Real-space motions of molecules and ions in an aqueous solution of sodium chloride are studied with high-resolution inelastic x-ray scattering. We focus on the correlated dynamics of water molecules and salt ions, instead of the instantaneous structure and the rotational and reorientation dynamics that have been studied using scattering techniques and optical spectroscopy, respectively. The real-space analysis in terms of the Van Hove function illustrates the effect of salt ions to slow down the local correlated dynamics of surrounding water in the hydration shells.

Perovskite photoelectrodes are being extensively studied in search for photocatalytic materials that can produce hydrogen through water splitting. The solar-to-hydrogen efficiency of these materials is critically dependent on the electrochemical state of their surface. Here, we develop an embedded quantum-mechanical approach using the self-consistent continuum solvation model to predict the relation between band alignment, electrochemical stability, and photocatalytic activity taking into account the long-range polarization of the semiconductor electrode under electrical bias. Using this comprehensive model, we calculate the charge-voltage response of various reconstructions of a solvated ${\mathrm{SrTiO}}_3$ surface, revealing that interfacial charge trapping exerts primary control on the electrical response and surface stability of the photoelectrode. Our results provide a detailed molecular-level interpretation of the enhanced photocatalytic activity of ${\mathrm{SrTiO}}_3$ upon voltage-induced restructuring of the semiconductor-solution interface.

Determining the three-dimensional (3D) crystallography of a material with subnanometer resolution is essential to understanding strain effects in epitaxial thin films. A scanning transmission electron microscopy imaging technique is demonstrated that visualizes the presence and strength of atomic movements leading to a period doubling of the unit cell along the beam direction, using the intensity in an extra Laue zone ring in the back focal plane recorded using a pixelated detector method. This method is used together with conventional atomic resolution imaging in the plane perpendicular to the beam direction to gain information about the 3D crystal structure in an epitaxial thin film of $\mathrm{LaFe}{\mathrm{O}}_3$ sandwiched between a substrate of (111) $\mathrm{SrTi}{\mathrm{O}}_3$ and a top layer of $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$. It is found that a hitherto unreported structure of $\mathrm{LaFe}{\mathrm{O}}_3$ is formed under the unusual combination of compressive strain and (111) growth, which is triclinic with a periodicity doubling from primitive perovskite along one of the three ⟨110⟩ directions lying in the growth plane. This results from a combination of La-site modulation along the beam direction, and modulation of oxygen positions resulting from octahedral tilting. This transition to the period-doubled cell is suppressed near both the substrate and near the $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ top layer due to the clamping of the octahedral tilting by the absence of tilting in the substrate and due to an incompatible tilt pattern being present in the $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ layer. This work shows a rapid and easy way of scanning for such transitions in thin films or other systems where disorder-order transitions or domain structures may be present and does not require the use of atomic resolution imaging, and could be done on any scanning transmission electron microscopy instrument equipped with a suitable camera.

We performed an optical spectroscopy study of electronic and magnetic excitations for a rare-earth system with a single electron quasilocalized in the $f$ shell on an ion at high-symmetry crystallographic site in application to ${\mathrm{CeB}}_6$ heavy-fermion metal. We carried out group-theoretical classification of the electronic crystal field (CF) transitions and assessed their coupling to light cross sections for polarization resolved Raman scattering processes. We discuss applicability of symmetrized Raman susceptibility to studies of exotic charge and spin high multiplet ordering phases in $f$-electron systems. We study temperature effects on intra- and intermultiplet CF transitions and also on the coupling between the CF excitations with the lattice vibrations. We acquired temperature dependence of the low-frequency polarization resolved Raman response for all Raman-allowed symmetry channels: ${\mathrm{A}}_{1g}$, ${\mathrm{E}}_g$, ${\mathrm{T}}_{1g}$, and ${\mathrm{T}}_{2g}$ of the cubic ${\mathrm{O}}_h$ point group. We demonstrate that ${\mathrm{T}}_{1g}$-symmetry static Raman susceptibility shows a temperature dependence which is consistent with the previously-reported magnetic susceptibility data. Such behavior in the ${\mathrm{T}}_{1g}$ channel signifies the presence of long wavelength magnetic fluctuations, which is interpreted as a manifestation of ferromagnetic correlations induced by tendency towards quadrupolar ordering.

