Kesterite ${\mathrm{I}}_2\text{−}\mathrm{II}\text{−}\mathrm{IV}\text{−}{\mathrm{V}}_4$ semiconductors are promising solar absorbers for photovoltaics applications. The band gap and its character, either direct or indirect, are fundamental properties determining photovoltaic-device efficiency. We use a combination of accurate first-principles calculations and machine learning to predict the properties of the band gap for a large number of kesterite ${\mathrm{I}}_2\text{−}\mathrm{II}\text{−}\mathrm{IV}\text{−}{\mathrm{V}}_4$ semiconductors. In determining the magnitude of the fundamental gap, we compare results for a number of machine-learning models, and achieve a root mean squared error as low as 283 meV; the best results are achieved using support-vector regression with a radial-bias kernel. This error is well within the uncertainty of even the most advanced first-principles methods for calculating semiconductor band gaps. Predicting the direct-indirect property of the band gap is more challenging. After significant feature engineering, we are able to train a classifier that predicts the nature of the band gap with an accuracy of 89% using logistic regression. Using these trained models, the band gap properties of 1568 kesterite ${\mathrm{I}}_2\text{−}\mathrm{II}\text{−}\mathrm{IV}\text{−}{\mathrm{V}}_4$ compounds are predicted. We find 717 compounds with band gaps in the range 0.5–2.5 eV that can potentially act as solar absorbers and 242 materials with a band gap in the “optimum range” of 1.2–1.8 eV. The stability of these 242 compounds is assessed by calculating the energy above hull using the Materials Project database, and the band gaps are verified using hybrid functional calculations; in the end, we identify 25 compounds that are expected to be synthesizable and have a band gap in the range 1.2–1.8 eV—most of which are previously unexplored. These results will be useful in the materials engineering of efficient photovoltaic devices.

Highly anisotropic interfaces play an important role in the development of material microstructure. Using the diffusive atomistic phase-field crystal (PFC) formalism, we determine the capability of the model to quantitatively describe these interfaces. Specifically, we coarse grain the PFC model to attain both its complex amplitude formulation and its corresponding phase-field limit. Using this latter formulation, in one-dimensional calculations, we determine the surface energy and the properties of the Wulff shape. We find that the model can yield Wulff shapes with missing orientations, the transition to missing orientations, and facet formation. We show that the corresponding phase-field limit of the complex amplitude model yields a self-consistent description of highly anisotropic surface properties that are a function of the surface orientation with respect to the underlying crystal lattice. The phase-field model is also capable of describing missing orientations on equilibrium shapes of crystals and naturally includes a regularizing contribution. We demonstrate, in two dimensions, how the resultant model can be used to study growth of crystals with varying degrees of anisotropy in the phase-field limit.

Single crystals of ${\mathrm{AlFe}}_2{\mathrm{B}}_2$ have been grown using the self-flux growth method, and then we measured the structural properties, temperature- and field-dependent magnetization, and temperature-dependent electrical resistivity at ambient as well as high pressure. The Curie temperature of ${\mathrm{AlFe}}_2{\mathrm{B}}_2$ is determined to be 274 K. The measured saturation magnetization and the effective moment for the paramagnetic Fe ion indicate the itinerant nature of the magnetism with a Rhode-Wohlfarth ratio $\frac{M_C}{M_{\text{sat}}}\approx 1.14$. Temperature-dependent resistivity measurements under hydrostatic pressure show that transition temperature $T_C$ is suppressed down to 255 K for $p=2.24$ GPa pressure with a suppression rate of $\sim -8.9$ K/GPa. The anisotropy fields and magnetocrystalline anisotropy constants are in reasonable agreement with density functional theory calculations.

