The role of the sublimation of the compound and of the evaporation of the constituents from the gold nanoparticle during the growth of semiconductor nanowires is exemplified with CdTe-ZnTe heterostructures. Operating close to the upper temperature limit strongly reduces the amount of Cd present in the gold nanoparticle and the density of adatoms on the nanowire sidewalls. As a result, the growth rate is small and strongly temperature dependent, but a good control of the growth conditions allows the incorporation of quantum dots in nanowires with sharp interfaces and adjustable shape, and it minimizes the radial growth and the subsequent formation of additional CdTe clusters on the nanowire sidewalls, as confirmed by photoluminescence. Uncapped CdTe segments dissolve into the gold nanoparticle when interrupting the flux, giving rise to a bulblike (pendant-droplet) shape attributed to the Kirkendall effect.

The adsorption of Ir adatoms on Al(100) has been investigated under various exposures and temperature conditions. The experimental and theoretical results reveal a diffusion of Ir adatoms within the Al(100) surface selvedge already at 300 K. Above 593 K, two domains of a $(\sqrt{5}\times \sqrt{5})R26.6^{\circ }$ phase are identified by low energy electron diffraction (LEED) and scanning tunneling microscopy measurements. This phase corresponds to the initial growth of an ${\mathrm{Al}}_9{\mathrm{Ir}}_2$ compound at the Al(100) surface. The ${\mathrm{Al}}_9{\mathrm{Ir}}_2$ intermetallic domains are terminated by bulklike pure Al layers. The structural stability of ${\mathrm{Al}}_9{\mathrm{Ir}}_2$(001) grown on Al(100) has been analyzed by density functional theory based calculations. Dynamical LEED analysis is consistent with an Ir adsorption leading to the growth of an ${\mathrm{Al}}_9{\mathrm{Ir}}_2$ intermetallic compound. We propose that the epitaxial relationship ${\mathrm{Al}}_9{\mathrm{Ir}}_2\left(001\right)\parallel \mathrm{Al}$(100) and ${\mathrm{Al}}_9{\mathrm{Ir}}_2\left[100\right]\parallel \mathrm{Al}\left[031\right]/\left[013\right]$ originates from a matching of Al atomic arrangements present both on Al(100) and on pure Al(001) layers present in the ${\mathrm{Al}}_9{\mathrm{Ir}}_2$ compound. Finally, the interface between ${\mathrm{Al}}_9{\mathrm{Ir}}_2$ precipitates and the Al matrix has been characterized by transmission electron microscopy measurements. The cross-sectional observations are consistent with the formation of ${\mathrm{Al}}_9{\mathrm{Ir}}_2$(001) compounds. These measurements indicate an important Ir diffusion within Al(100) near the surface region. The coherent interface between ${\mathrm{Al}}_9{\mathrm{Ir}}_2$ and the Al matrix is sharp.

Different from other magnetically frustrated spinel systems, ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ is a strongly tetragonal distorted spinel cuprate in which edge-sharing ${\mathrm{CuO}}_2$ ribbons are running along alternating directions perpendicular to the $c$ axis. Here, ${\mathrm{GeCu}}_2{\mathrm{O}}_4$ samples of high quality were prepared via high-pressure synthesis (at 4 GPa) and the corresponding magnetic and dielectric properties were investigated. We observed a ferroelectric polarization emerging at $T_N\sim 33$ K. Although the ferroelectric polarization is weak in ${\mathrm{GeCu}}_2{\mathrm{O}}_4\left(P\sim 0.2\mu \mathrm{C}/{\mathrm{m}}^2\right)$, the existence of spin-induced multiferroicity provides a strong constraint on the possible ground-state magnetic structures and/or the corresponding theoretical models of multiferroicity for ${\mathrm{GeCu}}_2{\mathrm{O}}_4$.

We report the successful bulk single-crystal growth of the hyperkagome lattice iridate $\mathrm{N}{\mathrm{a}}_4\mathrm{I}{\mathrm{r}}_3{\mathrm{O}}_8$ (Na438) by vapor transport using a sealed aluminum oxide tube as a container. Crystals were characterized by magnetization, x-ray diffraction, and energy-dispersive x-ray measurements, confirming their identity and properties. Single-crystal x-ray diffraction experiments revealed superlattice peaks indexed on a propagation vector $q=(1/3,1/3,1/3)$ based on the cubic substructure with cell parameter $a=8.986\left(1\right)\AA$. This superlattice is three-dimensional and fully coherent. Polarization analysis rules out spin and/or orbital order as the underlying origin of the modulation and points to long-range ordering of Na ions at the notionally disordered Na sites as a plausible origin for the observed superlattice.