Passivation of barium disilicide ($\mathrm{BaS}{\mathrm{i}}_2$) films is very important for their use in solar cell applications. In this paper, we demonstrated the effect of hydrogen (H) passivation on both the photoresponsivity and minority-carrier lifetime of $\mathrm{BaS}{\mathrm{i}}_2$ epitaxial films grown by molecular beam epitaxy. First, we examined the growth conditions of a 3-nm-thick hydrogenated amorphous silicon (a-Si) capping layer formed on a 500-nm-thick $\mathrm{BaS}{\mathrm{i}}_2$ film and found that an H supply duration ($t_{\mathrm{a}\text{−}\mathrm{Si}:\mathrm{H}}$) of 15 min at a substrate temperature of 180 °C sizably enhanced the photoresponsivity of the $\mathrm{BaS}{\mathrm{i}}_2$ film. We next supplied atomic H to $\mathrm{BaS}{\mathrm{i}}_2$ epitaxial films at 580 °C and changed supply duration ($t_{\mathrm{BaSi};\mathrm{H}}$) in the range of 1–30 min, followed by capping with an a-Si layer. The photoresponsivity of the films changed considerably depending on $t_{\mathrm{BaSi};\mathrm{H}}$ and reached a maximum of 2.5 A/W at a wavelength of 800 nm for the sample passivated for $t_{\mathrm{BaSi};\mathrm{H}}=15$ min under a bias voltage of 0.3 V applied to the front-surface indium-tin-oxide electrode with respect to the back-surface aluminum electrode. This photoresponsivity is approximately one order of magnitude higher than the highest value previously reported for $\mathrm{BaS}{\mathrm{i}}_2$. Microwave photoconductivity decay measurements revealed that the minority-carrier lifetime of the $\mathrm{BaS}{\mathrm{i}}_2$ film with the highest photoresponsivity was 14 μs, equivalent to its bulk carrier lifetime ever reported. We performed theoretical analyses based on a rate equation including several recombination mechanisms and reproduced the experimentally obtained decay curves. We also calculated the total density of states of $\mathrm{BaS}{\mathrm{i}}_2$ by ab initio studies when one Si vacancy existed in a unit cell and one, two, and three H atoms occupied Si vacancy or interstitial sites. A Si vacancy caused a localized state with two energy bands to appear close to the middle of the band gap. In certain cases, H passivation of the Si dangling bonds can markedly decrease trap concentration. From both experimental and theoretical viewpoints, we conclude that an atomic H supply is beneficial for $\mathrm{BaS}{\mathrm{i}}_2$ solar cells.

Rocksalt NdO (001) epitaxial thin films were synthesized. Contrary to paramagnetic NdO bulk polycrystal in previous studies, NdO epitaxial thin films showed ferromagnetism with a Curie temperature of 19.1 ± 1.8 K, originating from dominant direct Nd−Nd exchange interaction owing to the small intercationic distance. In addition, negative magnetoresistance and an anomalous Hall effect were observed.

Transparent conducting oxides (TCOs) are essential to many technologies including solar cells and transparent electronics. The search for high-performance $n$- or $p$-type TCOs has mainly focused on materials offering transport through band carriers instead of small polarons. In this work, we break this paradigm and demonstrate using well-known physical models that, in certain circumstances, TCOs exhibiting transport by small polarons offer a better combination of transparency and conductivity than materials conducting through band transport. We link this surprising finding to the fundamentally different physics of optical absorption for band and polaronic carriers. Our work rationalizes the good performances of recently emerging small-polaronic Cr-based $p$-type TCOs such as Sr-doped ${\mathrm{LaCrO}}_3$ and outlines design principles for the development of high-performance TCOs based on transport by small polarons. This opens new avenues for the discovery of high-performance TCOs especially $p$-type.

