Thermoelectricity in principle provides a pathway to put waste heat to good use. Motivated by this we investigate thermal and electrical transport properties of two new Fe-based Heusler alloys, FeTaSb and FeMnTiSb, by a first-principles approach and semiclassical Boltzmann transport theory within the constant relaxation-time approximation. We find a high power factor of $p$-doped FeTaSb, competitive with best performing Heusler alloy FeNbSb at 1100 K. The obtained power factor of $n$-doped FeMnTiSb at room temperature is higher than that of both FeNbSb and FeTaSb. Remarkably, FeMnTiSb can be used for both $n$-type and $p$-type legs in a thermoelectric module. The Seebeck coefficients of the two proposed systems are in line with those of earlier reported Heusler alloys. We also provide conservative estimates of the figure of merit for the two systems. Overall, our findings suggest a high temperature thermoelectric potential of FeTaSb while the low cost FeMnTiSb is a viable room-temperature thermoelectric candidate material.

The concept of a local linear elastic strain field is commonly used in the metallurgical research community to approximate the collective effect of atomic displacements around crystalline defects. Here we show that the elastic strain field approximation is a useful tool in colloidal systems. For colloidal crystals with repulsive particle interaction potentials, given similar mechanical properties, sharper potentials lead to (1) free energies of deformation dominated by entropy, (2) lower variance in strain field fluctuations, (3) increased tension-compression asymmetry near dislocation core regions, and (4) smaller windows of applicability of the linear elastic approximation. We show that the window of linear behavior for entropic colloidal crystals is broadened for pressures at which the interparticle separation sufficiently exceeds the range of steep repulsive interactions.

Constructed to satisfy 17 known exact constraints for a semilocal density functional, the strongly constrained and appropriately normed (SCAN) meta-generalized-gradient-approximation functional has shown early promise for accurately describing the electronic structure of molecules and solids. One open question is how well SCAN predicts the formation energy, a key quantity for describing the thermodynamic stability of solid-state compounds. To answer this question, we perform an extensive benchmark of SCAN by computing the formation energies for a diverse group of nearly 1000 crystalline compounds for which experimental values are known. Due to an enhanced exchange interaction in the covalent bonding regime, SCAN substantially decreases the formation energy errors for strongly bound compounds, by approximately 50% to 110 meV/atom, as compared to the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE). However, for intermetallic compounds, SCAN performs moderately worse than PBE with an increase in formation energy error of approximately 20%, stemming from SCAN's distinct behavior in the weak bonding regime. The formation energy errors can be further reduced via elemental chemical potential fitting. We find that SCAN leads to significantly more accurate predicted crystal volumes, moderately enhanced magnetism, and mildly improved band gaps as compared to PBE. Overall, SCAN represents a significant improvement in accurately describing the thermodynamics of strongly bound compounds.

We systematically investigated changes of a photoluminescence (PL) spectral shape by picking a tungsten disulfide monolayer (1L-${\mathrm{WS}}_2$) grown on a sapphire (SA) substrate and dropping it on various substrates such as SA, ${\mathrm{SiO}}_2$, and hexagonal boron nitride (hBN). An inhomogeneous contribution to the exciton PL linewidth was reduced by picking the CVD-grown 1L-${\mathrm{WS}}_2$, meaning potential energy disorder lowering in the 1L-${\mathrm{WS}}_2$. In addition, a defect-bound exciton peak was not seen at low temperature for all the samples with the pick-and-drop process although it was clearly observed for the as-grown sample. It was thought that the as-grown 1L-${\mathrm{WS}}_2$ was covalently bonded on the SA substrate surface due to the existence of crystal imperfections in the ${\mathrm{WS}}_2$ and the covalent bonds formed midgap states. A PL spectrum had to be carefully treated in order to fairly evaluate a crystal quality in a CVD-grown 1L-${\mathrm{WS}}_2$ since PL spectral shapes were determined by complexes of the defect density with other mechanisms such as strain fluctuation and a connection with the substrate.