Despite decades of efforts, achieving $p$-type conductivity in the wide band gap ZnO in its ground-state wurtzite structure continues to be a challenge. Here we detail how $p$-type ZnO can be realized in a known metastable, high-pressure rocksalt phase (also wide-gap) with Li as an external dopant. Using modern defect theory, we predict Li to dope the rocksalt phase $p$-type by preferentially substituting for Zn and introducing shallow acceptor levels, resulting in predicted hole concentrations to exceed ${10}^{19}{\mathrm{cm}}^{-3}$. Formation of compensating donors like interstitial Li and unintentional hydrogen, ubiquitous in wurtzite phase, is inhibited by the close-packed nature of the rocksalt polymorph. Also, relatively high absolute valence band edge of rocksalt ZnO benefits low hole effective masses and hole delocalization. In addition to the technological significance, our results reveal polymorphism as a promising route to overcome strong doping asymmetry of wide band gap oxides.

There has been a long-standing debate on the coexistence between itinerant electrons and localized spins in the ${\mathrm{PdCrO}}_2$ delafossite. By means of the charge self-consistent combination of density functional theory and dynamical mean-field theory, it is corroborated that despite the overall remarkable metallic response, the ${\mathrm{CrO}}_2$ layers are indeed Mott insulating as suggested by previous experimental work. The resulting $\mathbf{k}$-resolved spectral function in the paramagnetic phase is in excellent agreement with available photoemission data. Subtle coupling between the itinerant and Mott-localized degrees of freedom is revealed. Different doping scenarios are simulated in order to manipulate the electronic states within the inert layers. In particular, oxygen vacancies prove effective in turning the hidden Mott insulator into a strongly correlated itinerant subsystem. The present results may open a new venue in research on quantum materials beyond canonical classification schemes.

Filled skutterudites are workhorse materials in thermoelectric research. In recent years, double- or even triple-filling has emerged as a promising strategy to improve the thermoelectric figure of merit in skutterudites. For example, one type of filler is used to reduce thermal conductivity while the other filler is used to control the electron filling. However, in these studies, each filler atom is considered independent of each other and the interaction between the two filler atoms is completely ignored. Here we present our detailed investigation of the local structure of filler atoms in (${\mathrm{Yb},\mathrm{In})}_x{\mathrm{Co}}_4{\mathrm{Sb}}_{12}$ and show that significant interaction does exist between filler species. While Yb or In filler atoms at low concentration occupy the usual $2a$ filler site on their own, when Yb filler concentration goes above a critical value $x\approx 0.15$, Yb pushes In atoms from the $2a$ site and into the $24g$ substitutional site replacing Sb atoms. This behavior is in stark contrast to that of Ga, which forms a dual-site defect even in the absence of Yb fillers. The temperature-dependent, extended x-ray absorption fine structure (EXAFS) data further reveal distinct lattice dynamical properties around In, Ga, and Yb filler atoms. Our findings point out that the filler interaction should be an important design consideration for new thermoelectric skutterudites.

High perpendicular magnetic anisotropy is essential for the development of high-efficiency spintronics devices. In this paper, we investigate the structural and magnetic properties of Mn-Fe-Ga Heusler thin films with perpendicular magnetic anisotropy inherent to the tetragonal $D{0}_{22}$ structure. High quality films were heteroepitaxially grown on ${\mathrm{SrTiO}}_3$ substrates by magnetron sputtering technique. The magnetic properties such as saturation magnetization and coercive field are easily tunable by the variation of the Mn/Fe ratio, while retaining out-of-plane magnetization over large composition range. The uniaxial anisotropy was improved in ${\mathrm{Mn}}_{2.6-x}{\mathrm{Fe}}_x{\mathrm{Ga}}_{1.4}$ through Fe substitution at a fixed Ga excess. Films with composition ${\mathrm{Mn}}_{1.6}{\mathrm{Fe}}_1{\mathrm{Ga}}_{1.4}$ were found to be stable down to a thickness of 10 nm. Transmission electron microscopy investigations proved the high quality of the films as well as chemical homogeneity. The Hall effect exhibits an anomalous contribution that dominates over the normal part, leading to Hall angle as high as $3.4\%$. These findings suggest great potential for the integration of tetragonal Heusler materials into spintronic devices.