The high thermal conductivity of boron arsenide (BAs) makes it a promising material for optoelectronic applications in which thermal management is central. In this work, we study the finite temperature optoelectronic properties of BAs by considering both electron-phonon coupling and thermal expansion. The inclusion of electron-phonon coupling proves imperative to capture the temperature dependence of the optoelectronic properties of the material, while thermal expansion makes a negligible contribution due to the highly covalent bonding character of BAs. We predict that with increasing temperature the optical absorption onset is subject to a red-shift, the absorption peaks become smoother, and the phonon-assisted absorption at energies below those of the optical gap has a coefficient that lies in the range ${10}^{-3}$–${10}^{-4}{\mathrm{cm}}^{-1}$. We also show that good agreement with the measured indirect band gap of BAs is only obtained if exact exchange, electron-phonon coupling, and spin-orbit coupling effects are all included in the calculations.

The discovery of iron-based superconductors (FeSCs) has attracted worldwide attention in recent years due to their abundant properties. Recently, an air-stable high-$T_c$ FeSe-based superconductor, (Li,Fe)OHFeSe, was discovered, and it is thought to be an ideal platform for investigating the underlying physics of superconductivity. In this paper, we report on the solid-state synthesis of (Li,Fe)OHFeSe at low temperature. As-synthesized (Li,Fe)OHFeSe is nonsuperconducting, which can be attributed to the Fe vacancies in the FeSe layer. After annealing at high pressure or a solid-state reaction with lithium, the Fe vacancies in the FeSe layer can be eliminated, and superconductivity at 40 K is induced. X-ray photoelectron spectroscopy analysis also proves the decrease of the Fe valence, which indicates that the Fe vacancies are gradually eliminated upon annealing at high pressure or the solid-state reaction with lithium. Our finding suggests that the solid-state synthesis of (Li,Fe)OHFeSe after annealing at high pressure or the solid-state reaction with lithium can effectively eliminate the Fe vacancies in the FeSe layer, eventually inducing superconductivity in (Li,Fe)OHFeSe.

We theoretically propose and experimentally realize two types of open window structures that exhibit an unusual functionality of switchable omnidirectional acoustic insulation in two different ways. Both types of open window structures consist of an array of blades that are attached with ultrathin metasurfaces on both sides. Based on the acoustic manipulation ability of the metasurfaces, omnidirectional acoustic insulation can be realized in the open windows. Interestingly, such an effect is switchable by simply changing the configuration of the blade array. For the first type, omnidirectional acoustic insulation can be switched on or off by adjusting the distance between two adjacent blades. For the second type in which the open window structure is composed of an alternative arrangement of two types of blades, acoustic insulation can be controlled by moving one kind of blade horizontally. The proposed open windows have the advantage of broadband omnidirectional acoustic insulation and convenient switching mechanisms. Our work opens up a new approach for acoustic insulation in open structures, with potential applications in a variety of fields, such as noise control and architectural acoustics.

Using a combination of equilibrium classical molecular dynamics (within the Green-Kubo formalism) and the Boltzmann transport equation, we study the effect of strain on the ZnO thermal conductivity focusing in particular on the case of hydrostatic and uniaxial strain. The results show that in the case of hydrostatic strain up to $\pm 4\%$, we can obtain thermal conductivity variations of more than 100%, while for uniaxial strains the calculated thermal conductivity variations are comparatively less pronounced. In particular, by imposing uniaxial compressive strains up to $-4\%$, we estimate a corresponding thermal conductivity variation close to zero. The mode analysis based on the solution of the Boltzmann transport equation shows that for hydrostatic strains, the thermal conductivity variations are mainly due to a corresponding modification of the phonon relaxations times. Finally, we provide evidence that for uniaxial compressive strains the contribution of the phonon relaxations time is balanced by the increase of the group velocities leading to a thermal conductivity almost unaffected by strain.