Controlling multiferroic behavior in materials will enable the development of a wide variety of technological applications. However, the exact mechanisms driving multiferroic behavior are not well understood in most materials. Two such materials are the spinels ${\mathrm{MnV}}_2{\mathrm{O}}_4$ and ${\mathrm{Mn}}_3{\mathrm{O}}_4$, where mechanical strain is thought to play a role in determining magnetic behavior. Bulk studies of ${\mathrm{MnV}}_2{\mathrm{O}}_4$ have yielded conflicting and inconclusive results, due in part to the presence of mesoscale magnetic inhomogeneity, which complicates the interpretation of bulk measurements. To study the sub-micrometer-scale magnetic properties of Mn-based spinel materials, we performed magnetic force microscopy (MFM) on ${\mathrm{MnV}}_2{\mathrm{O}}_4$ samples subject to different levels of mechanical strain. We also used a crystal grain mapping technique to perform spatially registered MFM on ${\mathrm{Mn}}_3{\mathrm{O}}_4$. These local investigations revealed 100-nm-scale “stripe” modulations in the magnetic structure of both materials. In ${\mathrm{MnV}}_2{\mathrm{O}}_4$, the magnetization of these stripes is estimated to be ${\mathrm{M}}_{\mathrm{z}}\sim {10}^5\mathrm{A}/\mathrm{m}$, which is on the order of the saturation magnetization reported previously. Cooling in a strong magnetic field eliminated the stripe patterning only in the low-strain sample of ${\mathrm{MnV}}_2{\mathrm{O}}_4$. The discovery of nanoscale magnetostructural inhomogeneity that is highly susceptible to magnetic field control in these materials necessitates both a revision of theoretical proposals and a reinterpretation of experimental data regarding the low-temperature phases and magnetic-field-tunable properties of these Mn-based spinels.

The conversion of charge current into pure spin current based on the spin Hall effect provides a new way of spin injection and manipulation in novel spintronic architectures. The realization of a large conversion efficiency between the charge and spin currents is a crucial issue for its applications. Here we report a giant enhancement of spin Hall angle (SHA) in palladium through boron engineering. We measure a large SHA of 0.16 in palladium, with perpendicular magnetic anisotropy in the Pd/CoFeB-based structure compared to the Pd/CoFe-based structure ($\mathrm{SHA}=0.02$). Combined with theoretical calculations, it is found that both intrinsic and extrinsic spin Hall effects have been significantly enhanced through the introduction of boron. Taking into account the incorporation of boron in the thin films, the enhanced SHA is attributed to significant microscopic and electronic changes in the Pd host metal. This finding illustrates a crucial role of boron in manipulating spin-orbit torques. Together with high conductivity and long spin diffusion length, this work makes Pd a promising candidate for spintronic devices with magnetization switching by current pulse injection.

In modern GaAs/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor $\delta$ layers placed on both sides of the well. Each $\delta$ layer is located within a narrow GaAs well, flanked by narrow AlAs layers which capture excess electrons from donors. We show that each excess electron is localized in a compact dipole atom with the nearest donor. Nevertheless, excess electrons screen both the remote donors and background impurities. When the fraction of remote donors filled by excess electrons $f$ is small, the remote donor limited quantum mobility grows as $f^3$ and becomes larger than the background impurity limited one at a characteristic value $f_c$. We also calculate both the mobility and the quantum mobility limited by the screened background impurities with concentrations $N_1$ in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ and $N_2$ in GaAs, which allows one to estimate $N_1$ and $N_2$ from the measured mobilities. Taken together, our findings should help to identify avenues for further improvement of modern heterostructures.