Experiments on single crystal ${\mathrm{Li}}_X{\mathrm{FePO}}_4$ nanoparticles indicate rich nonequilibrium phase behavior, such as suppression of phase separation at high lithiation rates, striped patterns of coherent phase boundaries, and nucleation by binary-solid surface wetting and intercalation waves. These observations have been successfully predicted (prior to the experiments) by one-dimensional (1D) depth-averaged phase-field models, which neglect any subsurface phase separation. In this paper, using an electro-chemo-mechanical phase-field model, we investigate the coherent nonequilibrium subsurface phase morphologies that develop in the $ab$ plane of plateletlike single-crystal plateletlike ${\mathrm{Li}}_X{\mathrm{FePO}}_4$ nanoparticles. Finite element 2D plane-stress and plane-strain simulations are performed in the $ab$ plane and validated by 3D simulations, showing similar results. Using a realistic material model from previous work, we show that the anisotropy of the interfacial tension (or gradient penalty) tensor and its relation to electro-auto-catalytic surface intercalation reactions plays a crucial role in determining the subsurface phase morphology. With the standard assumption of an isotropic interfacial tension tensor, subsurface phase separation in the bulk is observed and its morphology is independent of the reaction kinetics at the surface, but for strong anisotropy, phase separation is controlled by surface reactions, as assumed in 1D models. Moreover, the driven intercalation reaction suppresses phase separation during lithiation, while enhancing it during delithiation, by electro-auto-catalysis, in quantitative agreement with in operando imaging experiments in single-crystalline nanoparticles, given measured reaction rate constants.

Due to the long-range oscillatory character of RKKY exchange interactions, for ${\mathrm{Dy}}_2{\mathrm{Co}}_3{\mathrm{Al}}_9$ there exist positive and negative couplings between the Dy magnetic moments that lead to magnetic frustration. As a result, the ground state can be easily perturbed, and the system displays a number of spontaneous and field-induced phase transitions. We performed magnetization, magnetic-susceptibility, specific-heat, and electrical-resistivity measurements as well as neutron-diffraction experiments on a single crystal. We find two transitions to distinct incommensurate antiferromagnetic phases at 6.2 and 5.2 K that evolve to a commensurate phase at 3.7 K. In applied magnetic field, new phases emerge. Field-dependent magnetization exhibits a multistep metamagnetic process with sharp transitions accompanied by pronounced magnetoresistance changes. The large number of phases and their complex magnetic structures suggest that the physical properties of ${\mathrm{Dy}}_2{\mathrm{Co}}_3{\mathrm{Al}}_9$ are ruled by exchange frustration in the presence of a large magnetocrystalline anisotropy.

We synthesized high-quality, single-crystalline thin films of layered rare-earth (R) palladates R${}_2{\mathrm{PdO}}_4$  ($R$ = La, Nd, and Sm) and Ce substituted ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{PdO}}_4$ by reactive molecular beam epitaxy. For ${\mathrm{La}}_2{\mathrm{PdO}}_4$, ${\mathrm{Nd}}_2{\mathrm{PdO}}_4$, and ${\mathrm{Sm}}_2{\mathrm{PdO}}_4$, we find that the electronic conduction is independent of R elements. Doping charge carriers into the ${\mathrm{PdO}}_2$ planes of ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{PdO}}_4$ increases the electronic conduction and this effect is enhanced by vacuum annealing. The enhanced electronic conduction originates not solely from the doped charge carriers but is superimposed by Pd vacancies. X-ray photoelectron spectroscopy combined with inductively coupled plasma mass spectrometry revealed Pd deficiencies in ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{PdO}}_4$  and these defects play a crucial role for the electronic conduction. We observe a monotonic enhancement of the electronic conduction in ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{PdO}}_4$  thin films induced by Ce substitution and vacuum annealing. The estimated charge carrier concentration necessary for metallic conduction ($x\approx 0.45$) at $T=0$ K is far beyond the experimentally accessible solubility limit.