Heuslers are a prominent family of multifunctional materials that includes semiconductors, half metals, topological semimetals, and magnetic superconductors. Owing to their same crystalline structure, yet quite different electronic properties and flexibility in chemical composition, Heusler-based heterostructures can be designed to show intriguing properties at the interface. Using electronic structure calculations, we show that two-dimensional electron or hole gases (2DEG or 2DHG) form at the interface of half-Heusler (HH) semiconductors without any chemical doping. We use CoTiSb/NiTiSn as an example, and show that the 2DEG at the TiSb/Ni(001) termination and the 2DHG at the Co/TiSn(001) termination are intrinsic to the interface, and hold rather high charge densities of $3\times {10}^{14}$ carriers/${\mathrm{cm}}^2$. These excess charge carriers are tightly bound to the interface plane and are fully accommodated in transition-metal $d$ subbands. The formation of 2DEG and 2DHG are not specific to the CoTiSb/NiTiSn system; a list of combinations of HH semiconductors that are predicted to form 2DEG or 2DHG is provided based on band alignment, interface termination, and lattice mismatch.

The cohesive energies and structural properties of recently predicted, and never synthesized, ${\mathrm{B}}_2{\mathrm{O}}_3$ polymorphs are investigated from first principles using density functional theory and high-accuracy many-body methods, namely, the random phase approximation and quantum Monte Carlo. We demonstrate that the van der Waals forces play a key role in making the experimentally known polymorph (${\mathrm{B}}_2{\mathrm{O}}_3\text{−}\mathrm{I}$) the lowest in energy, with many competing metastable structures lying only a few kcal/mol above. Remarkably, all metastable crystals are comparable in energy and density to the glass, while having anisotropic and mechanically soft structures. Furthermore, the best metastable polymorph according to our stability criteria has a structural motif found in both the glass and a recently synthesized borosulfate compound. Our findings provide a framework for understanding the ${\mathrm{B}}_2{\mathrm{O}}_3$ anomalous behavior, namely, its propensity to vitrify in a glassy structure drastically different from the known crystal.

Work on orthoferrite perovskites (e.g., ${\mathrm{GdFeO}}_3$) shows that $\mathit{AB}{\mathrm{O}}_3$ oxides with $A$ and $B$ magnetic cations present novel behaviors (e.g., magnetoelectric) not possible when only one cation has spin. Unfortunately, the magnetic $A$ cations are usually lanthanides whose $4f$ electrons interact weakly, restricting the most interesting effects to low temperatures. Here we explore the possibility of having both $A$ and $B$ sites occupied by $3d$ transition metals, by running a systematic first-principles investigation of 81 compositions, considering the polymorphs most likely to be stable in bulk or thin-film forms (perovskite, corundum derivatives). We predict specific compounds to be multiferroics at high temperature, multiferroics with large magnetic moments, “multiferroic metals” (i.e., as the much-sought “polar metals,” but also magnetic), and ferromagnetic insulators. Our results offer a compelling panoramic of these compounds and the possibilities they offer, providing plenty of directions for further original research, both theoretical and experimental.