A variety of conducting heterointerfaces have been made between ${\mathrm{SrTiO}}_3$ substrates and thin capping layers of distinctly different oxide materials that can be classified into polar band insulators (e.g., ${\mathrm{LaAlO}}_3$), polar Mott insulators (e.g., ${\mathrm{LaTiO}}_3$), apparently nonpolar band insulators (e.g., $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$), and amorphous oxides (e.g., amorphous ${\mathrm{SrTiO}}_3$). A fundamental question to ask is if there is a common mechanism that governs interfacial conductivity in all these heterointerfaces. Here, we examined the conductivity of different kinds of heterointerfaces by annealing in oxygen and surface treatment with water. It was found that the conductivity of all the heterointerfaces shows a strong dependence on annealing, and can be universally tuned by surface treatment whose effect is determined by the annealing condition. These observations, together with ambient-pressure x-ray photoelectron spectroscopy measurements, suggest that water chemistry at surface oxygen vacancies is a common mechanism that supplies electrons to the interface.

In this work, we use density functional theory calculations to demonstrate how spontaneous electric polarizations can be induced via a hybrid improper ferroelectric mechanism in iodide perovskites, a family well known to display solar-optimal band gaps, to create materials for photoferroic applications. We first assemble three chemically distinct $\left(A{A}^{\prime }\right)\left(B{B}^{\prime }\right){\mathrm{I}}_6$ double perovskites using centrosymmetric $AB{\mathrm{I}}_3$ perovskite iodides (where $A=\mathrm{Cs}$, Rb, K and $B=\mathrm{Sn}$, Ge) as building units. In each superlattice, we investigate the effects of three types of $A$- and $B$-site cation ordering schemes and three different $B{\mathrm{I}}_6$ octahedral rotation patterns. Out of these 27 combinations, we find that 15 produce polar space groups and display spontaneous electric polarizations ranging from 0.26 to $23.33\mu \mathrm{C}/{\mathrm{cm}}^2$. Furthermore, we find that a layered $A$-site/rock salt $B$-site ordering, in the presence of an $a^0{a}^0{c}^{+}$ rotation pattern, produces a chiral vortex-like $A$-site displacement pattern. We then investigate the effect of epitaxial strain on one of these compounds, (CsRb)(SnGe)${\mathrm{I}}_6$, in layered and rock salt ordered configurations. In both phases, we find strong competition between the cation ordering schemes as well as an enhancement of the spontaneous polarization magnitude under tensile strain. Finally, using a hybrid density functional with fractional Fock exchange, we find the iodide superlattices display semiconducting band gaps ranging from 0.2 to 1.3 eV. These results demonstrate that cation ordering and epitaxial strain are powerful ways to induce and control functionalities in technologically useful families of materials.

Unveiling both the presence and nature of point defects is one of the biggest challenges in condensed matter physics and materials science. Particularly in complex oxides, even a minute amount of unavoidable point defects could generate novel physical phenomena and functions, such as visible light emission and ferroelectricity, yet it remains elusive to clearly identify such point defects. Here, taking $\mathrm{SrTi}{\mathrm{O}}_3$ as a model system, we show that iterative feedback among atomic-layer- and stoichiometry-controlled thin-film epitaxy, hybrid density functional theory, and high-resolution cathodoluminescence spectroscopy allows for the identification of a functional cationic defect, the Ti antisite (${\mathrm{Ti}}_{\mathrm{Sr}}$) defect. Our cathodoluminescence measurements reveal sub-band-gap luminescence, whose spectral fine structures show excellent quantitative agreement, as well as a one-to-one correspondence, with the theoretically predicted optical transitions from intrinsic point defects such as ${\mathrm{Ti}}_{\mathrm{Sr}}$. Guided by the theory and spectroscopic results, we also control a cation stoichiometry, and it in turn results in good systematics in cathodoluminescence spectra. Not limited to the identification of ${\mathrm{Ti}}_{\mathrm{Sr}}$, this approach allows for more reliable, self-consistent defect study, and provides critical insight into a microscopic picture of point defects in complex oxides.

