Nickel oxide, NiO, is an important $p$-type oxide semiconductor that has been studied for applications in solar cells, junction diodes, and other optoelectronic devices. In a nominally undoped NiO, depending on its oxygen stoichiometry, it only has a modest $p$-type conductivity of ∼0.1 S/cm due to Ni vacancy acceptors. However, the overall transport can be improved by extrinsic doping. In this study, we carry out a combined experiment and computational study of the effects of acceptor dopants, including Li, Ag, and Cu on the properties of NiO. Our ab initio calculations show that among all the acceptors studied, substitutional Li (${\mathrm{Li}}_{\mathrm{Ni}}$) acceptor species has the lowest formation and ionization energies. Measured electrical properties of the undoped and doped oxygen-rich NiO ($\mathrm{Ni}{\mathrm{O}}_{1+\delta }$) show an increase in conductivity and hole concentration for the doped samples. In particular, Li is an efficient acceptor to achieve highly conducting $p$-type NiO with >40% transmittance in the visible range for a 100-nm-thick film. The improvement in the electrical properties with different dopant species studied is in good agreement with the calculated defect formation and ionization energies. A remarkable increase in the temperature-dependent Hall mobility is also observed in the doped samples. Based on the small-polaron hoping model, we analyzed the conduction mechanism in the doped samples, which revealed a hopping dominated activation with energies in the range of 172–208 meV.

Over the last few decades, extensive previous studies of the nonlinear response of metal nanoparticles (NPs) report a wide variation of nonlinear coefficients, thus, revealing a highly confused picture of the underlying physics. This naturally prevents rational design of these systems for practical devices. Here we provide a systematic study of the nonlinear response of metal spheres under continuous wave (CW) illumination within a purely thermal model, i.e., whereby the illumination only acts to modify the optical and thermal parameters via their dependence on the temperature. We characterize the strong dependence of the temperature rise and overall thermo-optic nonlinear response on the particle size and permittivity, on the optical and thermal host properties, as well as on the thermo-derivatives of these properties. This dependence on the nonintrinsic parameters explains why it is inappropriate to extract an intrinsic nonlinear coefficient from a specific system; equivalently, it explains the large differences in reported values for such systems, as well as for more complicated metal-dielectric systems and even pulsed illumination schemes. Despite the revealed complex multiparameter dependence, we managed to uncover a rather simple behavior of the nonlinear response. In particular, we show that the nonlinearity coefficients exhibit a dependence on the illumination intensity which mimics the dependence of the temperature itself on the illumination intensity, namely, it grows for small NP sizes, reaches a maximum and then decreases monotonically for larger NPs. The improved modeling allows us to demonstrate an overall nonlinear response which is about a 1000 times higher than in other strongly nonlinear systems (e.g., $\epsilon$-near-zero systems); it also provides an excellent match to experimental measurements of the scattering from a single metal NP, thus, confirming the dominance of the thermal nonlinear mechanism. Our work lays the foundations for an overall evaluation of previous studies of the nonlinear response of metal-dielectric system under general conditions.

The synthesis and single-crystal growth of a rare-earth high-entropy oxide ${\left({\mathrm{Yb}}_{0.2}{\mathrm{Tb}}_{0.2}{\mathrm{Gd}}_{0.2}{\mathrm{Dy}}_{0.2}{\mathrm{Er}}_{0.2}\right)}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ in the cubic pyrochlore phase $Fd\overline{3}m$ are achieved by solid-state reaction and optical floating-zone growth techniques. X-ray diffraction and a structure refinement analysis confirm a single-phase pyrochlore structure and lattice parameter of $a=10.1152\left(3\right)\AA$. Additional characterization techniques, including scanning transmission electron microscopy and nanoscale electron-energy loss spectroscopy mapping, support a single-phase pyrochlore structure with a homogeneous mixture down to ∼2.3 nm. Magnetization measurements on a single crystal reveal antiferromagnetic correlations with a spin-glass ground state. These results show that it is possible to grow large single crystals of a high entropy pyrochlore oxide, expanding the avenues for future high-entropy oxide research.