We report molecular dynamics simulations and their analysis for a mixed tilt and twist grain boundary vicinal to the $\mathrm{\Sigma }7$ symmetric tilt boundary of the type $\left\{123\right\}$ in aluminum. When minimized in energy at $0\mathrm{K}$, a grain boundary of this type exhibits nanofacets that contain kinks. We observe that at higher temperatures of migration simulations, given extended annealing times, it is energetically favorable for these nanofacets to coalesce into a large terrace-facet structure. Therefore, we initiate the simulations from such a structure and study as a function of applied driving force and temperature how the boundary migrates. We find the migration of a faceted boundary can be described in terms of the flow of steps. The migration is dominated at lower driving force by the collective motion of the steps incorporated in the facet, and at higher driving forces by the step detachment from the terrace-facet junction and propagation of steps across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in the facet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics. Since the mechanisms seem likely to be very general on kinked grain-boundary planes, the step-flow description is a promising approach to more quantitative modeling of general grain boundaries.

The structure factors of molten $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ and $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$ have been measured by high-energy x-ray and pulsed neutron diffraction. These are compared to transmission-mode x-ray diffraction through a self-supported 15-μm ion-beam sputtered amorphous tantala film. Atomistic models derived from the diffraction data by means of empirical potential structure refinement reveal that tantala and niobia liquids are very close to isomorphous, as confirmed by measurement of a molten mixture, $\mathrm{T}{\mathrm{a}}_{0.8}\mathrm{N}{\mathrm{b}}_{1.2}{\mathrm{O}}_5$. Nonetheless, peak Nb-O bond lengths are about $1\%$ shorter than those for Ta-O, at temperatures, $T^{*}=T/T_{\mathrm{melt}}$, scaled to the melting points. Mean coordination numbers are $n_{M\mathrm{O}}\simeq 5.6\left(1\right),n_{\mathrm{O}M}\simeq 2.23\left(4\right)$ in the liquid state, and $n_{\mathrm{TaO}}\simeq 6.6\left(2\right),n_{\mathrm{OTa}}\simeq 2.63\left(8\right)$ in the solid. The liquids are built from five- and six-fold $M$-O polyhedra which connect principally by corner sharing, with a minority of edge sharing; a-$\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ on the other hand has a local structure more akin to the crystalline polymorphs, built primarily from six- and seven-fold polyhedra, with a larger degree of edge sharing. The structural differences between liquid and amorphous $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$, coupled with observations of increasing peak bond lengths upon cooling, are consistent with the interpretation that the amorphous film reaches a supercooled liquidlike metastable equilibrium during deposition. In other words, the amorphous film shares a common progenitor state with a hypothetical glass quenched from a fragile melt. In addition, we show that recent classical interatomic potentials do not fully reproduce the diffraction data, and infer that inclusion of attractive (non-Coulombic) Ta-Ta interactions is important, particularly for obtaining the correct degree of edge sharing, coordination numbers, and densities. Nanoscale inhomogeneity of the amorphous film is confirmed by the observation of small-angle x-ray scattering.

An important challenge in the field of topological materials is to carefully disentangle the electronic transport contribution of the topological surface states from that of the bulk. For ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ topological insulator samples, bulk single crystals and thin films exposed to air during fabrication processes are known to be bulk conducting, with the chemical potential in the bulk conduction band. For ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films grown by molecular beam epitaxy, we combine structural characterization (transmission electron microscopy), chemical surface analysis as function of time (x-ray photoelectron spectroscopy) and magnetotransport analysis to understand the low defect density and record high bulk electron mobility once charge is doped into the bulk by surface degradation. Carrier densities and electronic mobilities extracted from the Hall effect and the quantum oscillations are consistent and reveal a large bulk carrier mobility. Because of the cylindrical shape of the bulk Fermi surface, the angle dependence of the bulk magnetoresistance oscillations is two dimensional in nature.

We present a comprehensive study of the magnetic properties of ${\mathrm{Sr}}_{3-x}{\mathrm{Y}}_x\left({\mathrm{Fe}}_{1.25}{\mathrm{Ni}}_{0.75}\right){\mathrm{O}}_{7-\delta }\left(0\le x\le 0.75\right)$. Experimentally, the magnetic properties are investigated using superconducting quantum interference device (SQUID) magnetometry and neutron powder diffraction (NPD). This is complemented by a theoretical study based on density functional theory as well as the Heisenberg exchange parameters. Experimental results show an increase in the Néel temperature $\left(T_N\right)$ with an increase of Y concentrations and O occupancy. The NPD data reveal that all samples are antiferromagnetically ordered at low temperatures, which has been confirmed by our theoretical simulations for the selected samples. Our first-principles calculations suggest that the three-dimensional magnetic order is stabilized due to finite interlayer exchange couplings. The latter give rise to finite interlayer spin-spin correlations, which disappear above $T_N$.

