Using first-principles methods, we have studied structural and electronic properties of ternary sulfide $A\mathrm{Bi}{\mathrm{S}}_2$ ($A=\mathrm{Na},\mathrm{K}$) compounds for low cost and environmentally friendly optoelectronic applications. We find that the most stable crystal structure of these compounds at low temperature forms a 1 × 1 rocksalt based ${\mathrm{Bi}}^{3+}$ and $A^{+}$ superlattice along the [111] direction. The ordered phase will transform to the disordered phase at high temperature, with the calculated order-disorder transition temperature for $\mathrm{K}\mathrm{Bi}{\mathrm{S}}_2$ ($T_c=\sim 1200\mathrm{K}$), higher than that for $\mathrm{Na}\mathrm{Bi}{\mathrm{S}}_2$ ($T_c=\sim 900\mathrm{K}$) due to the larger size mismatch between K and Bi than between Na and Bi. The disordered phase has higher optical absorption than the ordered one with the light absorption threshold at 1.33 and 1.57 eV, respectively, for $\mathrm{Na}\mathrm{Bi}{\mathrm{S}}_2$ and $\mathrm{K}\mathrm{Bi}{\mathrm{S}}_2$. The good agreement between our calculated properties such as x-ray diffraction and light absorption of the partially disordered phase and the experimental observations suggests that the partially disordered phase, instead of the ordered phase, was formed in experimental samples. Our study provides theoretical insights into the order-disorder transition and the corresponding variation of the crystal and electronic structures of rocksalt based $A\mathrm{Bi}{\mathrm{S}}_2$ ($A=\mathrm{Na},\mathrm{K}$) compounds, which is helpful for future synthesis and applications of these materials.

Favorable optoelectronic properties and ease of fabrication make NiO a promising hole transport layer for perovskite solar cells. To achieve maximum efficiency, the electronic levels of NiO need to be optimally aligned with those of the perovskite absorber. Applying surface modifiers by adsorbing species on the NiO surface is one of the most widespread strategies to tune its energy levels. Alkali halides are simple inorganic surface modifiers that have been used extensively in organic optoelectronics, but rarely studied in perovskite solar cells. Using density functional theory calculations, we investigate the effect of single-layer adsorption of 20 different alkali halides on the electronic levels of NiO. Our results show that alkali halides can shift the position of the valence-band maximum (VBM) of NiO to a surprisingly large extent in both directions, from $-3.10$ to $+1.59\mathrm{eV}$. We interpret the direction and magnitude of the shift in terms of the surface dipoles, formed by the adsorbed cations and anions, where the magnitude of the VBM shift is a monotonic function of the surface coverage. Our results indicate that with alkali halide surface modifiers, the electronic levels of NiO can be tuned robustly and potentially match those of many perovskite compositions in perovskite solar cells.

Both engineered and natural materials may be subjected to extremes of elevated pressure, temperature, and strain rate due to shock loading; examples include ballistic impact of projectiles on armor, planetary impacts, and high-speed machining operations. Experimental techniques for ascertaining the macroscopic response of materials to shock loading are well established, but insight into fundamental mechanisms of deformation requires the ability to characterize the evolution of microstructure in real time. Experiments in which specimens are recovered and characterized after shock loading have been widely used to understand structure-property relationships. But these shock recovery experiments only reveal information at the end state of the material, from which the evolution of the structure during unloading must be inferred. There is always the possibility that the structure of the material continues to evolve during unloading, leading to a potential misunderstanding of the structural evolution during shock compression and release. In this paper we describe the results of shock recovery and time-resolved in situ x-ray diffraction studies of deformation twinning in an extruded fine-grained AMX602 magnesium alloy. The samples were shock compressed along the plate normal and extrusion directions, then released back to ambient conditions. Analysis of the microstructure before and after shock loading indicates a substantial change in crystallographic texture reflecting substantial deformation twinning. Texture evolution from in situ synchrotron x-ray diffraction measurements show significant twinning during shock compression followed by detwinning during stress release. These results not only provide insight into the complex twinning-detwinning behavior of this particular alloy, but also illustrate the utility of in situ characterization for bridging the knowledge gap between shock recovery experiments and the transient behavior of materials during shock loading more generally.

