Effective defect passivation of semiconductor surfaces and interfaces is indispensable for the development of high efficiency solar cells. In this study we systematically investigated the surface and grain boundary properties of ${\mathrm{CuInSe}}_2$ (CISe) with scanning tunneling microscopy (STM) and spectroscopy (STS) after different surface treatments such as potassium cyanide (KCN) etching, pre-electrolyte treatment with cadmium ions, and annealing in ultrahigh vacuum (UHV). We show that air exposed samples with a subsequent KCN etching step exhibits a highly defective surface. However, a Cd pre-electrolyte treatment passivates most of these defects, which manifests itself by a reduction of the high conductance in the STS measurements at positive sample biases. The origin of the improvement can be traced back to an increase in surface band bending, which leads to a type inversion, induced by a change in the concentration of Cu vacancies. We observe a defect passivation at the CISe surface and at the grain boundaries. Our results give a direct explanation of why the CdS buffer layer in CISe thin film solar cells is of utmost importance for high efficiency devices.

We study the optical properties of thin flakes of InSe encapsulated in hexagonal boron nitride. More specifically, we investigate the photoluminescence (PL) emission and its dependence on sample thickness and temperature. Through the analysis of the PL line shape, we discuss the relative weights of the exciton and electron-hole contributions. Thereafter we investigate the PL dynamics. Two contributions are distinguishable at low temperature: direct band-gap electron-hole and defect-assisted recombination. The two recombination processes have lifetimes of ${\tau }_1\sim 8\mathrm{ns}$ and ${\tau }_2\sim 100\mathrm{ns}$, respectively. The relative weights of the direct band-gap and defect-assisted contributions show a strong layer dependence due to the direct-to-indirect band-gap crossover. Electron-hole PL lifetime is limited by population transfer to lower-energy states and no dependence on the number of layers was observed. The lifetime of the defect-assisted recombination gets longer for thinner samples. Finally, we show that the PL lifetime decreases at high temperatures as a consequence of more efficient nonradiative recombinations.

A recently discovered 73-K cuprate superconductor ${\mathrm{Ba}}_2{\mathrm{CuO}}_{4-\delta }$ implies that intact ${\mathrm{CuO}}_2$ planes are not absolutely necessary in achieving high-$T_c$ superconductivity. Featuring an exceptional Jahn-Teller distortion, wherein the ${\mathrm{CuO}}_6$ octahedrons are compressed along the $c$ axis, O vacancies in this material prefer to reside in the ${\mathrm{CuO}}_2$ plane, which significantly modifies the two-dimensional (2D) square lattice. By combining first-principles total-energy calculation with the automated structure inversion method, the effective cluster interactions of O vacancies are mapped out. Around $\delta =0.8$, where the superconductivity was observed experimentally, we predict that the O vacancies form a long-range order, which slice the ${\mathrm{CuO}}_2$ plane into 1D chains and two-leg ladders. The latter was not known to exist in other cuprates. A Monte Carlo simulation is performed based on the effective cluster interaction model, showing that such an ordering pattern is stable up to 900 K. Our results put forth a concrete structural basis to discuss the underlying superconducting mechanism.

A characteristic of a Fermi liquid is the $T^2$ dependence of its resistivity, sometimes referred to as the Baber law. However, for most metals, this behavior is only restricted to very low temperatures, usually below 20 K. Here, we experimentally demonstrate that for the single-crystal van der Waals layered material $\mathrm{MoOC}{\mathrm{l}}_2$, the Baber law holds in a wide temperature range up to ∼120 K, indicating that the electron-electron scattering plays a dominant role in this material. Combining with the specific heat measurement, we find that the modified Kadowaki-Woods ratio of the material agrees well with many other strongly correlated metals. Furthermore, in the magnetotransport measurement, a colossal magnetoresistance is observed, which reaches ∼350% at 9 T and displays no sign of saturation. With the help of first-principles calculations, we attribute this behavior to the presence of open orbits on the Fermi surface. We also suggest that the dominance of electron-electron scattering is related to an incipient charge density wave state of the material. Our results establish $\mathrm{MoOC}{\mathrm{l}}_2$ as a strongly correlated metal and shed light on the underlying physical mechanism, which may open a new path for exploring the effects of electron-electron interaction in van der Waals layered structures.