The effects of anneal-tuning of single-crystalline samples of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ synthesized via a molten Sn-flux method are investigated using x-ray diffraction, chemical composition, electrical transport, and magnetic susceptibility measurements in order to understand the role of growth conditions on the resultant phase diagram. Previous studies of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ crystals synthesized using a self-flux (FeAs) method revealed an ability to tune the structural and magnetic properties of this system by control of post-synthesis annealing conditions, resulting in an ambient pressure phase diagram that spans from tetragonal/orthorhombic antiferromagnetism to the collapsed tetragonal phase of this system. In this work, we compare previous results to those obtained on crystals synthesized via Sn flux, finding similar tunability in both self- and Sn-flux cases, but less sensitivity to annealing temperatures in the latter case, resulting in a temperature-shifted phase diagram.

Electronic and structural properties of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and its extension to copper doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3:\mathrm{Cu}$ were studied using combined ab initio simulations and transmission electron microscopy based techniques, including electron energy loss spectroscopy, energy filtered transmission electron microscopy, and energy dispersive x-ray spectroscopy. The stability of the mixed phases was investigated for substitutional and intercalation changes of basic $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ structure. Four systems were compared: $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, structures obtaining by Cu intercalation of the van der Waals gap, by substitution of Bi by Cu in quintuple layers, and $\mathrm{C}{\mathrm{u}}_2\mathrm{Se}$. The structures were identified and their electronic properties were obtained. Transmission electron microscopy measurements of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ and the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3:\mathrm{Cu}$ system identified the first structure as uniform and the second as composite, consisting of a nonuniform lower-Cu-content matrix and randomly distributed high-Cu-concentration precipitates. Critical comparison of the ab initio and experimental data identified the matrix as having a $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ dominant part with randomly distributed Cu-intercalated regions having 1Cu-$\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ structure. The precipitates were determined to have 3Cu-$\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ structure.

We report experimental and theoretical evidence for the formation of chiral bobbers—an interfacial topological spin texture—in FeGe films grown by molecular beam epitaxy. After establishing the presence of skyrmions in FeGe/Si(111) thin-film samples through Lorentz transmission electron microscopy and the topological Hall effect, we perform magnetization measurements that reveal an inverse relationship between the film thickness and the slope of the susceptibility $\left(d\chi /dH\right)$. We present evidence for the evolution as a function of film thickness $L$ from a skyrmion phase for $L<L_{\mathrm{D}}/2$ to a cone phase with chiral bobbers at the interface for $L>L_{\mathrm{D}}/2$, where $L_{\mathrm{D}}\sim 70\mathrm{nm}$ is the FeGe pitch length. We show using micromagnetic simulations that chiral bobbers, earlier predicted to be metastable, are in fact the stable ground state in the presence of an additional interfacial Rashba Dzyaloshinskii-Moriya interaction.

The rich polymorphism exhibited by inorganic/organic interfaces is a major challenge for materials design. In this work, we present a method to efficiently explore the potential energy surface and predict the formation energies of polymorphs and defects. This is achieved by training a machine learning model on a list of only 100 candidate structures that are evaluated via dispersion-corrected density functional theory (DFT) calculations. We demonstrate the power of this approach for tetracyanoethylene on Ag(100) and explain the anisotropic ordering that is observed experimentally.

We report the observation of room-temperature magnetic skyrmions in Pt/Co/Os/Pt thin-film heterostructures and their response to electric currents. The magnetic properties are extremely sensitive to inserting thin Os layers between the Co-Pt interface, resulting in reduced saturation magnetization, magnetic anisotropy, and Curie temperature. The observed skyrmions exist in a narrow temperature, applied-field and layer-thickness range near the spin-reorientation transition from perpendicular to in-plane magnetic anisotropy. The skyrmions have an average diameter of $2.3\mu \mathrm{m}$ and transport measurements demonstrate these features can be displaced by means of spin-orbit torques with current densities as low as $J=2\times {10}^8\mathrm{A}/{\mathrm{m}}^2$ and display a skyrmion Hall effect.

