We report that in a $\beta$-Mn-type chiral magnet ${\mathrm{Co}}_9{\mathrm{Zn}}_9{\mathrm{Mn}}_2$, skyrmions are realized as a metastable state over a wide temperature range, including room temperature, via field cooling through the thermodynamic equilibrium skyrmion phase that exists below a transition temperature $T_{\mathrm{c}}\sim 400$ K. The once-created metastable skyrmions survive at zero magnetic field both at and above room temperature. Such robust skyrmions in a wide temperature and magnetic field region demonstrate the key role of topology and provide a significant step toward technological applications of skyrmions in bulk chiral magnets.

A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.

High-throughput first-principles computational methods, such as density functional theory (DFT), offer the ability to predict the properties of materials, provided their crystal structures are known. However, there are many compounds for which the structure is unknown and, consequently, many potentially useful materials cannot be assessed by high-throughput DFT searches. Here, we demonstrate an automated tool to solve the structures of materials from powder diffraction patterns based on the first-principles-assisted structure solution (FPASS) method. We validate this tool by using it to solve 95 already-known crystal structures and find that FPASS can determine the correct structure in all cases. We then tuned FPASS to improve its performance on the most-difficult test cases, which include structures with larger numbers of symmetrically unique atoms. We also used FPASS to solve the structures of 10 materials and found, using DFT, several are interesting candidates for semiconductor applications.

Hematite $\left(\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\right)$ is known for poor electronic transport properties, which are the main drawback of this material for optoelectronic applications. In this study, we investigate the concept of enhancing electrical conductivity by the introduction of oxygen vacancies during temperature treatment under low oxygen partial pressure. We demonstrate the possibility of tuning the conductivity continuously by more than five orders of magnitude during stepwise annealing in a moderate temperature range between 300 and 620 K. With thermoelectric power measurements, we are able to attribute the improvement of the electrical conductivity to an enhanced charge-carrier density by more than three orders of magnitude. We compare the oxygen vacancy doping of hematite thin films with hematite nanoparticle layers. Thereby we show that the dominant potential barrier that limits charge transport is either due to grain boundaries in hematite thin films or due to potential barriers that occur at the contact area between the nanoparticles, rather than the potential barrier within the small polaron hopping model, which is usually applied for hematite. Furthermore, we discuss the transition from oxygen-deficient hematite $\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_{3-x}$ towards the magnetite $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ phase of iron oxide at high density of vacancies.

The metal/semiconductor hybrid system Tl/Si(111)-$\left(1\times 1\right)$ exhibits a unique Tl-derived surface state with remarkable properties. It lies within the silicon band gap and forms spin-momentum-locked valleys close to the Fermi energy at the $\overline{K}$ and ${\overline{K}}^{\prime }$ points. These valleys are completely spin polarized with opposite spin orientation at $\overline{K}$ and ${\overline{K}}^{\prime }$ and show a giant spin splitting of more than 0.5 eV. We present a detailed preparation study of the surface system and demonstrate that the electronic valleys are extremely robust, surviving exposure to 100 L hydrogen and 500 L oxygen. We investigate the influence of additional Tl atoms on the spin-polarized valleys. By combining photoemission and inverse photoemission, we prove the existence of fully spin-polarized valleys crossing the Fermi level. Moreover, these metallic valleys carry opposite Berry curvature at $\overline{K}$ and ${\overline{K}}^{\prime }$, very similar to ${\mathrm{WSe}}_2$, promising a large spin Hall effect. Thus, Tl/Si(111)-$\left(1\times 1\right)$ possesses all necessary key properties for spintronic applications.