We found superconductivity in ${\mathrm{CaSb}}_2$ with the transition temperature of 1.7 K by means of electrical-resistivity, magnetic-susceptibility, and specific-heat measurements. This material crystallizes in a nonsymmorphic structure and is predicted to have multiple Dirac nodal lines in the bulk electronic band structure protected by symmetry even in the presence of spin-orbit coupling. We discuss a possible topological superconductivity for the quasi-two-dimensional band originating mainly from one of the antimony sites.

Diffusion is the underlying mechanism for many complicated materials phenomena, and understanding it is basic to the discovery of novel materials with desired physical and mechanical properties. Certain groups of solid phases, such as the bcc phase of IIIB and IVB metals and their alloys, which are only stable when they reach high enough temperatures and experience anharmonic vibration entropic effects, exhibit “anomalously fast diffusion.” However, the underlying reason for the observed extraordinary fast diffusion is poorly understood and due to the existence of harmonic vibration instabilities in these phases the standard models fail to predict their diffusivity. Here, we indicate that the anharmonic phonon-phonon coupling effects can accurately describe the anomalously large macroscopic diffusion coefficients in the bcc phase of IVB metals and therefore yield understanding on the underlying mechanism for diffusion in these phases. We utilize temperature-dependent phonon analysis by combining ab initio molecular dynamics with lattice dynamics calculations to provide an approach to use the transition state theory beyond the harmonic approximation. We validate the diffusivity predictions for the bcc phase of titanium and zirconium with available experimental measurements, while we show that predictions based on harmonic transition state theory severely underestimate diffusivity in these phases.

MnPtGa crystallizes in the hexagonal ${\mathrm{Ni}}_2\mathrm{In}$ structure type in space group $P{6}_3/mmc$ and has been reported to display a ferromagnetic Curie temperature near 220K. Here we find a transition near $T_C=236$ K to a ferromagnetic state, albeit with a reduced moment from what is expected for collinear ordering. The peak magnetocaloric entropy change was determined to be $\mathrm{\Delta }{S}_M=-1.9\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ for an applied magnetic field of $H=5\mathrm{T}$ at the ferromagnetic ordering temperature. Magnetostructural coupling manifests as a change in the slope of the thermal expansion coefficients of the $c$ lattice parameter near $T_C$, with a negative spontaneous volume magnetostriction; $\omega =-300$ ppm at 190K. Neutron powder diffraction studies of the magnetic ground state reveal an evolution in complexity as temperature decreases: from a ferromagnet, to a canted antiferromagnet, to the eventual formation of a spin-density-wave state at low temperatures.

Antiperovskites have recently been attracting considerable attention because of their intriguing physical properties. We theoretically investigate polymorphism of mixed-anion antiperovskites $M_3^{2+}{X}^{3-}{\mathrm{N}}^{3-}(M$ = Mg, Ca, Sr, Ba; $X$ = P, As, Sb, Bi) using the seven representative crystal-structure prototypes of $AB{Z}_3$ compounds. Stable crystal-structure exploration for four unreported compounds $({\mathrm{Mg}}_3\mathrm{PN},{\mathrm{Sr}}_3\mathrm{PN},{\mathrm{Ba}}_3\mathrm{PN}$, and ${\mathrm{Mg}}_3\mathrm{BiN})$ is also performed using ab initio evolutionary crystal-structure-search and lattice-dynamics calculations, which have consistently identified antiperovskite phases as their ground states. As a result of crystal-structure exploration, the orthorhombic perovskite $Pbnm$ phases of ${\mathrm{Mg}}_3\mathrm{PN},{\mathrm{Sr}}_3\mathrm{PN}$, and ${\mathrm{Ba}}_3\mathrm{PN}$ are obtained, while the cubic perovskite $Pm\overline{3}m$ phase is stable in ${\mathrm{Mg}}_3\mathrm{BiN}$. We show that the octahedral rotational distortions in $M_3\mathrm{PN}$ and $M_3\mathrm{AsN}$ reduce their Madelung energies, which is not common in conventional perovskites. Further, we estimate the ionic radii of anionic nitrogen ${\mathrm{N}}^{3-}$ and pnictogen $X^{3-}$ to explain the chemical trends for the phase stability, lattice distortion amplitude, total energies, valence bandwidths, and band gaps of $M_{3}X\mathrm{N}$ using the Goldschmidt tolerance factor. We also report the electronic structures of $M_{3}X\mathrm{N}$ with and without the rotational distortions and spin-orbit coupling. The valence bands of ${\mathrm{Mg}}_{3}X\mathrm{N}$ are created mainly from $p\text{−}p$ hybridization, whereas those of $M_3^{\prime }X\mathrm{N}(M^{\prime }$ = Ca, Sr, Ba) are constructed mainly from $d\text{−}p$ hybridization. We then explain the mechanism of the band-gap changes owing to the rotational distortions and spin-orbit coupling. The effective masses for the relevant ground-state phases of $M_{3}X\mathrm{N}$ are found to be comparable to preexisting compound semiconductors. Finally, we propose potential applications of earth-abundant semiconductors ${\mathrm{Mg}}_3\mathrm{PN}$ and ${\mathrm{Sr}}_3\mathrm{PN}$ as light absorbers and emitters utilizing their direct-type band structures and high optical absorption coefficients. The present study provides a clear picture and recipe for the understanding and design of mixed-anion antiperovskites.

