Transparent conductors (TCs), which bring two seemingly contraindicated properties, conductivity and transparency, together into one material, enable many critical technologies. Significant successes have been achieved for $n$-type TCs, such as Sn-doped $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$, but developing their $p$-type counterparts has still encountered a big challenge, mainly due to the intrinsic band structure of conventional semiconductors. Here, we propose that a class of wide-gap inorganic metal halide perovskites have great potential as ideal $p$-type TCs, because of their inverted band structure compared to $n$-type TCs, i.e., they have $s$-like wave-function character at the top of the valence band, and $p$-like wave-function character at the lower valence bands and the bottom of the conduction band, which results in significantly improved $p$-type dopability, high hole mobility, and good optical transparency. This insight of designing $p$-type TCs from an inverted band structure opens an avenue for the future design of critical optoelectronic materials.

Earth-abundant copper barium thioselenostannate, $\mathrm{C}{\mathrm{u}}_2\mathrm{BaSn}{\left(\mathrm{S},\mathrm{Se}\right)}_4$, absorbers have recently demonstrated promising optoelectronic and defect resistance properties for solar harvesting applications. The highest photovoltaic device efficiencies have been achieved in vacuum-based co-sputter deposited films, yet there is a tendency for a multilayer formation consisting of large, plateletlike surface grains and a smaller grain-sized underlayer (often accompanied by voids). In this work we use a combination of in situ and ex situ x-ray diffraction and scanning electron microscopy to unravel the coupling of phase evolution to film morphology. We find that Cu surface migration and associated Cu-rich phases play a defining role in determining the overall film structure.

The transition metal dichalcogenide ${\mathrm{TiSe}}_2$ has received significant research attention over the past four decades. Different studies have presented ways to suppress the 200 K charge-density-wave transition, vary low-temperature resistivity by several orders of magnitude, and stabilize magnetism or superconductivity. Here we give the results of a synthesis technique whereby samples were grown in a high-pressure environment with up to 180 bar of argon gas. Above 100 K, properties are nearly unchanged from previous reports, but a distinct hysteretic resistance region begins around 80 K, accompanied by insulating low-temperature behavior. An accompanying decrease in carrier concentration is seen in Hall effect measurements, and photoemission data show a removal of an electron pocket from the Fermi surface in an insulating sample. We conclude that high inert gas pressure synthesis accesses an underlying nonmetallic ground state in a material long speculated to be an excitonic insulator.

Based on high-throughput density-functional theory calculations, we investigated the stability and physical properties of the MAX phases with the chemical formula ${\mathrm{M}}_2\mathrm{AX}$. Starting from 1080 possible compositions with the M element being one of the $3d$, $4d$, or $5d$ transition metal elements, the A element being a main group element, and X being either C or N, we identified 82 compounds satisfying all three stability criteria (thermodynamic, mechanical, and dynamic stabilities), in addition to 48 synthesized systems as a validation of our workflow. The trend in the stability was analyzed based on the crystal orbital Hamilton population, concluding that increasing the number of electrons on the A and M sites destabilizes the MAX phase. The mechanical, thermoelectric, and topological properties of the candidate compounds are characterized by detailed calculations, resulting in anisotropic thermoelectric effect for all MAX compounds and a promising candidate topological insulator.

We self-consistently derive a formalism that couples a phase field crystal (PFC) density field to thermal transport. It yields a theory for nonuniform transient temperature and density evolution, and includes local latent heat release during atomic rearrangements of the PFC density field. The basic formalism is applied to the original PFC model, demonstrating its capacity to capture heat transfer and recalescence in solidification. With an aim towards consistently incorporating temperature and other thermodynamic variables into PFC modeling, a new classical density field theory for solid/liquid/vapor systems is then derived. It presents a different approach to those used in the PFC literature while retaining the major advantages that have become the hallmark of PFC modeling; the new model is also based entirely on physical density, temperature, and pressure scales. We end the paper by applying the thermal-density coupling formalism to this new multiphase density functional theory/PFC model.

