor materials for high-power electronics applications. The heat dissipation, which mainly depends on the thermal conductivity of the constituent material in the device structures, is the key for device performance and reliability. However, the reports on the thermal conductivity of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ alloys are very limited. Here, we present a comprehensive study of the thermal conductivity of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ in the entire Al composition range. Thick ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ layers grown by metal-organic chemical vapor deposition on GaN/sapphire and GaN/SiC templates are examined. The thermal conductivity measurements are done by the transient thermoreflectance method at room temperature. The effects of the Al composition, dislocation density, Si doping, and layer thickness on the thermal conductivity of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ layers are thoroughly investigated. All experimental data are fitted by the modified Callaway model within the virtual crystal approximation, and the interplay between the different phonon scattering mechanisms is analyzed and discussed.

The reduction of surface energy is a well-known driving force for the fragmentation of nanowires. Fragmentation can limit the service life of those small low-dimensional building blocks or can be controlled to induce beneficial shape changes. For isotropic surface energy, fragmentation is analogous to the classic Rayleigh-Plateau instability of liquid jets. However, commonly synthesized crystalline nanowires have strongly anisotropic surface energies and exhibit facets coinciding with cusps in the Wulff plot. Depending on growth orientation, different fragmentation behaviors have been seen in nanowires. Using phase-field simulations, we show that fragmentation of faceted nanowires with cubic crystal symmetry may occur by a finite-amplitude nonlinear instability, as opposed to a Rayleigh-Plateau-like linear instability, depending on nanowire growth orientation. We carry out a weakly nonlinear analysis based on sharp interface theory to characterize the faceted nanowire shape corresponding to a surface-energy saddle point. The analysis predicts that the minimum amplitude of a periodic shape perturbation to trigger fragmentation increases with cusp strength but decreases inversely proportionally to the perturbation wavelength for long wavelengths, in good quantitative agreement with phase-field predictions. The results provide the theoretical foundation to predict nanowire stability as a function of length and surface energy in diverse applications.

Double-perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. In this paper, we report the electronic structure and magnetic properties of an ordered double perovskite material, ${\mathrm{Ho}}_2{\mathrm{CoMnO}}_6$. In the double perovskites with general class $A_{2}BB{}^{\prime }{\mathrm{O}}_6$ ($A$ = rare-earth ions; $B$, $B{}^{\prime }$ = transition metal ions), the octahedral $B$ and $B{}^{\prime }$ sites have a distinct crystallographic site. The Rietveld refinement of x-ray diffraction data reveals that ${\mathrm{Ho}}_2{\mathrm{CoMnO}}_6$ crystallizes in the monoclinic $P{2}_1/n$ space group. X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibits a long-range ferromagnetic ordering at $T_{\mathrm{c}}\sim 76$ K owing to superexchange interaction between ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms the ferromagnetic behavior of this double perovskite. We observed a reentrant glassy state in the intermediate-temperature regime, which is attributed to inherent antisite disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. Temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above $T_{\mathrm{c}}$ in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infer the compound to be a narrow-band-gap insulator with the gap arising between the lower and upper Hubbard Co $d$ subbands. Our results demonstrate that antisite disorder and complex $3d\text{−}4f$ exchange interactions in the spin lattice account for the observed electronic and magnetic properties in this promising double perovskite material.

Recovery processes in metals with a high defect concentration are at the heart of a structural alloy's usability in engineering applications. However, such processes take place via nonconservative mechanisms mediated by point defect diffusion and are governed by their interactions with the underlying microstructure. Indeed, one of the main challenges in studying recovery processes is their intrinsic multiscale complexity, which often negates the direct use of experimental techniques. In this Research Letter, we study the mechanism of dislocation loop coalescence in metals by simulating the interaction dynamics between vacancies and prismatic loops using a stochastic framework that seamlessly bridges point defect kinetics with dislocation mechanics. Our simulations reveal two fundamental discoveries: first, that climb processes are governed by local fluctuations that are not captured by continuum elastodiffusion models, and, second, that prismatic loops create internal traps for vacancies, leading to the formation of rings around the perimeter of the loops that affect how coalescence takes place. Our results show an excellent agreement with in situ experimental observations of loop coalescence in Fe under irradiation.

A magnetic droplet soliton is a localized dynamic spin state which can serve as a nanoscale information carrier and nonlinear oscillator. The present opinion is that the formation of droplet solitons requires the compensation of magnetic damping by a torque created by a spin-polarized electric current or pure spin current. Here, we demonstrate theoretically that nanoscale droplet solitons can be generated and routed in ferromagnetic nanostructures with voltage-controlled magnetic anisotropy in the presence of uncompensated magnetic damping. Performing micromagnetic simulations for the MgO/Fe/MgO trilayer with almost perpendicular-to-plane magnetization, we reveal the formation of the droplet soliton under a nanoscale gate electrode subjected to a subnanosecond voltage pulse. The soliton lives up to 50 ns at room temperature and can propagate over micrometer distances in a ferromagnetic waveguide due to nonzero gradient of the demagnetizing field. Furthermore, we show that electrical routing of the soliton to different outputs of a spintronic device can be realized with the aid of an additional semiconducting nanostripe electrode creating controllable gradient of the perpendicular magnetic anisotropy.