Ceramic materials have become widely used in various fields of material science, and ceramic oxides such as cubic zirconia (c-${\mathrm{ZrO}}_2)$ and magnesia (MgO) are candidate materials for nuclear energy applications. For the corresponding in-service conditions of these materials, there is a crucial need in studies at moderate or high temperatures of the physical phenomena underlying the damage buildup under irradiation. In the present work, we show, using x-ray diffraction, that these two materials exhibit a similar damage accumulation process under ion irradiation at fixed temperature. However, they display an unexpected opposite damaging rate to changes in the irradiation temperature. In fact, as the temperature is increased, the final damage state is reached earlier in c-${\mathrm{ZrO}}_2$ while it is delayed in MgO. Rate equation cluster dynamics simulations show that defect clustering is favored over defect recombination in c-${\mathrm{ZrO}}_2$, but the situation is reversed for MgO, explaining the observed opposite response to temperature of the two materials. This contrasting behavior can be rationalized in terms of nonequivalent interstitial versus vacancy defect migration energies in MgO. We finally demonstrate that these results allow for a qualitative prediction of the evolution of the experimental irradiation-induced disorder with temperature, henceforth potentially reducing the cost in selecting and developing ad hoc materials.

Ferroelectric field-effect doping has emerged as a powerful approach to manipulate the ground state of correlated oxides, opening the door to a different class of field-effect devices. However, this potential is not fully exploited so far, since the size of the field-effect doping is generally much smaller than expected. Here we study the limiting factors through magnetotransport and scanning transmission electron and piezoresponse force microscopy in ferroelectric/superconductor $\left(\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7\text{−}\delta }/\mathrm{BiFe}{\mathrm{O}}_3\right)$ heterostructures, a model system showing very strong field effects. Still, we find that they are limited in the first place by an incomplete ferroelectric switching. This can be explained by the existence of a preferential polarization direction set by the atomic terminations at the interface. More importantly, we also find that the field-effect carrier doping is accompanied by a strong modulation of the carrier mobility. Besides making quantification of field effects via Hall measurements not straightforward, this finding suggests that ferroelectric poling produces structural changes (e.g., charged defects or structural distortions) in the correlated oxide channel. Those findings have important consequences for the understanding of ferroelectric field effects and for the strategies to further enhance them.

We present diffusion Monte Carlo (DMC) results for equation of state and quasiparticle gaps of manganese binary oxides MnO and ${\mathrm{MnO}}_2$ and the ternary oxide ${\mathrm{LaMnO}}_3$. Owing to the limited approximations made and the direct treatment of electronic correlations, our DMC-based study correctly describes structural properties such as the lattice constant, bulk moduli, and cohesive energies. It correctly predicts the ground-state phase of these oxides, which have different valences. Our study demonstrates the capability of DMC methods to predict the structural properties of highly correlated systems, which have been identified as suitable candidates for many applications ranging from catalysis to electronic devices. Our study also serves as a benchmark for both the manganese pseudopotential and other methodological choices to be used in calculations of similar oxides.

We model the kinetics of nanoring (NR) formation in stressed thin films and explore systematically the resulting nanoring structures upon variation of the film thermal processing parameters. We find that the thermal stress induced in the films during their thermal annealing can trigger the transformation of quantum dots on the film surface to simple NRs or multiple concentric NRs. Our analysis provides a comprehensive explanation of experimentally observed nanoring formation on stressed film surfaces, as well as a design rule for fabricating NRs with precisely controlled features and sizes.

Strongly correlated materials show unique solid-state phase transitions with rich nanoscale phenomenology that can be controlled by external stimuli. Particularly interesting is the case of light–matter interaction in the proximity of the metal–insulator transition of heteroepitaxial nickelates. In this work, we use near-infrared laser light in the high-intensity excitation regime to manipulate the nanoscale phase separation in ${\mathrm{NdNiO}}_3$. By tuning the laser intensity, we can reproducibly set the coverage of insulating nanodomains, which we image by photoemission electron microscopy, thus semipermanently configuring the material state. With the aid of transport measurements and finite element simulations, we identify two different timescales of thermal dynamics in the light–matter interaction: a steady-state and a fast transient local heating. These results open interesting perspectives for locally manipulating and reconfiguring electronic order at the nanoscale by optical means.