Reflection high-energy electron diffraction (RHEED) is a ubiquitous in situ molecular beam epitaxial (MBE) characterization tool. Although RHEED can be a powerful means for crystal surface structure determination, it is often used as a static qualitative surface characterization method at discrete intervals during a growth. A full analysis of RHEED data collected during the entirety of MBE growths is made possible using principle component analysis (PCA) and $k$-means clustering to examine significant boundaries that occur in the temporal clusters grouped from RHEED data and identify statistically significant patterns. This process is applied to data from homoepitaxial $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ growths, heteroepitaxial $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ grown on scandate substrates, $\mathrm{Ba}\mathrm{Sn}{\mathrm{O}}_3$ films grown on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates, and ${\mathrm{LaNiO}}_3$ films grown on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates. This analysis may provide additional insights into the surface evolution and transitions in growth modes at precise times and depths during growth, and that video archival of an entire RHEED image sequence may be able to provide more insight and control overgrowth processes and film quality.

The topological kagome magnet (TKM) ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ exhibits unusual topological properties, flat electronic bands, and chiral spin textures, making it an exquisite materials platform to explore the interplay between topological band structure, strong electron correlations, and magnetism. Here we report the synthesis of high-quality epitaxial (0001) ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ films with large intrinsic anomalous Hall effect close to that measured in bulk single crystals. In addition, we measured a large, anisotropic anomalous Nernst coefficient $S_{yx}$ of $1.26\mu \mathrm{V}{\mathrm{K}}^{-1}$, roughly 2–5 times greater than that of common ferromagnets, suggesting the presence of Berry curvature sources near the Fermi level in this system. Crucially, the realization of high-quality ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ films opens the door to explore emergent interfacial physics and create novel spintronic devices based on TKMs by interfacing ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ with other quantum materials and by nanostructure patterning.

We present experimental evidence showing that the effective carrier diffusion length $L_d$ and lifetime τ depend on the carrier density in $\mathrm{MAPb}{\mathrm{Br}}_3$ single crystals. Independent measurements reveal that both $L_d$ and τ decrease with an increase in photocarrier density. Scanning photocurrent microscopy is used to extract the characteristic photocurrent $I_{\mathrm{ph}}$ decay-length parameter $L_d$, which is a measure of effective carrier diffusion. The $L_d$ magnitudes for electrons and holes are determined to be $\sim 13.3$ and $\sim 13.8\mu \mathrm{m}$, respectively. A marginal increase in uniform light bias $(\le 5\times {10}^{15}\mathrm{photons}/\mathrm{c}{\mathrm{m}}^2)$ increases the modulated photocurrent magnitude and reduces the $L_d$ parameter by a factor of 2 and 3 for electrons and holes, respectively, indicating that the recombination is not monomolecular. The $L_d$ variations are correlated to the features in photoluminescence lifetime studies. Analysis of lifetime variation shows intensity-dependent monomolecular and bimolecular recombination trends with recombination constants determined to be $\sim 9.3\times {10}^6{\mathrm{s}}^{-1}$ and $\sim 1.4\times {10}^{-9}\mathrm{c}{\mathrm{m}}^3{\mathrm{s}}^{-1}$, respectively. Based on the trends of $L_d$ and lifetime, it is inferred that the sub-band-gap trap recombination influences carrier transport in the low-intensity excitation regime, while bimolecular recombination and transport dominate at high intensity.

Reflection high-energy electron diffraction is a widely used tool to study the growth dynamics of thin films, in situ and operando. Here it is applied after deposition, revealing that the morphological state of the surface is connected to the amount of material underneath. We focus on the growth of ${\mathrm{LaAlO}}_3$ thin films on a ${\mathrm{SrTiO}}_3$ substrate by pulsed laser deposition, utilizing a geometrically anisotropic plasma plume. After deposition, oscillations in the intensity and width of the specular spot as a function of position are observed along the short axis of the plasma plume. Each complete oscillation represents a unit-cell difference in film thickness, as confirmed by local x-ray reflectivity measurements. We apply this approach to demonstrate the critical thickness for conductivity at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface on a single sample. Our results introduce a straightforward method to engineer and study thickness-dependent properties of epitaxial thin films on a highly detailed level.

The functioning of organic optoelectronic devices such as organic light-emitting diodes (OLEDs) is determined in part by the dielectric permittivity $\epsilon$ of the organic materials used, at frequencies that vary from quasistatic to the optical range. The difference between the dielectric constants at these extremes of the frequency scale is due to contributions of vibrational and (for some materials) dipole rotational modes and can depend on the detailed molecular packing. Studies of these contributions are therefore expected to sensitively probe differences in thin-film structures that affect their long-term stability. The absolute value of the dielectric constant affects key processes, such as charge transport, exciton generation, and exciton dissociation. As a first step toward disentangling the various contributions to $\epsilon$, we present in this paper the results of first-principles calculations of the vibrational mode contribution to $\epsilon$ for a large number of small-molecule organic semiconducting materials that are relevant to OLEDs. We find that this contribution is significant for molecules with polar groups and strongly infrared-active vibrational modes, but also for molecules without such groups but with very-low-frequency vibrational modes, below $\sim 2$ THz ($\sim 10$ meV). A comparison with available experimental data reveals good overall agreement concerning the order of magnitude of this contribution, but also indicates the need for detailed material-specific studies of the sensitivity to the thin-film structure.