Single-layer (SL) phosphorus (phosphorene) has drawn considerable research attention recently as a new two-dimensional (2D) material with great application promises. Blue phosphorus (blueP), an allotrope of black phosphorus, has been suggested to exist in the SL form on some substrates. Here, we report the growth of blueP by molecular-beam epitaxy and reveal an explicit sequential growth behavior that involves a dewetting process of the first adsorbed P atoms in the $\left(\sqrt{3}\times \sqrt{3}\right)R30^{\circ }$ patches for the nucleation and growth of SL blueP-like islands. We also reveal a one-dimensional (1D) chain structure at low coverage. Interestingly, over a large coverage range, a composite surface containing locally high-P coverage blueP islands surrounded by locally low-coverage 1D chains prevails over the homogeneous $\left(\sqrt{3}\times \sqrt{3}\right)R30^{\circ }$ or P-trimer covered surfaces. Such a phase-separated composite surface is found to be favorable from an energy-minimization point of view and can be general for the growth of other systems.

Enhanced van der Waals (vdW) epitaxy of semiconductors on a layered vdW substrate is identified as the formation of dative bonds. For example, despite that ${\mathrm{NbSe}}_2$ is a vdW layered material, first-principles calculations reveal that the bond strength at a CdTe-${\mathrm{NbSe}}_2$ interface is five times as large as that of vdW interactions at a CdTe-graphene interface. The unconventional chemistry here is enabled by an effective net electron transfer from Cd dangling-bond states at a CdTe surface to metallic nonbonding ${\mathrm{NbSe}}_2$ states, which is a necessary condition to activate the Cd for enhanced binding with Se.

The crystallographic and magnetic properties of the cleavable $4d^3$ transition metal compound $\alpha \text{−}{\mathrm{MoCl}}_3$ are reported, with a focus on the behavior above room temperature. Crystals were grown by chemical vapor transport and characterized using temperature dependent x-ray diffraction, Raman spectroscopy, and magnetization measurements. A structural phase transition occurs near 585 K, at which the Mo-Mo dimers present at room temperature are broken. A nearly regular honeycomb net of Mo is observed above the transition, and an optical phonon associated with the dimerization instability is identified in the Raman data and in first-principles calculations. The crystals are diamagnetic at room temperature in the dimerized state, and the magnetic susceptibility increases sharply at the structural transition. Moderately strong paramagnetism in the high-temperature structure indicates the presence of local moments on Mo. This is consistent with results of spin-polarized density functional theory calculations using the low- and high-temperature structures. Above the magnetostructural phase transition the magnetic susceptibility continues to increase gradually up to the maximum measurement temperature of 780 K, with a temperature dependence that suggests two-dimensional antiferromagnetic correlations.

Electronic instabilities in transition-metal compounds may lead to ground states containing orbital molecules when direct metal-metal orbital interactions occur. The spinel $\mathrm{Al}{\mathrm{V}}_2{\mathrm{O}}_4$ was reported to contain ${\mathrm{V}}_7^{17+}$ orbital heptamers that emerge below a 700 K charge ordering transition. Our x-ray total scattering analysis of $\mathrm{Al}{\mathrm{V}}_2{\mathrm{O}}_4$ between 300 and 1100 K reveals a very different picture as the postulated heptamers are found to be pairs of spin-singlet ${\mathrm{V}}_3^{9+}$ trimers and ${\mathrm{V}}_4^{8+}$ tetramers, and these orbital molecules persist to at least 1100 K in a disordered high-temperature cubic phase.

Two improvements to the binary structural phase field crystal (XPFC) theory are presented. The first is an improvement to the phenomenology for modelling density-density correlation functions and the second extends the free energy of the mixing term in the binary XPFC model beyond ideal mixing to a regular solution model. These improvements are applied to study kinetics of precipitation from solution. We observe a two-step nucleation pathway similar to recent experimental work [N. D. Loh, S. Sen, M. Bosman, S. F. Tan, J. Zhong, C. A. Nijhuis, P. Král, P. Matsudaira, and U. Mirsaidov, Nat. Chem. 9, 77 (2017); A. F. Wallace, L. O. Hedges, A. Fernandez-Martinez, P. Raiteri, J. D. Gale, G. A. Waychunas, S. Whitelam, J. F. Banfield, and J. J. De Yoreo, Science 341, 885 (2013)] in which the liquid solution first decomposes into solute-poor and solute-rich regions, followed by precipitate nucleation of the solute-rich regions. Additionally, we find a phenomenon not previously described in the literature in which the growth of precipitates is accelerated in the presence of uncrystallized solute-rich liquid regions.