The discovery of ferroelectricity in both pure and doped ${\mathrm{HfO}}_2$-based thin films have revitalized interest in using ferroelectrics for nanoscale device applications. To take advantage of this silicon-compatible ferroelectric, fundamental questions such as the origin of ferroelectricity and better approach to controlled realization of ferroelectricity at the nanoscale need to be addressed. The emergence of robust polarization in ${\mathrm{HfO}}_2$-based thin films is considered as the cumulative effect of various extrinsic factors such as finite-size effects and surface/interface effects of small grains, compressive stress, dopants, oxygen vacancies, and electric fields. The kinetic effects of phase transitions and their potential impacts on the emergence of ferroelectricity in ${\mathrm{HfO}}_2$ at the nanoscale are not well understood. In this paper, we construct the transition paths between different polymorphs of hafnia with density-functional-theory calculations and variable-cell nudged elastic band technique. We find that the transition barriers depend strongly on the mechanical boundary conditions and the transition from the tetragonal phase to the polar orthorhombic phase is a fast process kinetically under clamping. The effects of growth orientations and epitaxial strains on the relative stability of different phases of ${\mathrm{HfO}}_2$ are investigated. The two orthorhombic phases, polar $Pca{2}_1$ and nonpolar $Pbca$, become thermodynamically stable in (111)-oriented thin films over a wide range of epitaxial strain conditions. This paper suggests a potential avenue to better stabilize the ferroelectric phase in ${\mathrm{HfO}}_2$ thin films through substrate orientation engineering.

Materials showing light-induced voltages (LIVs) are a flourishing class characterized by anisotropic charge transport. The LIVs’ functionality could be tuned by local perturbations and structural symmetry, but achieving large LIVs is still challenging and a clear guiding principle to this end thus remains to be established. Here, a long-range lattice strain approach that enables manipulating the crystal field and symmetry breaking is proposed to realize colossal LIVs. The tailored tensile and compressive strains are introduced into superconducting $\left(\mathrm{L}{\mathrm{a}}_{1.45}\mathrm{N}{\mathrm{d}}_{0.4}\right)\mathrm{S}{\mathrm{r}}_{0.15}\mathrm{Cu}{\mathrm{O}}_4$ epitaxial films that are fabricated by the pulsed laser deposition method. The experimental characterization indicates that long-range lattice compressive strain results in the epitaxially mosaic structures and the local crystal field transition from tetragonal to octahedral configurations that spontaneously breaks the symmetry. Such a structural transition and change through modulated compressive strain could greatly favor in-plane transport that contributes to a large anisotropy of the Seebeck coefficient $\mathrm{\Delta }S$ of over $100\mu \mathrm{V}{\mathrm{K}}^{-1}$, achieving strikingly colossal LIVs of over 70 V, many times larger than those of existing materials reported. This would broaden LIVs’ promising application in light or photovoltage response and also exemplifies a steady route to exploring new high-performance LIV materials with structural or transport anisotropy.

Using first-principles calculations, we explore the electron properties of ${\text{Y}D}_3$ ($D=\text{In}$, Sn, Tl, Pb) and predict that ${\mathrm{YIn}}_3$, ${\mathrm{YTl}}_3$, and ${\mathrm{YPb}}_3$ are topological superconductor candidates. In the presence of spin-orbit coupling effect, continuous band gaps for ${\mathrm{YIn}}_3$, ${\mathrm{YSn}}_3$, and ${\mathrm{YPb}}_3$ are opened between their highest occupied bands ($N$) and the lowest unoccupied bands ($N+1$), where the different $Z_2$ invariants are obtained. Differently and specially, there are type-II Dirac points (DPs) at the high-symmetry lines in ${\mathrm{YTl}}_3$, indicating one possibility of topological Dirac semimetal. Furthermore, the nontrivial Rashba-like topological surface states are achieved at the $\overline{\mathrm{X}}$ point on the (001) surface for ${\mathrm{YIn}}_3$, ${\mathrm{YTl}}_3$, and ${\mathrm{YPb}}_3$, as well as the Fermi arcs in ${\mathrm{YTl}}_3$ connecting the DPs. In addition to the topological properties, our electron-phonon coupling calculations indicate clearly that these four intermetallics are all phonon-mediated superconductors. The calculated superconducting transition temperatures of $T_c=0.96$, 6.34, 2.17, and 4.37 K respectively for ${\mathrm{YIn}}_3$, ${\mathrm{YSn}}_3$, ${\mathrm{YPb}}_3$, and ${\mathrm{YTl}}_3$ agree well with experiments.