As a new family of quasicrystals, oxide quasicrystals (OQCs) were recently discovered in Ba-Ti-O films on Pt(111). Here we report on the growth and composition of an ultrathin Ba-Ti-O OQC film and its crystalline approximant, studied by scanning tunneling microscopy (STM), low-energy electron diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy. STM images reveal a wide-scale ultrathin OQC film without BaO islands, and magnified STM images display typical OQC clusters. The ultrathin OQC film has been prepared by annealing an oxide crystalline approximant with decreasing Ti concentration. We also found that the ultrathin Ba-Ti-O film forms a $(2\times 2)$ superstructure on annealing the ultrathin OQC film. For quantitative analysis, the XPS peak intensity was measured as a function of elemental atomic density. The elemental atomic densities for the ultrathin OQC film were determined to be $8\times {10}^{14}$, $4\times {10}^{14}$, and $3\times {10}^{14}\mathrm{atoms}/{\mathrm{cm}}^2$ for Ba, Ti, and O, respectively.

Topological transport phenomena in magnetic materials are a major topic of current condensed matter research. One of the most widely studied phenomena is the topological Hall effect (THE), which is generated via spin-orbit interactions between conduction electrons and topological spin textures such as skyrmions. We report a comprehensive set of Hall effect and magnetization measurements on epitaxial films of the prototypical ferromagnetic metal ${\mathrm{SrRuO}}_3$ the magnetic and transport properties of which were systematically modulated by varying the concentration of Ru vacancies. We observe Hall effect anomalies that closely resemble signatures of the THE, but a quantitative analysis demonstrates that they result from inhomogeneities in the ferromagnetic magnetization caused by a nonrandom distribution of Ru vacancies. As such inhomogeneities are difficult to avoid and are rarely characterized independently, our results call into question the identification of topological spin textures in numerous prior transport studies of quantum materials, heterostructures, and devices. Firm conclusions regarding the presence of such textures must meet stringent conditions such as probes that couple directly to the noncollinear magnetization on the atomic scale.

Organic-inorganic hybrid metal-halide perovskite (OIHP) materials provide a tunable platform for engineering their optoelectronic properties. Although several high-pressure studies have been conducted from the OIHP family of single crystals and films, the exact nature of the dynamic coupling of the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3$ (MA) cation with the octahedral lattice framework and the mechanisms responsible for the structural phase transformation under pressure are not well captured. By combined photoluminescence (PL), synchrotron-based x-ray diffraction, and Raman-scattering studies as a function of pressure from methylammonium lead bromide ($\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_3$), we shed light on an isostructural phase transition due to the coupling of the MA cation and the $\mathrm{Pb}{\mathrm{Br}}_6$ lattice through hydrogen bonding. The sharp discontinuities at $\sim$ 1 GPa and $\sim$ 3 GPa in the PL peak positions correlate with the structural changes observed in high-pressure XRD and Raman-scattering studies. The electronic band edge as a function of pressure is calculated within density-functional theory. The PL peak position, intensity and width of the excitonic peak show significant changes at 2 GPa, which corroborate the changes observed in high-pressure Raman-scattering studies. The frequencies of the lattice modes and the C–H/N–H bending and stretching modes of the MA cation show anomalous changes and other nuances at 2 GPa. The suppression of rotational and orientational disorder of the organic moiety is initiated at 2 GPa and the ordering is completed by 3.0 GPa, leading to an order-disorder type cubic II ($Im\overline{3}$) to orthorhombic ($Pnma$) phase transition. Along with the revelation of an isostructural transformation at 2 GPa, this paper highlights the impact of molecular vibrations on the electronic properties of $\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_3$ under pressure.

We have studied the effects of electrical current pulses on skyrmion formation in a series of Co/Ni/Pt-based multilayers. Transmission x-ray microscopy reveals that by applying electrical current pulses of duration and current density on the order of 50 μs and $1.7\times {10}^{10}\mathrm{A}/{\mathrm{m}}^2$, respectively, in an applied magnetic field of 50 mT, stripe-to-skyrmion transformations are attained. The skyrmions formed by such pulses remain stable across a wide range of magnetic fields, including zero field. We attribute the transformation primarily to current-induced Joule heating on the order of ∼128 K. Reducing the magnetic moment and perpendicular anisotropy using thin rare-earth spacers reduces the pulse duration, current density, and magnetic field necessary to 25 µs, $2.4\times {10}^9\mathrm{A}/{\mathrm{m}}^2$, and 27 mT, respectively. These findings show that energetic inputs allow for the formation of skyrmion phases in a broad class of materials, and that material properties can be tuned to yield more energy-efficient access to skyrmion phases.