We demonstrate inversion symmetry breaking induced by oxygen octahedral rotations in layered perovskite oxides $\mathrm{K}{A}_R{\mathrm{TiO}}_4$ ($A_R$ = rare earth) using a combined experimental and theoretical approach including synchrotron x-ray diffraction, optical second harmonic generation, and first-principles lattice dynamics calculations. We experimentally find an interesting but counterintuitive phenomenon, i.e., the acentric-to-centric phase transition temperatures for K family are higher than those for previously reported Na family, in contrast to expectations based on the Goldschmidt tolerance factor, where the octahedral rotation instability toward the acentric phases would reduce with an increase in the radius of $A$-site alkali metal ions. Our detailed analysis of first-principles calculations for $A_A{A}_R{\mathrm{TiO}}_4$ ($A_A=\mathrm{Na}$, K, Rb) reveals that the alkali metal and rare-earth ions play quite different roles in driving the octahedral rotations. Since rare-earth ions attract oxide ions more strongly than alkali metal ions due to the higher valence of the former in comparison with the latter ($A_R^{3+}$ vs $A_A^{+}$), the optimization of coordination environment of rare-earth ions is the primary driving force of the octahedral rotations. Alkali metal ions serve to impose “bond strains” parallel to the layers, playing a secondary role in the octahedral rotations. Incorporation of large alkali metal ions generates a significant in-plane biaxial bond strain in $A_R\mathrm{O}$ and ${\mathrm{TiO}}_2$ layers through the expanded $A_A\mathrm{O}$ layers, and thereby facilitates the octahedral rotations because of the otherwise highly underbonding of rare-earth ions. Thus, the effect of $A$-site alkali metal size on the octahedral rotation instability can be explained in terms of the interlayer lattice mismatch. This understanding allows us to propose a geometric descriptor governing the structural instability in $A_A{A}_R{\mathrm{TiO}}_4$ layered perovskites. We believe that control over the interlayer lattice mismatch could be a useful strategy to tune the octahedral rotations in layered compounds.

The structure of thin terminated Bi(1 1 1) films of approximately $1\mathrm{nm}$ thickness is investigated from first principles. Our density functional theory calculations show that covalent bonds to the surface can change the orientation of the films completely. For thicker films, the effect is limited to the surface only. Based on these observations, we further present a simple model structure for the native oxide and chemically similar oxides, which form a protective capping layer, leaving the orientation of the films unchanged. The advantages of this energetically favorable layered termination are discussed in the context of the films' technological exploitation in nanoelectronic devices.

We found that an Al-Si-Ru cubic quasicrystalline approximant has a semiconducting band structure by performing an orbital analysis based on density functional theory. These semiconducting transport properties have been confirmed in an experimentally synthesized sample. The temperature dependences of the electrical conductivity and the Seebeck coefficient were consistent with the trends of an intrinsic semiconductor with a band gap of $0.15\mathrm{eV}$ above $350\mathrm{K}$. The lattice thermal conductivity had a low value of approximately $1.0\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ above $400\mathrm{K}$, which is close to the theoretical minimum.

Bismuth-antimony alloys (${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$) are topological insulators between 7 and 22% Sb in bulk crystals, with an unusually high conductivity suitable for spin-orbit torque applications. Reducing the thickness of epitaxial ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ films is expected to increase the maximum band gap through quantum confinement, which may improve isolation of topological surface-state transport. Like Bi(001) on Si(111), ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ has been predicted to form a black phosphoruslike allotrope with unique electronic properties in nanoscale films; however, the impact of Sb alloying on both the bulklike and nanoscale crystal structures on Si(111) is currently unknown. Here we demonstrate that the allotropic transition in ultrathin epitaxial ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ films on Si(111) is suppressed above 8–9% Sb, resulting in an unexpected (012) orientation within the topologically insulating regime. The metallic temperature-dependent conductivity associated with surface states in Bi(001) was not observed in the ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\left(012\right)$ films, suggesting that the (012) orientation may significantly reduce surface-state transport. Growth on a Bi(001) buffer layer may prevent this orientation transition. Finally, we demonstrate that Sb alloying improves the continuity and quality of nanoscale ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\left(012\right)$ films in the thickness regime expected for the black phosphorus allotrope, suggesting a promising route to large-area growth of puckered-layer two-dimensional ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$, which will be necessary to harness its unique electronic properties in practical applications.