The Mn-based antiperovskite ${\mathrm{Mn}}_3\mathrm{GaC}$ is an interesting material for magnetocaloric cooling applications below room temperature. To optimize its use, knowledge of the anisotropic magnetic properties is required. Here, we use a single crystal to study the magnetic anisotropy by the ferromagnetic resonance technique and magnetic-response simulations. The anisotropy constant $K_4=-5.49\mathrm{kJ}/{\mathrm{m}}^3$ obtained in combination from experiment and simulations is about an order of magnitude smaller than that of bcc Fe. The magnetically soft nature of this material practically in all crystallographic directions is favorable for using it in polycrystalline form for magnetic refrigeration.

In this work, we present and apply schemes to determine from first-principles calculations the relevant effective parameters used in phase field theory simulations of battery compounds. In particular, we derive that a consistent free energy density can be obtained by mean-field sampling, which is especially suited for materials with different configurations on a lattice, such as alloys or Li intercalation batteries. In addition, it is demonstrated that mean-field sampling can be performed very effectively with the use of special quasi random structures and that experimentally determined free energy density parameters for ${\mathrm{LiFePO}}_4$ are reproduced by density functional theory calculations. The additional computation of interfacial and strain energy parameters allows us to present a consistent phase field parametrization of ${\mathrm{Li}}_2{\mathrm{FeSiO}}_4$ without relying on experimental data.

Transition-metal dichalcogenide monolayers provide an emerging material system to implement quantum photonics with intrinsic two-dimensional excitons or embedded zero-dimensional localized states. Here, we demonstrate mesoscopic localized states between two and zero dimensions, which is a many-body system with electron-electron Coulomb interactions. A fine-structure splitting is observed, which is similar to quantum dots. Meanwhile, the polarization degree is changed by the magnetic field, due to the nature of the two-dimensional monolayers. Furthermore, a large quadratic diamagnetism with a coefficient of around $100\mu \mathrm{eV}/{\mathrm{T}}^2$ is observed, as a unique consequence of the mesoscopic scale. The many-body effect also results in the emission energy variation and linewidth narrowing in the spectrum, which corresponds well to the theoretical analysis. These unique properties indicate the great potential of mesoscopic localized states in many-body physics and quantum photonics.

Femtosecond laser exposure of fused silica in the nonablative regime can lead to various localized bulk modifications of the material structure. In this paper, we show that these laser-induced modifications can be used to tune silica thermal expansion properties permanently. In particular, we demonstrate that a given exposer regime leads to lower thermal expansion than the bulk, while other exposure conditions yield the opposite results. This remarkable property enables the possibility to engineer a given thermal expansion behavior by selectively exposing a material volume to a femtosecond laser beam. This finding opens up opportunities for a variety of integrated precision instruments and optical devices for which inertness to thermal fluctuations is essential.

Epitaxial ${\mathrm{BiMnO}}_3$ (BMO) thin films were prepared on (001)-oriented ${\mathrm{SrTiO}}_3$ substrates with a ${\mathrm{SrRuO}}_3$ buffer layer using pulsed laser deposition. The correlation between ferroelectric and structural properties of BMO has been investigated in detail. By high-resolution transmission electron microscopy measurement, we observed a structural transition from pseudocubic to monoclinic structure with increasing thickness of BMO films, which can be ascribed to the intermediate layer and small composition fluctuation, i.e., Bi deficiency. The ferroelectricity of the BMO films has been characterized by piezoresponse force microscope, demonstrating that pseudocubic BMO thin films show intrinsic ferroelectricity, while the ferroelectricity is absent or very weak in the monoclinic BMO thin films at room temperature. The results of density-functional theory calculations also support the experimental results.