Coarsening of bicontinuous microstructures is observed in a variety of systems, such as nanoporous metals and mixtures that have undergone spinodal decomposition. To better understand the morphological evolution of these structures during coarsening, we compare the morphologies resulting from two different coarsening mechanisms, surface and bulk diffusion. We perform phase-field simulations of coarsening via each mechanism in a two-phase mixture at nominal volume fractions of 50%-50% and 36%-64%, and the simulated structures are characterized in terms of topology (genus density), the interfacial shape distribution, structure factor, and autocorrelations of phase and mean curvature. We observe self-similar evolution of morphology and topology and agreement with the expected power laws for dynamic scaling, in which the characteristic length scale increases over time proportionally to $t^{1/4}$ for surface diffusion and $t^{1/3}$ for bulk diffusion. While we observe the expected difference in the coarsening kinetics, we find that differences in self-similar morphology due to coarsening mechanism are relatively small, although typically they are larger at 36% volume fraction than at 50% volume fraction. In particular, we find that bicontinuous structures coarsened via surface diffusion have lower scaled genus densities than structures coarsened via bulk diffusion. We also compare the self-similar morphologies to those in literature and to two model bicontinuous structures, namely, constant-mean-curvature surfaces based on the Schoen G minimal surface and random leveled-wave structures. The average scaled mean curvatures of these model structures agree reasonably with those of the coarsened structures at both 36% and 50%, but we find substantial disagreements in the scaled genus densities and the standard deviations of mean curvature.

One of the most ubiquitous and important defects in solids is oxygen. Knowledge about the solubility and diffusivity of oxygen in materials is crucial to understand a number of important technological processes, such as oxidation, corrosion, and heterogeneous catalysis. Density-functional theory calculations of the thermodynamics and kinetics of oxygen in cobalt show that oxygen diffusing into the two close-packed phases, namely $\alpha$ (hcp) and $\beta$ (fcc), strongly interacts with vacancies. We observe the formation of oxygen split-vacancy centers (V-${\mathrm{O}}_{\mathrm{i}}$-V) in both phases, and we show that this defect complex exhibits a similar migration energy barrier to the vacancy and oxygen interstitials. In contrast to the vacancy and oxygen interstitials, the oxygen split-vacancy centers exhibit an anisotropic strain field that couples to applied stress, making it possible to observe them through an internal friction experiment on quenched cobalt.

A phase-field crystal model based on the density-field approach incorporating high-order interparticle direct correlations is developed to study vapor-liquid-solid coexistence and transitions within a single continuum description. Conditions for the realization of the phase coexistence and transition sequence are systematically analyzed and shown to be satisfied by a broad range of model parameters, demonstrating the high flexibility and applicability of the model. Both temperature-density and temperature-pressure phase diagrams are identified, while structural evolution and coexistence among the three phases are examined through dynamical simulations. The model is also able to produce some temperature and pressure related material properties, including effects of thermal expansion and pressure on equilibrium lattice spacing, and temperature dependence of saturation vapor pressure. This model can be used as an effective approach for investigating a variety of material growth and deposition processes based on vapor-solid, liquid-solid, and vapor-liquid-solid growth.

Using density functional theory combined with a Hubbard model ($\mathrm{DFT}+$ $U$), the electronic band structure of $\mathrm{Cr}{\mathrm{I}}_3$ multilayers, both freestanding and enclosed between graphene contacts, is calculated. We show that the $\mathrm{DFT}+$ $U$ approach, together with the “around-mean-field” correction scheme, is able to describe the vertical magnetotransport in line with the experimental measurements of magnetoresistance in multilayered $\mathrm{Cr}{\mathrm{I}}_3$ enclosed between graphene contacts. Moreover, by interpolating between different double-counting correction schemes, namely the around-mean-field correction and the fully localized limit, we show their importance for consistently describing both the band structure and the ground-state total energy. Our description of the magnetic exchange interaction is compatible with the experimentally observed antiferromagnetic ground state in the bilayer $\mathrm{Cr}{\mathrm{I}}_3$ and the transition to a ferromagnetic arrangement in a small external magnetic field. Thus, using spin-polarized $\mathrm{DFT}+$ $U$ with an around-mean-field correction, a consistent overall picture is achieved.