Obtaining a combination of high toughness and strength is crucial for most structural materials, but unfortunately these tend to be mutually exclusive. The search for strong and tough damage-resistant materials has thus typically been based on achieving an acceptable compromise between hardness and crack resistance. Focusing here on brittle oxide glasses, we propose a new strategy for overcoming this conflict by identifying new structural motifs for designing hard and crack-resistant glasses. Specifically, we report that surprisingly there is no decrease in the densification contribution to deformation of a mixed network ${\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{B}}_2{\mathrm{O}}_3-{\mathrm{P}}_2{\mathrm{O}}_5-\mathrm{Si}{\mathrm{O}}_2$ bulk glass following predensification of the glass at elevated temperature. Hitherto unique to this glass composition, the treatment reduces the residual stress during subsequent sharp contact loading, which in turn leads to a simultaneous increase in hardness and crack resistance. Based on structural characterization, we show that the more densified medium-range order of the hot compressed glass results in formation of certain structural states (e.g., nonring trigonal boron), which could not be reached through any composition or thermal path. This work thus shows that accessing such “forbidden” structural states through the identified densification at elevated temperatures offers a way forward to overcome the conflict of strength versus toughness in structural materials.

As a first step to porting scanning tunneling microscopy methods of atomic-precision fabrication to a strained-Si/SiGe platform, we demonstrate post-growth P atomic-layer doping of SiGe heterostructures. To preserve the substrate structure and elastic state, we use a $T\le 800{}^{\circ }\mathrm{C}$ process to prepare clean ${\mathrm{Si}}_{0.86}{\mathrm{Ge}}_{0.14}$ surfaces suitable for atomic-precision fabrication. P-saturated atomic-layer doping is incorporated and capped with epitaxial Si under a thermal budget compatible with atomic-precision fabrication. Hall measurements at $T=0.3$ K show that the doped heterostructure has ${\mathrm{R}}_{\square }=570\pm 30\mathrm{\Omega }$, yielding an electron density $n_e=2.1\pm 0.1\times {10}^{14}{\mathrm{cm}}^{-2}$ and mobility ${\mu }_e=52\pm 3{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, similar to saturated atomic-layer doping in pure Si and Ge. The magnitude of ${\mu }_e$ and the complete absence of Shubnikov–de Haas oscillations in magnetotransport measurements indicate that electrons are overwhelmingly localized in the donor layer, and not within a nearby buried Si well. This conclusion is supported by self-consistent Schrödinger-Poisson calculations that predict electron occupation primarily in the donor layer.

Polymer fibers, including natural silk and synthetic fibers, exhibit universal viscoelastic response. On stretching below yield, they show logarithmic stress decay. On unloading fibers with a glassy amorphous phase, the stress recovers. A simple phenomenological model accurately describes data from independent mechanical experiments and provides insights into the microstructural origins of the fiber response. Counter to intuition, the model indicates that it is the crystalline regions, rather than the amorphous glass, that deform first on stretching fibers at high strain rates. On holding a stretched fiber, stress decays as a consequence of relaxations in amorphous regions. Finally, unloading the fiber transfers stress from the amorphous to crystalline regions resulting in stress recovery. Model parameters correlate well with the fiber microstructure. Crystal and amorphous moduli from the model match those from x-ray diffraction. Activation energies for the temperature dependence of the peak relaxation time are similar to those reported in the literature. Thus, a simple model that invokes only crystal-amorphous coexistence can successfully model the mechanical response of a wide variety of polymer fibers.