In antiferromagnetic spintronics where manipulation of the antiferromagnetic spins is a central technological challenge, it is important to understand the dynamic properties, especially their THz spin dynamics and the magnetic damping. Here, we investigate thoroughly the antiferromagnetic spin dynamics in NiO by broadband THz spectroscopy and find the remarkable difference in the damping constants $\alpha =5.0\pm 0.4\times {10}^{-4}$ and $7.4\pm 0.4\times {10}^{-4}$, respectively, for mono- and polycrystalline NiO, indicating the existence of strong extrinsic contributions to the damping, such as two-magnon scattering. Our results shed light on the mechanism of the antiferromagnetic dissipative dynamics and give important insights for developing antiferromagnetic and THz spintronics devices.

Ultrawide bandgap semiconductors are important for a broad range of electronic and photonic devices, but their practical application has been limited by poor current conduction. Here, we demonstrate that with controlled tuning of the Fermi level by an in situ metal-semiconductor junction during epitaxy, efficient $p$-type conduction, which was otherwise impossible, can be achieved for large bandgap aluminum gallium nitride (AlGaN). During epitaxy, the Fermi level is pinned away from the valence band edge, which fundamentally reduces the formation energy for substitutional Mg-dopant incorporation while simultaneously increasing the formation energy for compensating defects. We have demonstrated that Mg-doped ${\mathrm{Al}}_{0.9}{\mathrm{Ga}}_{0.1}\mathrm{N}$ can exhibit free hole concentration $\sim 4.5\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$, with a resistivity $<5\mathrm{\Omega }·\mathrm{cm}$, which is nearly three orders of magnitude lower compared to previous reports. Ultraviolet 280-nm light emitting diodes grown using this method exhibited nearly an order of magnitude improvement in external quantum efficiency compared to those grown using conventional molecular beam epitaxy. Such a unique technique can be extended for the epitaxy/synthesis of a broad range of wide bandgap semiconductors to achieve efficient current conduction that was not previously possible.

The low ductility of lightweight magnesium prevents it from wide engineering applications, making it essential to improve its ductility. To provide guidelines to accelerate this improvement, we propose and test a hierarchical high-throughput screening approach for alloy design that is based on the quantum-mechanics-derived full general stacking fault (GSF) energy surface (γ surface), but simplified to examine only the unstable stacking fault energy (${\gamma }_{\mathrm{us}}$). We apply this approach to determine the promising dopant elements in Mg-$X$ binary alloys which may activate the pyramidal dislocations by decreasing GSF energy and thus improve the ductility of Mg. Ten elements, including Hg, Tl, Sn, Sb, Bi, Te, As, Pb, In, and Ca, are sifted out as promising alloying elements. Particularly, the Mg-Te alloy has the lowest ${\gamma }_{\mathrm{us}}$ of $156\mathrm{mJ}/{\mathrm{m}}^2$ among all binary alloys, which is 25% lower than that of Mg ($209\mathrm{mJ}/{\mathrm{m}}^2$), suggesting that it is the most promising alloy to enhance the ductility of Mg.

Superconductivity in group IV semiconductors is desired for hybrid devices combining both semiconducting and superconducting properties. Following boron-doped diamond and Si, superconductivity has been observed in gallium-doped Ge; however, the obtained specimen is in polycrystalline form [Phys. Rev. Lett. 102, 217003 (2009)]. Here we present superconducting single-crystalline Ge hyperdoped with gallium or aluminum by ion implantation and rear-side flash lamp annealing. The maximum concentration of Al and Ga incorporated into substitutional positions in Ge is 8 times higher than the equilibrium solid solubility. This corresponds to a hole concentration above ${10}^{21}\mathrm{c}{\mathrm{m}}^{-3}$. Using density functional theory in the local-density approximation and pseudopotential plane-wave approach, we show that the superconductivity in $p$-type Ge is phonon mediated. According to the ab initio calculations, the critical superconducting temperature for Al- and Ga-doped Ge is in the range of 0.45 K for $6.25\mathrm{at}.\%$ of dopant concentration, being in qualitative agreement with experimentally obtained values.