We performed neutron-diffraction experiments and density functional theory calculations to study the magnetostructural coupling in $\mathrm{MnCoGe}{\mathrm{B}}_x$ ($x=0$, 0.01, and 0.05) alloys. By varying the amount of boron addition, we are able to freely switch the magnetostructural coupling on and off in the MnCoGe alloys. It is found that the boron addition stabilizes the high-temperature hexagonal phase due to the reduced interatomic distances and the enhanced covalent bonding. The hexagonal-orthorhombic structural transition shifts to low temperatures with the boron addition and coincides with the paramagnetic-ferromagnetic (PM-FM) transition in the $\mathrm{MnCoGe}{\mathrm{B}}_{0.01}$ alloy. With a further increase in the boron addition, the structural and magnetic transitions are decoupled again. The hexagonal-orthorhombic structural transition is significantly suppressed in the $\mathrm{MnCoGe}{\mathrm{B}}_{0.05}$ alloy, although subtle distortions in the hexagonal structure are evidenced by a canted spin arrangement below 75 K. The MnCoGe and $\mathrm{MnCoGe}{\mathrm{B}}_{0.01}$ alloys show a collinear FM structure, having a much larger Mn moment than the $\mathrm{MnCoGe}{\mathrm{B}}_{0.05}$ alloy. The relatively small Mn moment in the $\mathrm{MnCoGe}{\mathrm{B}}_{0.05}$ alloy can be attributed to the shortened Mn-Mn distance and the enhanced overlap of the $3d$ orbitals between the neighboring Mn atoms. The uncovered relationship between the structural evolution and the sizable magnetic moment in the present work offers more insight into the magnetostructural coupling in the MnCoGe-based alloys.

We searched for stable crystal structures of ${\mathrm{YFe}}_{12}$ using a crystal structure prediction technique based on a genetic algorithm and first-principles calculations. We obtained two monoclinic $C2/m$ structures as metastable phases that are different from the well-known ${\mathrm{ThMn}}_{12}$ structure. These two phases have advantages in their magnetism over the ${\mathrm{ThMn}}_{12}$ structure: The total magnetization $M$ is increased from $25.6{\mu }_{\mathrm{B}}$/f.u. up to $26.8{\mu }_{\mathrm{B}}$/f.u. by the transformations. We also calculated Curie temperature $T_{\mathrm{C}}$ for these structures within the mean-field approximation, and we predicted the increase of $T_{\mathrm{C}}$ from 792 up to 940 K, which is mainly caused by the increase of intersite magnetic couplings within the distance of 2.3–3.1 Å. The similar enhancements of $M$ and $T_{\mathrm{C}}$ are also obtained in the pseudobinary system $\mathrm{Y}{\left({\mathrm{Fe}}_{1\text{−}x}{\mathrm{Co}}_x\right)}_{12}$ with $x$ of 0–0.7.

$\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ is emerging as an excellent potential semiconductor for high power and optoelectronic devices. However, the successful development of $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ in a wide range of applications requires a full understanding of the role and nature of its point and extended defects. In this work, high quality epitaxial $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ films were grown on sapphire substrates by metal-organic chemical vapor deposition and fully characterized in terms of structural, optical, and electrical properties. Then defects in the films were investigated by a combination of depth-resolved Doppler broadening and lifetime of positron annihilation spectroscopies and thermally stimulated emission (TSE). Positron annihilation techniques can provide information about the nature and concentration of defects in the films, while TSE reveals the energy level of defects in the bandgap. Despite very good structural properties, the films exhibit short positron diffusion length, which is an indication of high defect density and long positron lifetime, a sign for the formation of Ga vacancy related defects and large vacancy clusters. These defects act as deep and shallow traps for charge carriers as revealed from TSE, which explains the reason behind the difficulty of developing conductive $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ films on non-native substrates. Positron lifetime measurements also show nonuniform distribution of vacancy clusters throughout the film depth. Further, the work investigates the modification of defect nature and properties through thermal treatment in various environments. It demonstrates the sensitivity of $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ microstructures to the growth and thermal treatment environments and the significant effect of modifying defect structure on the bandgap and optical and electrical properties of $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$.