The development of thermoelectric devices faces not only the challenge of optimizing the Seebeck coefficient, the electrical and thermal conductivity of the active material, but also further bottlenecks when going from the thermoelectric material to an actual device, e.g., the dopant diffusion at the hot contact. We show that for large bandgap thermoelectrics another aspect can dramatically reduce the efficiency of the device: the formation of Schottky barriers. Understanding the effect, it can then be fixed rather cheaply by a two-metal contact solution.

The (Li,Al)-codoped magnesium spinel $\left({\mathrm{Li}}_x{\mathrm{Mg}}_{1-2x}{\mathrm{Al}}_{2+x}{\mathrm{O}}_4\right)$ is a solid lithium-ion electrolyte with potential use in all-solid-state lithium-ion batteries. The spinel structure means that interfaces with spinel electrodes, such as ${\mathrm{Li}}_y{\mathrm{Mn}}_2{\mathrm{O}}_4$ and ${\mathrm{Li}}_{4+3z}{\mathrm{Ti}}_5{\mathrm{O}}_{12}$, may be lattice matched, with potentially low interfacial resistances. Small lattice parameter differences across a lattice-matched interface are unavoidable, causing residual epitaxial strain. This strain potentially modifies lithium diffusion near the electrolyte-electrode interface, contributing to interfacial resistance. Here, we report a density functional theory study of strain effects on lithium diffusion pathways for (Li,Al)-codoped magnesium spinel, for $x_{\mathrm{Li}}=0.25$ and $x_{\mathrm{Li}}=0.5$. We have calculated diffusion profiles for the unstrained materials, and for isotropic and biaxial tensile strains of up to $6\%$, corresponding to $\left\{100\right\}$ epitaxial interfaces with ${\mathrm{Li}}_y{\mathrm{Mn}}_2{\mathrm{O}}_4$ and ${\mathrm{Li}}_{4+3z}{\mathrm{Ti}}_5{\mathrm{O}}_{12}$. We find that isotropic tensile strain reduces lithium diffusion barriers by as much as $0.32\mathrm{eV}$, with typical barriers reduced by $\sim 0.1$ eV. This effect is associated with increased volumes of transitional octahedral sites, and broadly follows qualitative changes in local electrostatic potentials. For biaxial (epitaxial) strain, which more closely approximates strain at a lattice-matched electrolyte-electrode interface, changes in octahedral site volumes and in lithium diffusion barriers are much smaller than under isotropic strain. Typical barriers are reduced by only $\sim 0.05$ eV. Individual effects, however, depend on the pathway considered and the relative strain orientation. These results predict that isotropic strain strongly affects ionic conductivities in (Li,Al)-codoped magnesium spinel electrolytes, and that tensile strain is a potential route to enhanced lithium transport. For a lattice-matched interface with candidate spinel-structured electrodes, however, epitaxial strain has a small, but complex, effect on lithium diffusion barriers.

This paper focuses on effects of isovalent La substitution on the crystal structure and dielectric properties of ferroelectric $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$. Polycrystalline samples of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ are synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses with a composition range of $0\le x\le 0.1$. The crystal structure changes from monoclinic to tetragonal with increasing La-substitution rate $x$ at room temperature. This structural variation stems from the change in orientation of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra that form one-dimensional chains when they are in the ordered configuration, thus suggesting that lone-pair electrons play an important role in sustaining one-dimensional chains of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra. Synchronizing with the disordering of $\mathrm{Si}{\mathrm{O}}_4$ chains, ferroelectric phase transition temperature of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ sharply decreases as $x$ increases, and ferroelectricity finally vanishes at around $x=0.03$. The present results demonstrate that lone-pair electrons of Bi play an important role in the ferroelectricity of $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$ through propping the ordered structure of one-dimensional $\mathrm{Si}{\mathrm{O}}_4$ chains with stereochemical activity. Furthermore, an additional phase transition has been first discovered in the low-temperature region of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ with $x\le 0.01$, where the ordered one-dimensional $\mathrm{Si}{\mathrm{O}}_4$ chains remain.

We evaluate the accuracy of electron densities and quasiparticle energy gaps given by hybrid functionals by directly comparing these to the exact quantities obtained from solving the many-electron Schrödinger equation. We determine the admixture of Hartree-Fock exchange to approximate exchange-correlation in our hybrid functional via one of several physically justified constraints, including the generalized Koopmans' theorem. We find that hybrid functionals yield strikingly accurate electron densities and gaps in both exchange-dominated and correlated systems. We also discuss the role of the screened Fock operator in the success of hybrid functionals.