We report the dependence of antiferromagnetism on Zn-doping concentration in the newly synthesized $\mathrm{CeRh}{\left(\mathrm{I}{\mathrm{n}}_{1\text{−}x}\mathrm{Z}{\mathrm{n}}_x\right)}_5$ single crystal with $x\le 0.023$. X-ray-diffraction measurements showed a smooth decrease of lattice parameters with an increasing Zn concentration, indicating a positive chemical pressure effect. The electrical resistivity, specific heat, and magnetic susceptibility measurements revealed that the antiferromagnetic transition temperature $T_{\mathrm{N}}$ initially decreases from 3.8 K for pure $\mathrm{CeRhI}{\mathrm{n}}_5$ to 3.1 K at $x=0.012$; then, it becomes flat, remaining at approximately 3.1 K between Zn concentrations of 0.012 and 0.017, and finally, it increases to 3.3 K at 0.023 Zn concentration. These results suggest that the change in the electronic structure induced by Zn doping is more important than the chemical pressure effects with regard to tuning the magnetic order. A study on the electronic structure and pressure tuning of the newly synthesized heavy-fermion compound $\mathrm{CeRh}{\left(\mathrm{I}{\mathrm{n}}_{1\text{−}x}\mathrm{Z}{\mathrm{n}}_x\right)}_5$, which does not include a toxic element, is expected to further enhance our understanding of the competing ground states emerging in heavy-fermion systems.

By rigorously accounting for mesoscale spatial correlations in donor/acceptor surface properties, we develop a scale-spanning model for same-material tribocharging. We find that mesoscale correlations affect not only the magnitude of charge transfer but also the fluctuations—suppressing otherwise overwhelming charge-transfer variability that is not observed experimentally. We furthermore propose a generic theoretical mechanism by which the mesoscale features might emerge, which is qualitatively consistent with other proposals in the literature.

We report epitaxial growth of vanadium diselenide (${\mathrm{VSe}}_2$) thin films in the octahedrally coordinated (1T) structure on GaAs(111)B substrates by molecular beam epitaxy. Film thickness from a single monolayer (ML) up to 30 ML is demonstrated. Structural and chemical studies using x-ray diffraction, transmission electron microscopy, scanning tunneling microscopy, and x-ray photoelectron spectroscopy indicate high-quality thin films. Further studies show that monolayer ${\mathrm{VSe}}_2$ films on GaAs are not air stable and are susceptible to oxidation within a matter of hours, which indicates that a protective capping layer should be employed for device applications. This work demonstrates that ${\mathrm{VSe}}_2$, a candidate van der Waals material for possible spintronic and electronic applications, can be integrated with III-V semiconductors via epitaxial growth for two- and three-dimensional hybrid devices.

Graphenylene and naphthylene have recently been proposed as two-dimensional carbon allotropes conceptually defined by the assembly of phenyl-like (P) or naphthyl-like (N) units with the creation of side squarelike rings. These structures can be cut into one-dimensional (1D) nanoribbons displaying electronic properties ranging from metals to semiconductors. Even though they have not been synthesized to date, recent advances in preprogrammed atomically precise chemical routes are compatible with their future synthesis. Here, an alternative naphthylene form is proposed. It is named as the $\gamma$ type, which is theoretically constructed by combining P and N blocks joined by 4-membered links. This naphthylene-$\gamma$ has a semiconductor behavior and theory predicts that this property is shared by its nanoribbon counterparts. Computational analysis also indicates the electronic structure of these systems is dictated by quasi-1D states due to the interplay between the system's geometry (e.g., chirality and edge structure) and its electronic properties.