We use ab initio calculations to study the role of stacking faults in connecting the high-temperature B2 and the theoretically predicted low-temperature B33 NiTi phases. In contrast with prior work, we describe the $\mathrm{B}2\to \mathrm{B}33$ phase transformation in terms of alternate bilayer shifts by $\frac{1}{2}$ [100] on the ${\left(011\right)}_{\mathrm{B}2}$ plane, obtaining a viable pathway; the same mechanism could also work with the B19 parent phase. We then examine B33-like structures built from alternate stacking sequences of B19 bilayers, constructed to have monoclinic tilt angles close to the experimentally reported NiTi B19′ martensite, and find four low-energy stacking-fault variants with energies 5.8–8.5 meV/atom above the calculated B19′ martensite structure, suggesting that such structures might appear as a part of the NiTi martensite phase at low temperatures. Investigating further the occurrence of specific coordinated planar shifts in NiTi systems, we report a dynamically stable NiTi B27 phase and find that it is only 1.2 meV/atom above the calculated B33 ground-state structure, thus having a potential to also play a role in NiTi martensitic phase transformation.

The influence of martensitic-transformation-induced plastic deformation in steel is studied using phase field simulations. A phase field framework that incorporates elastic as well as plastic effects is used. A high-carbon steel is considered as an example to illustrate the coupling of martensitic transformations and plasticity. In this work, all 24 transformation variants associated with the Kurdjumov-Sachs orientational relationship are considered to realistically describe the martensitic transformation in steel. Temperature-induced as well as stress-induced transformations are studied. The role of plasticity is investigated by performing simulations with and without the plastic flow. It is found that transformation plasticity plays a dual role: (1) in the case of temperature-induced transformations it helps in the initial nucleation of the martensite by stabilizing the initial embryo, and (2) once the martensite starts to grow, transformation-induced plasticity resists the further growth and results in stabilization of retained austenite. In contrast, the simulations in the absence of transformation-induced plasticity show that the entire system transforms into martensite. For stress-induced transformations, it is found that transformation induces plastic deformations, even though the applied (macroscopic) stress is lower than the yield stress. Although the dominant contribution to the stress-induced strain arises due to the formation of favored martensite variants, transformation-induced plasticity generates significant additional deformation that can influence the mechanical properties and the resulting martensite domain pattern.

${\mathrm{La}}_2\mathrm{Li}\mathrm{Mo}{\mathrm{O}}_6$ is a double perovskite (DP) with $P{2}_1/n$ symmetry based on the ${\mathrm{Mo}}^{5+}$ ion, $4d^1$, ${t_{2\mathrm{g}}}^1$, $S=1/2$. It is isostructural with ${\mathrm{Sr}}_2\mathrm{Y}\mathrm{Mo}{\mathrm{O}}_6$, the magnetic ground state of which is apparently a very unusual collective spin singlet or valence-bond glass state as is the case for cubic ($Fm\text{−}3m$) ${\mathrm{Ba}}_2\mathrm{Y}\mathrm{Mo}{\mathrm{O}}_6$. Initial studies of ${\mathrm{La}}_2\mathrm{Li}\mathrm{Mo}{\mathrm{O}}_6$ suggested a different ground state from the other DPs but no clear conclusions could be drawn. A more detailed study is presented here including magnetic susceptibility, heat capacity, and elastic neutron-scattering results. This DP is now well characterized as an antiferromagnet, $T_{\mathrm{N}}=18\mathrm{K}$, via observation of magnetic Bragg peaks in neutron scattering and an anomaly in the magnetic heat capacity. The ordering wave vector is $\mathit{k}=(1/21/20)$, consistent with a type I face-centered-cubic magnetic structure, and the ordered moment on ${\mathrm{Mo}}^{5+}$ is $0.32\left(11\right){\mu }_{\mathrm{B}}$, much reduced from the spin-only value of $1{\mu }_{\mathrm{B}}$. The index, $f=|{\vartheta }_c|/T_{\mathrm{N}}\sim 3$, indicates a low level of frustration. The heat-capacity data above $T_{\mathrm{N}}$ can be interpreted in terms of a one-dimensional spin-correlation model, as can the low-temperature data which follow a $T^1$ power law. This is consistent with an earlier suggestion. The difference with isostructural ${\mathrm{Sr}}_2\mathrm{Y}\mathrm{Mo}{\mathrm{O}}_6$ is attributed to differences in the local distortion of the Mo–O octahedron and the resulting orbital ordering.