Technological developments in high-resolution time-of-flight neutron detectors have raised the prospect of tomographic reconstruction of elastic strain fields from Bragg-edge strain images. This approach holds the potential to provide a unique window into the full triaxial stress field within solid samples. Recent work has provided an algorithm to achieve tomographic reconstruction for systems subject to in situ loads in the absence of residual stress. This paper details an experimental proof of concept for this algorithm involving the full reconstruction of a biaxial strain field within a nontrivial steel sample. Aside from a small systematic error present within individual measurements, the experiment was a success and now serves as a practical demonstration of Bragg-edge transmission strain tomography for systems under in situ loads.

A quantum spin liquid might be realized in $\alpha -{\mathrm{RuCl}}_3$, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, $\alpha -{\mathrm{RuCl}}_3$ is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spectroscopy, and density functional theory calculations. We obtain a stable stoichiometry at ${\mathrm{K}}_{0.5}{\mathrm{RuCl}}_3$. This gives rise to a peculiar charge disproportionation into formally ${\mathrm{Ru}}^{2+}$ ($4d^6$) and ${\mathrm{Ru}}^{3+}$ ($4d^5$). Every Ru $4d^5$ site with one hole in the $t_{2g}$ shell is surrounded by nearest neighbors of $4d^6$ character, where the $t_{2g}$ level is full and magnetically inert. Thus, each type of Ru site forms a triangular lattice, and nearest-neighbor interactions of the original honeycomb are blocked.

Semicrystalline polyethylene (PE) is attractive for a variety of mechanically demanding applications, where shock compression can occur. Although often highly crystalline, PE invariably contains nanoscale amorphous domains that influence shock propagation. Our objective in this work is to study the effects of such domains. To this end, we adopt a novel approach wherein we parametrize a simple continuum-level theory based on the shock impedance from molecular dynamics (MD) simulations. Using this theory, we predict how crystalline/amorphous interfaces attenuate shocks via energy reflection due to the impedance mismatch between the phases. The theory predicts that these interfaces attenuate weak shocks more effectively than strong shocks. We compare the theory to explicit nonequilibrium MD simulations of compressive shocks in semicrystalline PE containing nanometer-scale amorphous regions of varying size, where we analyze the pressure response and reflection of energy. The theory and simulations show good agreement for strong shocks $\left(\ge 1.0\mathrm{km}/\mathrm{s}\right)$, but for weak shocks $\left(<1.0\mathrm{km}/\mathrm{s}\right)$ the simulations show enhanced energy reflection relative to the continuum predictions. Furthermore, the simulations show an effect not captured by the continuum theory: the size of amorphous regions is important. The theory assumes a sharp (discontinuous) interface between two bulk phases and a sharp change in thermodynamic and hydrodynamic quantities at the shock front. However, the simulations show that when amorphous domains are narrow—with widths comparable to the shock front—reflection is reduced compared to the predictions. We identify several nanoscale mechanisms that reduce the impedance mismatch, and thus reduce reflection, at thin amorphous domains. First, the two-wave elastic-plastic structure of shocks in crystalline PE allows the faster-moving elastic precursor wave to compress small amorphous domains before the plastic wave arrives. Second, confinement between stiff, ordered crystalline domains increases the stiffness and chain ordering in small amorphous regions. Moreover, in terms of stiffness the interfaces are similar in width to the shock front, which may contribute to the underprediction of the theory for weak shocks, where the shock front is widest. We conclude by discussing the significance of these results, namely, how they can be applied to tune shock attenuation for particular applications.