Layered quasi-two-dimensional transition-metal dichalcogenides (TMDCs), which can be readily made in ultrathin films, offer excellent opportunities for studying how dimensionality affects electronic structure and physical properties. Among all TMDCs, ${\mathrm{NbSe}}_2$ is of special interest; bulk ${\mathrm{NbSe}}_2$ hosts a charge-density-wave phase at low temperatures and has the highest known superconducting transition temperature, and these properties can be substantially modified in the ultrathin film limit. Motivated by these effects, we report herein a study of few-layer ${\mathrm{NbSe}}_2$ films, with a well-defined single-domain orientation, epitaxially grown on GaAs. Angle-resolved photoemission spectroscopy was used to determine the electronic band structure and the Fermi surface as a function of layer thickness; first-principles band-structure calculations were performed for comparison. The results show interesting changes as the film thickness increases from a monolayer (ML) to several layers. The most notable changes occur between a ML and a bilayer, where the inversion symmetry in bulk ${\mathrm{NbSe}}_2$ is preserved in the bilayer but not in the ML. The results illustrate some basic dimensional effects and provide a basis for further exploring and understanding the properties of ${\mathrm{NbSe}}_2$.

An important aspect of successful hydrogen storage materials is their hydrogen release mechanism, which—unfortunately—for many promising materials occurs at impractically high or low temperatures. In light of recent improvements observed when complexing borohydrides with ${\mathrm{NH}}_3$, we performed an ab initio investigation into the effects of complexing the borohydrides $\mathrm{Al}{\left({\mathrm{BH}}_4\right)}_3, \mathrm{Mg}{\left({\mathrm{BH}}_4\right)}_2$, and $\mathrm{Zn}{\left({\mathrm{BH}}_4\right)}_2$ with new additive molecules ${\mathrm{CH}}_4, {\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$, and ${\mathrm{H}}_2\mathrm{O}$, as well as ${\mathrm{NH}}_3$ as a benchmark. To find candidate ground-state structures for these borohydride complexes, we first performed an in-depth evolutionary structure search. These ground-state structures were then used to both evaluate their thermodynamic properties as well as to investigate nearby metastable states that facilitate a favorable hydrogen release using evolutionary metadynamics. We found that all of these additives markedly affect—to a varying degree—the nearby metastable states relative to the unaltered borohydride. Furthermore, we found that complexing borohydrides with ${\mathrm{CH}}_4$ and ${\mathrm{H}}_2\mathrm{O}$ results in particularly favorable properties for improving their hydrogen release. Our study not only investigates new and promising hydrogen storage materials, but also elucidates the hydrogen release mechanism and the effect of additives in processes that are difficult to characterize experimentally.

The rapid increase in power conversion efficiencies of photovoltaic devices incorporating lead halide perovskites has resulted in intense interest in the cause of their excellent properties. In the present paper, resonant ultrasound spectroscopy has been used to determine the elastic and anelastic properties of ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Pb}{X}_3$ (where $X=\mathrm{Cl}$, Br, or I) and ${\mathrm{CD}}_3{\mathrm{ND}}_3\mathrm{Pb}{\mathrm{I}}_3$ perovskites in the 5–380 K temperature range. This is coupled with differential scanning calorimetry, variable temperature neutron powder diffraction, and variable temperature photoluminescence studies to provide insights into the underlying processes and structural instabilities in the crystal structure. By comparing measurements on ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Pb}{\mathrm{I}}_3$ with the deuterated equivalent, it has been possible to distinguish processes which are related to the hydrogen bonding between the methylammonium cation and the perovskite framework. We observe that replacing hydrogen with deuterium has a significant impact on both the elastic and photophysical properties, which shows that hydrogen bonding plays a crucial role in the material performance. Temperature-dependent photoluminescence studies show that the light emission is unaffected by the tetragonal-orthorhombic phase transition, but a blueshift in the emission and a steep increase in photoluminescence quantum yield are seen at temperatures below 150 K. Finally, observations of peaks in acoustic loss occurring in ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Pb}{\mathrm{Cl}}_3$ have revealed freezing processes in the vicinity of $\sim 150-170\mathrm{K}$, with activation energies in the range of 300 to 650 meV. These processes are attributed to freezing of the motion of methylammonium cations, and could explain the changes in photoluminescence seen in ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Pb}{\mathrm{I}}_3$ at the same temperature.