We investigate the magnetic properties of ${\mathrm{Ru}}_2\mathrm{Mn}Z$ ($Z=\mathrm{Sn},\mathrm{Sb},\mathrm{Ge},\mathrm{Si}$) chemically ordered, full Heusler compounds for zero as well as finite temperatures. Based on first-principles calculations we derive the interatomic isotropic bilinear and biquadratic couplings between Mn atoms from the paramagnetic state. We find frustrated isotropic couplings for all compounds and, in the case of $Z=\mathrm{Si}$ and Sb, a nearest-neighbor biquadratic coupling that favors perpendicular alignment between the Mn spins. By using an extended classical Heisenberg model in combination with spin dynamics simulations we obtain the magnetic equilibrium states. From these simulations we conclude that the biquadratic coupling, in combination with the frustrated isotropic interactions, leads to noncollinear magnetic ground states in the ${\mathrm{Ru}}_2\mathrm{Mn}\mathrm{Si}$ and ${\mathrm{Ru}}_2\mathrm{Mn}\mathrm{Sb}$ compounds. In particular, for these alloys we find two distinct, noncollinear ground states which are energetically equivalent and can be identified as $3Q$ and $4Q$ states on a frustrated fcc lattice. Investigating the thermal stability of the noncollinear phase we find that, in the case of ${\mathrm{Ru}}_2\mathrm{Mn}\mathrm{Si}$, the multiple-$Q$ phase undergoes a transition to the single $Q$ phase, while in case of ${\mathrm{Ru}}_2\mathrm{Mn}\mathrm{Sb}$ the corresponding transition is not obtained due to the larger magnitude of the nearest-neighbor biquadratic coupling.

Nonwoven fibrous materials are a unique class of porous materials that are widely used for filtration and separation applications. The complex pore structure in fibrous media, with high connectivity and low tortuosity, enhances filtration efficiency while preventing filter fouling. The pore size distribution is a key to predicting and improving transport properties by tailoring the microstructure of fibrous media. However, the statistical properties of the pore space are not easily accessible in experiments, and often only effective geometric properties such as hydraulic radius and specific surface area are reported. In this study, we employ a computational framework and generate realistic nonwoven fibrous media with a wide range of porosities. We obtain the pore network by extracting a subnetwork of the watershed segmentation of pore space and analyze the statistical distribution of pore and throat sizes and their connectivity. We find that a random fiber orientation has a pronounced effect on the effective geometric properties and their dependence on porosity. We investigate the influence of processing parameters such as fiber size, orientation, and overlaps between fibers as a prerequisite to predicting transport properties of nonwoven fibrous media. The computational approach can be combined with experimental imaging techniques, which enables rapid characterization of porous microstructures in terms of the statistical pore space properties resulting from specific manufacturing processes.

Flow phenomena in porous media are relevant in many technological applications including wastewater filtration, chemical separations, and oil recovery. Nonwoven fibrous membranes are widely used as filtration media, and tailoring the microstructure of the pore space is a key to improving filtration efficiency and preventing membrane fouling. However, predicting effective properties such as permeability and tortuosity for complex porous microstructures remains a challenging task as most available models do not incorporate the influence of random fiber arrangement and orientation on the structure-property relations. This study presents a computational analysis of the permeability and tortuosity of realistic nonwoven fibrous media with regular and random fiber arrangements over a wide range of porosities. We conduct pore-scale lattice Boltzmann simulations to predict the fluid flow through nonwoven fibrous media and use the results to test the accuracy of semiempirical scaling relations. Combining morphological properties of the pore space with pore-scale flow simulations enables us to determine the influence of the statistical pore size distribution on the effective fluid transport properties over a wide range of macroscopic porosities. We find that semiempirical coefficients such as the Kozeny-Carman coefficient depend markedly on the statistical distribution of pores and throats. This dependence can be quantified as a constriction factor that incorporates the pore size distribution to yield accurate predictions of permeability of random fibrous media. In combination with experimental imaging techniques, the ability to quantify the connection between microstructure and fluid transport paves the way to computational characterization and design of porous media with tailored properties for filtration and separation applications.

We present a framework for quantifying the uncertainty that results from the choice of exchange-correlation (XC) functional in predictions of phonon properties and thermal conductivity that use density functional theory to calculate the atomic force constants. The energy ensemble capabilities of the Bayesian error estimation functional with van der Waals correlation XC functional are first applied to determine an ensemble of interatomic force constants, which are then used as inputs to lattice dynamics calculations and a solution of the Boltzmann transport equation. The framework is applied to isotopically pure silicon. We find that the uncertainty estimates bound property predictions (e.g., phonon dispersions, specific heat, thermal conductivity) from other XC functionals and experiments. We distinguish between properties in silicon that are correlated with the predicted thermal conductivity [e.g., the transverse-acoustic branch sound speed (squared Pearson correlation coefficient, $R^2$, of 0.89) and average Grüneisen parameter ($R^2=0.85$)] and those that are not [e.g., longitudinal-acoustic branch sound speed ($R^2=0.23$) and specific heat ($R^2=0.00$)]. We find that differences in ensemble predictions of thermal conductivity are correlated with the behavior of transverse-acoustic phonons with mean free paths between 100 and 300 nm. The framework systematically accounts for XC uncertainty in phonon calculations and should be used whenever it is suspected that the choice of XC functional is influencing physical interpretations.