All-inorganic halide perovskites have attracted a great interest as a promising light harvester of perovskite solar cells due to their enhanced chemical stability. In this work we investigate the material properties of solid solutions $\mathrm{CsPb}{\left({\mathrm{I}}_{1-x}{\mathrm{Br}}_x\right)}_3$ in cubic phase by applying the virtual crystal approximation approach within a density functional theory framework. First we check the validity of constructed pseudopotentials of the virtual atoms ($\text{X}={\text{I}}_{1-x}{\text{Br}}_x$) by verifying that the lattice constants follow the linear function of mixing ratio. We then suggest an idea of using the hybrid HSE functional with linear increasing value of exact exchange term as increasing the Br content $x$, which produces the band gaps of $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_3$ in good agreement with the available experimental data. The calculated light absorption coefficients and reflectivity show the systematic varying tendency to the Br content. We calculate the phonon dispersions of $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_3$, CsX, and ${\mathrm{PbX}}_2$ as slightly changing their volumes, revealing the phase instability of $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_3$ and calculating the thermodynamic potential function differences. By projecting Gibbs free energy differences onto the plane of $\mathrm{\Delta }G=0$, we determine the $P-T$ diagram for $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_3$ to be stable against the chemical decomposition, highlighting that the area of being stable extends gradually as the Br content increases.

The quasi-one-dimensional structure of ${\mathrm{Li}}_2{\mathrm{Mn}}_2{\left({\mathrm{MoO}}_4\right)}_3$ consists of three mutually distinct chains of ${\mathrm{Li}}_{1-x}{\mathrm{Mn}}_x$-centered polyhedra in which Mn ostensibly adopts a $J=5/2{\mathrm{Mn}}^{2+}$ configuration. In situ x-ray scattering experiments carried out as crystallites emerge from a molten oxide solution facilitate the synthesis of large single crystals. Ex situ x-ray diffraction finds no evidence of long-range Li/Mn occupancy ordering, suggesting that the structure is effectively composed of finite chains of Mn moments of statistically varying lengths. UV/visible diffuse reflectance spectroscopy measurements establish a wide 3.43(12)-eV direct charge gap consistent with the local polyhedral coordination of the nominally ${\mathrm{Mn}}^{2+}$ species. The temperature $T$ dependence of the DC magnetic susceptibility $\chi$ reveals a fluctuating moment of only $2.74{\mu }_B\pm 0.01{\mu }_B$/Mn, dramatically reduced from the $5.9{\mu }_B$/Mn expected for ${\mathrm{Mn}}^{2+}$. Meanwhile, the Weiss temperature ${\mathrm{\Theta }}_W=-89\pm 1$ K reveals antiferromagnetic fluctuations that are stymied from reaching an ordered state apparently by the chemical disorder intrinsic to the polyhedral chains. Measurements of magnetization vs field $H$ at $T\le 10$ K are far from saturation even at $H=5$ T and are strongly non-Brillouin-like, instead scaling as $H/T^{0.24\left(3\right)}$ and suggesting the presence of quantum fluctuations associated with an eventual quasi-one-dimensional, disordered magnetic phase.