Organic-inorganic coupling in the hybrid lead-halide perovskite is a central issue in rationalizing the outstanding photovoltaic performance of these emerging materials. Here, we compare and contrast the evolution of the structure and dynamics of hybrid ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ and inorganic ${\mathrm{CsPbBr}}_3$ lead-halide perovskites with temperature, using Raman spectroscopy and single-crystal x-ray diffraction. Results reveal a stark contrast between their order-disorder transitions, which are abrupt for the hybrid whereas smooth for the inorganic perovskite. X-ray diffraction observes an intermediate incommensurate phase between the ordered and the disordered phases in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$. Low-frequency Raman scattering captures the appearance of a sharp soft mode in the incommensurate phase, ascribed to the theoretically predicted amplitudon mode. Our work highlights the interaction between the structural dynamics of organic cation ${\mathrm{CH}}_3{\mathrm{NH}}_3{}^{+}$ and the lead-halide framework, and unravels the competition between tendencies for the organic and inorganic moieties to minimize energy in the incommensurate phase of the hybrid perovskite structure.

Results are reported for the $f$-electron intermetallic ${\mathrm{CeAuAl}}_4{\mathrm{Ge}}_2$, where the atomic arrangement of the cerium ions creates the conditions for possible geometric frustration. The magnetic susceptibility follows a Curie-Weiss temperature dependence at elevated temperatures, revealing that the cerium ions are trivalent. At lower temperatures the crystal electric field splits the Hund's rule multiplet, resulting in a weak low-temperature magnetic exchange interaction and ordering near $T_M\approx 1.4\mathrm{K}$. This occurs within a metallic Kondo lattice, where electrical resistivity and heat capacity measurements show that the Kondo-driven electronic correlations are negligible. Quantum oscillations are detected in ac-magnetic susceptibility measurements and uncover small charge carrier effective masses. Electronic structure calculations reveal that inclusion of an on-$f$-site Coulomb repulsion (Hubbard) $U$ results in antiferromagnetic order and causes the $f$-electron bands to move away from the Fermi level, resulting in electronic behavior that is dominated by the $s,p$, and $d$ bands, which are all characterized by light electron masses. Thus, ${\mathrm{CeAuAl}}_4{\mathrm{Ge}}_2$ may provide a starting point for investigating geometric magnetic frustration in a cerium lattice without strong Kondo hybridization, where calculations provide useful guidance.

We discuss a parameter-free and computationally efficient ab initio simulation approach for moderately and strongly correlated materials, the multitier self-consistent $GW+\text{EDMFT}$ method. This scheme treats different degrees of freedom, such as high-energy and low-energy bands, or local and nonlocal interactions, within appropriate levels of approximation, and provides a fully self-consistent description of correlation and screening effects in the solid. The ab initio input is provided by a one-shot $G^0{W}^0$ calculation, while the strong-correlation effects originating from narrow bands near the Fermi level are captured by a combined $GW$ plus extended dynamical mean-field (EDMFT) treatment. We present the formalism and technical details of our implementation and discuss some general properties of the effective EDMFT impurity action. In particular, we show that the retarded impurity interactions can have noncausal features, while the physical observables, such as the screened interactions of the lattice system, remain causal. As a first application, we present ab initio simulation results for ${\mathrm{SrMoO}}_3$, which demonstrate the existence of prominent plasmon satellites in the spectral function not obtainable within LDA+DMFT, and provide further support for our recent reinterpretation of the satellite features in the related cubic perovskite ${\mathrm{SrVO}}_3$. We then turn to stretched sodium as a model system to explore the performance of the multitier self-consistent $GW+\text{EDMFT}$ method in situations with different degrees of correlation. While the results for the physical lattice spacing $a_0$ show that the scheme is not very accurate for electron-gas-like systems, because nonlocal corrections beyond $GW$ are important, it does provide physically correct results in the intermediate correlation regime, and a Mott transition around a lattice spacing of $1.5a_0$. Remarkably, even though the Wannier functions in the stretched compound are less localized, and hence the bare interaction parameters are reduced, the self-consistently computed impurity interactions show the physically expected trend of an increasing interaction strength with increasing lattice spacing.