Resonant soft x-ray scattering at the carbon $K$ edge is shown to be an unambiguous approach to evaluate the phase segregation between donor and acceptor phases across the grooves of a laser-induced periodic surface nanostructure on a paradigmatic poly(3-hexylthiophene)/fullerene bulk heterojunction. By taking advantage of the use of polarized soft x rays, the results provide direct evidence that the formation of the nanostructure on the surface of the heterojunction induces not only additional phase separation of the two components but also a preferential directional arrangement of the different phases.

In sharp contrast to the well-documented irradiation-hardness property of ZnO, in situ transmission electron microscopy investigations reveal an unexpected nucleation and rotation of Zn clusters in single-crystal ZnO nanosheets (enclosed by the $\left\{0001\right\}$ planes) subjected to the 200-keV electron beam irradiation. Detailed classical molecular dynamics simulations mirror the experimental findings and suggest that the nucleation of Zn clusters is dependent on the surface planes. More importantly, a Schottky barrier may form at the Zn/ZnO interface, whereas the Zn can act as an electron sink and thus may facilitate the $p$-type doping of the ZnO, consistent with the former experimental findings. In addition, the as-nucleated Zn cluster shows a transient hexagonal phase with an expansion in the a-b plane ($a_{\mathrm{Zn}}\sim 2.72\AA$) compared with its bulk counterpart ($a_{\mathrm{Zn}}\sim 2.66\AA$), and exhibits an unreported relationship with ZnO: ${\left[0001\right]}_{\mathrm{Zn}}\parallel {\left[0001\right]}_{\mathrm{ZnO}},(01\overline{1}0{)}_{\mathrm{Zn}}\parallel (1\underline{25}\underline{\overline{26}}0{)}_{\mathrm{ZnO}}$. It is interesting to note that the enlarged value of $a_{\mathrm{Zn}}\sim 2.72\AA$ has been predicted by the theoretical calculation [N. Gaston et al., Phys. Rev. Lett. 100, 226404 (2008)], while never reported in the experiments. Our results contribute to the basic understanding of point defects behavior in ZnO and shed light on controllable structural modification of Zn/ZnO heterostructures for industrial applications.

The high-temperature phase of ${\mathrm{LiBH}}_4$ (HT-${\mathrm{LiBH}}_4$) exhibits a promisingly high lithium ion conductivity but is unstable at room temperature. We use density functional theory (DFT) calculations to investigate the stabilization effect of halogen and alkali cation/anion substitutions on HT-${\mathrm{LiBH}}_4$ as well the underlying mechanism for the high lithium ion conductivity. We find that increasing the substituent concentration enhances the stabilization of HT-${\mathrm{LiBH}}_4$ (i.e., the DFT energy difference between the low- and high-temperature forms of substituted ${\mathrm{LiBH}}_4$ is reduced). Cation/anion substitution also leads to a higher Li defect (vacancy, interstitial, and Frenkel) formation energy, thereby reducing the Li defect (vacancy, interstitial, and Frenkel) concentrations. Using DFT migration barriers input into kinetic Monte Carlo simulations and the Materials INTerface (MINT) framework, we calculate the room-temperature lithium ion conductivities for $\mathrm{Li}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_{1-x}{\mathrm{I}}_x$ ($x=0.25$ and 0.5) and $\mathrm{L}{\mathrm{i}}_{1-y}{\mathrm{K}}_y\mathrm{B}{\mathrm{H}}_4$ ($y=0.25$). Our calculations suggest that the lower I concentration leads to a higher lithium ion conductivity of $5.7\times {10}^{-3}$ S/cm compared with that of $\mathrm{Li}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_{0.5}{\mathrm{I}}_{0.5}$ ($4.2\times {10}^{-5}$ S/cm) because of the formation of more Li-related defects. Based on our findings, we suggest that the stabilization of HT-${\mathrm{LiBH}}_4$-based lithium ion conductors can be controlled by carefully tuning the cation/anion substituent concentrations to maximize the lithium ionic conductivities of the specific system.