We report on a systematic computational study of ideal strengths, i.e., the lowest stresses needed to destabilize a perfect crystal under a variety of loading conditions for six stable and metastable tungsten nitrides identified by advanced crystal structure search algorithms. We employ first-principles calculations to determine stress-strain relations and examine the corresponding atomistic bonding changes for a microscopic understanding of the structural deformation modes. The obtained results show that, in stark contrast to many previously studied transition-metal borides, carbides, and also most nitrides, the tungsten nitrides exhibit surprisingly broad and common patterns of strain-stiffening effects under indentation strains. These extraordinary behaviors are attributed to the favorable bonding arrangements in these materials, including the strong and well-connected W-W metallic bonds and $\mathrm{N}\equiv \mathrm{N}$ bonds in combination with W-N covalent bonds, which together create a strong three-dimensional bonding network that is capable of resisting large-indentation shear deformations, thereby greatly enhancing the corresponding mechanical strength that is directly responsible for the superior experimentally measured hardness results. These findings provide insights for elucidating unusual indentation strength enhancements in these materials and offer guidelines for exploring experimental synthesis and optimization of the presently identified and potentially additional very hard to superhard materials. These insights are also crucial for understanding the fundamental relationship between crystal and chemical bonding structures and the associated mechanical properties under a wide range of loading conditions in diverse application environments.

A systematic method for building an extensible tight-binding model from ab initio calculations has been developed and tested on two hexagonal metals: Zr and Mg. The errors introduced at each level of approximation are discussed and quantified. For bulk materials, using a limited basis set of $spd$ orbitals is shown to be sufficient to reproduce with high accuracy bulk energy versus volume curves for fcc, bcc, and hcp lattice structures, as well as the electronic density of states. However, the two-center approximation introduces errors of several tenths of eV in the pair potential, crystal-field terms, and hopping integrals. Environmentally dependent corrections to the former two have been implemented, significantly improving the accuracy. Two-center hopping integrals were corrected by taking many-center hopping integrals for a set of structures of interest, rotating them into the bond reference frame, and then fitting a smooth function through these values. Finally, a pair potential was fitted to correct remaining errors. However, this procedure is not sufficient to ensure transferability of the model, especially when point defects are introduced. In particular, it is shown to be problematic when interstitial elements are added to the model, as demonstrated in the case of octahedral self-interstitial atoms.

Ion track formation, amorphization, and the formation of porosity in crystalline GaSb induced by 185 MeV ${}^{197}\mathrm{Au}$ swift heavy ion irradiation is investigated as a function of fluence and irradiation angle relative to the surface normal. Rutherford backscattering spectrometry in channeling configuration and small-angle x-ray scattering reveal an ion track radius between 3 and 5 nm. The observed pore morphology and saturation swelling of GaSb films shows a strong irradiation angle dependence. Raman spectroscopy and scanning electron microscopy show that the ion tracks act as a source of strain in the material, leading to macroscopic plastic flow at high fluences and off-normal irradiation. The results are consistent with the ion hammering model for glasses. Furthermore, wide-angle x-ray scattering reveals the formation of nanocrystallites inside otherwise amorphous GaSb after the onset of porosity.

Large anharmonic vibrations often play a crucial role in dynamically stabilizing crystalline phases whose structures are unstable at low temperature. Although the average structure of such phases can be measured through diffraction experiments, their local structure remains a challenge to characterize and understand. Dynamically stabilized phases are often classified as order/disorder or displacive based on the qualitative nature of their local structure. A robust understanding of how chemistry determines this distinction in behavior, however, is lacking. This article presents a parametric study of an anharmonic vibrational model that describes the transition from a cooperative to a noncooperative Jahn-Teller distortion in layered oxides—a class of materials widely used in Li-ion and Na-ion batteries. The results illustrate how the shape of the energy landscape determines the extent to which the high-temperature phase has order/disorder vs displacive character. We find that the nature of the high-temperature phase is determined by a competition between the strength of the elastic coupling between Jahn-Teller distortions at nearby sites and the energy scale driving the Jahn-Teller distortion. A comparison of the model to energy landscapes calculated with density-functional theory suggests that the high-temperature phases of the Jahn-Teller active layered compounds $\mathrm{LiNi}{\mathrm{O}}_2$, $\mathrm{NaNi}{\mathrm{O}}_2$, $\mathrm{LiMn}{\mathrm{O}}_2$, and $\mathrm{NaMn}{\mathrm{O}}_2$ exhibit order/disorder character.