$\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ is often doped with Y and/or Mn to realize a range of desired properties. Yet, existing canonical models of their defect chemistry cannot explain various observed phenomena outside of the high temperature electrical conductivity measurements to which they were fit. Existing models assume Y substitutes exclusively for Ba or Ti despite experiments showing Y is amphoteric, substituting predominantly for Ba or Ti depending on the cation nonstoichiometry. Existing models assume Mn forms isolated ${\mathrm{Mn}}_{\text{Ti}}$ exclusively, but experiments have shown complexes of ${\mathrm{Mn}}_{\mathrm{Ti}}$ with native oxygen vacancies (${\mathrm{Mn}}_{\mathrm{Ti}}\text{−}{\mathrm{v}}_{\text{O}}$) form in significant concentrations. Additionally, recent computational works in ${\mathrm{SrTiO}}_3$ suggest A-site substitutional defects may form in greater concentrations in a configuration with reduced symmetry relative to the on-site geometry. To address these inconsistencies, we developed a hybrid functional density functional theory informed grand canonical defect model with the ability to simulate specific nonstoichiometries without ad hoc assumptions about the bulk chemical potentials. Using this model, the site preference of Y and Mn in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ as a function of the Ba/Ti ratio was evaluated in the context of a more complete set of defects including: native cation vacancies, on-site and reduced symmetry A-site defects, isolated B-site defects, and ${\mathrm{X}}_{\text{B}}\text{−}{\mathrm{v}}_{\text{O}}$ defects. The results reproduce experimental observations of yttrium's amphotericity and significant concentrations of ${\mathrm{Mn}}_{\mathrm{Ti}}\text{−}{\mathrm{v}}_{\text{O}}$. Both and Y and Mn are found to substitute predominantly for Ba at Ba/Ti = 0.99 and Ti at Ba/Ti = 1.01, but neither are found to substitute exclusively for Ba or Ti within the range of experimentally accessible Ba/Ti ratios at 1400 ${}^{\circ }\mathrm{C}$.

Single crystals of the metallic Ruddlesden-Popper trilayer nickelates $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) were successfully grown using an optical-image floating zone furnace under oxygen pressure $\left(p{\mathrm{O}}_2\right)$ of 20 bar for ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and 140 bar for ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. A combination of synchrotron and laboratory x-ray single-crystal diffraction, high-resolution synchrotron x-ray powder diffraction and measurements of physical properties revealed that $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) crystallizes in the monoclinic $P{2}_1/a$ ($Z=2$) space group at room temperature, and that a metastable orthorhombic phase (Bmab) can be trapped by postgrowth rapid cooling. Both ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, the MMT is found at 157.6 K. For ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, the MMT depends on the lattice symmetry: 147.5 K for Bmab vs 138.6 K for $P{2}_1/a$. Lattice anomalies were found at the MMT that, when considered together with the pronounced dependence of the transition temperature on subtle structural differences between Bmab and $P{2}_1/a$ phases, demonstrate a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.

Alloying metals with other elements is often done to improve the material strength or hardness. A key microscopic mechanism is precipitation hardening, where precipitates impede dislocation motion, but the role of such obstacles in determining the nature of collective dislocation dynamics remains to be understood. Here, three-dimensional discrete dislocation dynamics simulations of fcc single crystals are performed with fully coherent spherical precipitates from zero precipitate density up to ${\rho }_p={10}^{21}{\text{m}}^{-3}$ and at various dislocation-precipitate interaction strengths. When the dislocation-precipitate interactions do not play a major role, the yielding is qualitatively the same as for pure crystals, i.e., dominated by “dislocation jamming,” resulting in glassy dislocation dynamics exhibiting critical features at any stress value. We demonstrate that increasing the precipitate density and/or the dislocation-precipitate interaction strength creates a true yield or dislocation assembly depinning transition, with a critical yield stress. This is clearly visible in the statistics of dislocation avalanches observed when quasistatically ramping up the external stress, and it is also manifested in the response of the system to constant applied stresses. The scaling of the yielding with precipitates is discussed in terms of the Bacon-Kocks-Scattergood relation.