The reliable calculation of electronic structures and understanding of electrical properties depends on an accurate model of the crystal structure. Here we have reinvestigated the crystal structure of the high-$zT$ thermoelectric material tin selenide, SnSe, between 4 and 1000 K using high-resolution neutron powder diffraction. Symmetry analysis reveals the presence of four active structural distortion modes, one of which is found to be active over a relatively wide range of more than $\pm 200$ K around the symmetry-breaking $Pnma\text{−}Cmcm$ transition at 800 K. Density functional theory calculations on the basis of the experimental structure parameters show that the unusual, steplike temperature dependencies of the electrical transport properties of SnSe are caused by the onset of intrinsic bipolar conductivity, amplified and shifted to lower temperatures by a rapid reduction of the band gap between 700 and 800 K. The calculated band gap is highly sensitive to small out-of-plane Sn displacements observed in the diffraction experiments. SnSe with a sufficiently controlled acceptor concentration is predicted to produce simultaneously a large positive and a large negative Seebeck effect along different crystal directions.

Particulate films obtained by drying colloidal dispersions often displaying a network of cracks are ubiquitous. In particulate films, drying-induced internal stresses are accumulated as a result of the consolidation of particles during solvent evaporation and are released by the formation of cracks. The morphology of crack patterns can be controlled by externally applied fields, modulation of drying kinetics, or by exploiting the shape anisotropy of the colloids. Here, we explore the role of particle shape anisotropy and externally applied thermal and magnetic fields on the evolution of cracks by performing desiccation experiments, i.e., by drying colloidal-ellipsoid-laden sessile drops on solid substrates. We determine the critical aspect ratio $\left({\alpha }_c\right)$, where the cracks in the dried particulate deposit drastically change their orientation from a radial to circular pattern. It is observed that the drying of drops consisting of ellipsoids at an elevated temperature enhances the randomness in the particle arrangement and brings nontrivial modifications to the crack pattern. In addition, we show that the randomness in the particle arrangement due to the thermal field can be compensated by the application of an external magnetic field and the orientation of cracks can be completely restored.

Since oxide materials like ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ are usually applied as thin films, we studied the effect of biaxial strain, resulting from the substrate, on the electronic and magnetic properties and, in particular, on the formation energy of point defects. From our first-principles calculations, we determined that the probability of forming point defects, like vacancies or substitutions, in ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ could be adjusted by choosing a proper substrate. For example, the amount of antisite disorder can be reduced with compressive strain in order to obtain purer ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ as needed for spintronic applications, while the formation of oxygen vacancies is more likely for tensile strain, which improves the functionality of ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ as a basis material of solid oxide fuel cells. In addition, we were also able to include the oxygen partial pressure in our study by using its thermodynamic connection with the chemical potential. Strontium vacancies become, for example, more likely than oxygen vacancies at a pressure of $1\mathrm{bar}$. Hence, this degree of freedom might offer in general another potential method for defect engineering in oxides aside from, e.g., experimental growth conditions like temperature or gas pressure.

Tail state formation in solar cell absorbers leads to a detrimental effect on solar cell performance. Nevertheless, the characterization of the band tailing in experimental semiconductor crystals is generally difficult. In this paper, to determine the tail state generation in various solar cell materials, we have developed a quite general theoretical scheme in which the experimental Urbach energy is compared with the absorption edge energy derived from density-functional theory (DFT) calculation. For this purpose, the absorption spectra of solar cell materials, including CdTe, ${\mathrm{CuInSe}}_2$ (CISe), ${\mathrm{CuGaSe}}_2$ (CGSe), ${\mathrm{Cu}}_2{\mathrm{ZnSnSe}}_4$ (CZTSe), ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ (CZTS), and hybrid perovskites, have been calculated by DFT particularly using very-high-density $k$ meshes. As a result, we find that the tail state formation is negligible in CdTe, CISe, CGSe, and hybrid perovskite polycrystals. However, coevaporated CZTSe and CZTS layers exhibit very large Urbach energies, which are far larger than the theoretical counterparts. Based on DFT analysis results, we conclude that the quite large tail state formation observed in the CZTSe and CZTS originates from extensive cation disordering. In particular, even a slight cation substitution is found to generate unusual band fluctuation in CZTS(Se). In contrast, ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ hybrid perovskite shows the sharpest absorption edge theoretically, which agrees with experiment.