Redox-active $3d$ transition-metal oxides (TMOs) are crucial ingredients for multiple sustainable energy applications, including solar cells, batteries, catalysis, and solar thermochemical water splitting. However, any predictive modeling, such as that employing density functional theory, needs to describe accurately the energetics of redox reactions involving transition metals, if new candidate materials are to be identified in a reliable fashion. Recently, we demonstrated that the state-of-the-art, strongly constrained and appropriately normed (SCAN) exchange-correlation functional requires a Hubbard $U$ correction (determined, e.g., from experimental oxidation enthalpies) to reproduce the ground-state structure, lattice parameters, magnetic moments, and electronic properties of Ce-, Mn-, and Fe-based oxides. In the present work, we extend our approach to identify optimal $U$ values for other $3d$ TMOs, specifically Ti, V, Cr, Co, Ni, and Cu, within the $\mathrm{SCAN}+U$ framework. We determine optimal $U$ values of 2.5, 1.0, 3.0, and 2.5 eV for Ti, V, Co, and Ni oxides, respectively, while Cr and Cu oxides best reproduce redox thermodynamics without any $U$ correction at all. While the $U$ values required for Ti, V, Co, and Ni are lower than those needed within the generalized gradient approximation (GGA) + $U$ or local density approximation (LDA) + $U$ approaches, inclusion of $U$ makes non-negligible improvements in ground-state property evaluations of these oxides. Here we also validate our optimal $U$ values by performing a number of transferability checks for each $3d$ material. A $\mathrm{SCAN}+U$ framework (with an appropriately determined $U$) therefore is needed to assess accurately the ground-state energies and qualitatively consistent electronic structures for (most) first-row TMOs.

The dynamics of Na ions in amorphous $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_5$, a potential solid electrolyte material for Na battery, have been investigated by employing. the quasielastic neutron scattering (QENS) technique in the temperature range 300 to 748 K. To understand the diffusion pathways and relaxation timescales of Na ionic diffusion, the experimental studies are complemented by ab initio and force-field molecular dynamics simulations. The QENS data are fairly well described by a jump-diffusion model with a mean jump length of about 3 Å and residence time about 9 ps. Our molecular dynamics simulations have predicted that the diffusion of $\mathrm{N}{\mathrm{a}}^{+}$ ions occurs in the amorphous phase of $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_5$ while absent in the crystalline orthorhombic phase even up to 1100 K. The molecular dynamics simulations have revealed that in the amorphous phase, due to different orientations of silicon polyhedral units, several accessible pathways are opened up for $\mathrm{N}{\mathrm{a}}^{+}$ diffusions. These pathways are not available in the crystalline phase of $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_5$ due to rigid spatial arrangement of silicon polyhedral units.

Controlling the crystalline structure of hafnium zirconate and its epitaxial relationship to a semiconducting electrode has high technological interest, as ferroelectric materials are key ingredients for emerging electronic devices. Using pulsed laser deposition, a phase-pure, ultrathin film of $\mathrm{HfZr}{\mathrm{O}}_4$ is grown epitaxially on a GaN(0001)/Si(111) template. Since standard microscopy techniques do not allow us to determine with certitude the crystalline structure of the film due to the weak scattering of oxygen, differentiated differential phase contrast scanning transmission electron microscopy is used to allow the direct imaging of oxygen columns in the film. Combined with x-ray diffraction analysis, the polar nature and rhombohedral $R3$ symmetry of the film are demonstrated.

We parametrize Hubbard and spin models for ${\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ from broken symmetry density functional calculations. This gives a scalene triangular model where the largest net exchange interaction is three times larger than the mean interchain coupling. The chain random phase approximation shows that the difference in the interchain couplings is equivalent to a bipartite interchain coupling, favoring long-range magnetic order. This competes with ring exchange, which favors quantum disorder. Ring exchange wins. We predict that the thermal conductivity $\kappa$ along the chain direction is much larger than that along the crystallographic axes and that $\kappa /T\to 0$ as $T\to 0$ along the crystallographic axes, but that $\kappa /T\to \mathrm{aconstant}>0$ as $T\to 0$ along the chain direction.

The different occupancy of electronic orbitals, the so-called orbital polarization, is a key parameter determining electric and magnetic properties of materials. Here we report on the demonstration of in operando voltage-controlled tuning of the orbital occupation in $\mathrm{LaNi}{\mathrm{O}}_3$ epitaxial thin films grown on piezoelectric substrates. The different static contributions to the orbital occupation are disentangled, namely the epitaxial strain and the surface symmetry breaking, and the superimposed electric-field controlled orbital polarization are determined by x-ray linear dichroism at the Ni $L_{2,3}$ edges. The voltage-controlled orbital polarization allows changing the orbital polarization by about an additional 50%.