Spacer thickness $t_{\mathrm{N}}$ and annealing temperature $T_{\mathrm{anneal}}$ dependence of current perpendicular-to-plane giant magnetoresistance effects were investigated in junctions using $L{1}_2{\mathrm{Ag}}_3\mathrm{Mg}$ spacer and half-metallic ${\mathrm{Co}}_2{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ (CFMS) Heusler alloy electrodes. $t_{\mathrm{N}}$ was changed from 2–12 nm and $T_{\mathrm{anneal}}$ was changed from $450-650{}^{\circ }\mathrm{C}$ to promote the chemical ordering of the CFMS electrodes. Concerning the $t_{\mathrm{N}}$ dependence, the magnetoresistance (MR) ratio and the change of the areal resistance $\left(\mathrm{\Delta }RA\right)$ exhibited the maximum values at $t_{\mathrm{N}}=5\mathrm{nm}$. The reasons for the decrease of the MR ratio for $t_{\mathrm{N}}<5$ nm and for $t_{\mathrm{N}}>5$ nm were possibly due to an unstable antiparallel magnetization configuration and to the increased occurrence of spin scattering inside the ${\mathrm{Ag}}_3\mathrm{Mg}$ spacer, respectively. The spin-diffusion length of the ${\mathrm{Ag}}_3\mathrm{Mg}$ spacer was also estimated using the $t_{\mathrm{N}}$ dependence of $\mathrm{\Delta }RA$ and it was found to be of the order of 10–30 nm. Concerning the $T_{\mathrm{anneal}}$ dependence, $\mathrm{\Delta }RA$ and the MR ratio exhibited the maximum values at $550{}^{\circ }\mathrm{C}$, which was the optimum point in terms of the degrees of order in the CFMS layers and the ${\mathrm{Ag}}_3\mathrm{Mg}$ layer, and the (001) orientation of the layered structure. The maximum $\mathrm{\Delta }RA$ and the intrinsic MR ratio in which parasitic resistance contribution was eliminated were $25\mathrm{m}\mathrm{\Omega }\mu {\mathrm{m}}^2$ and 63%, respectively, at room temperature.

We investigate the mechanical response of a compressed monolayer of large and dense particles at a liquid-fluid interface: a granular raft. Upon compression, rafts first wrinkle; then, as the confinement increases, the deformation localizes in a unique fold. This characteristic buckling pattern is usually associated with floating elastic sheets, and as a result, particle laden interfaces are often modeled as such. Here, we push this analogy to its limits by comparing quantitative measurements of the raft morphology to a theoretical continuous elastic model of the interface. We show that, although powerful to describe the wrinkle wavelength, the wrinkle-to-fold transition, and the fold shape, this elastic description does not capture the finer details of the experiment. We describe an unpredicted secondary wavelength, a compression discrepancy with the model, and a hysteretic behavior during compression cycles, all of which are a signature of the intrinsic discrete and frictional nature of granular rafts. It suggests also that these composite materials exhibit both plastic transition and jamming dynamics.

We examine the step instability observed in Fe growth on a Cu(100) vicinal substrate in which deposition of a relatively low coverage of Fe leads to a dramatic increase in the step roughness, with a change in the step morphology from relatively straight [100] steps to a mixture of [100] and [110] steps. Our temperature-accelerated dynamics (TAD) simulations and energetics calculations indicate that the dramatic change in the step morphology is due to a variety of unexpected complex multiatom interlayer diffusion (MID) processes near step edges which lead to a competition between [100] and [110] steps, whose barriers are significantly reduced due to the existence of strong Fe-Fe and Fe-Cu interactions as well as strain effects. In contrast, TAD simulations of vicinal Cu/Cu(100) growth do not lead to an instability, in good agreement with experiments. Our results also indicate that while the instability is driven by energetics, kinetics plays a crucial role. We also present the results of TAD simulations and energetics calculations for the case of Fe/Cu(100) growth on vicinal surfaces with [110] steps. Our results in this case indicate that a variety of effects, including MID processes as well as pinning of edge-diffusing atoms, lead to the formation of protrusions with [110] steps perpendicular to the original [110] steps. These results may also provide an explanation for the experimentally observed instabilities observed in growth on vicinal Ni/Cu(100) and Co/Cu(100) surfaces with [110] steps.

The electronic and optoelectronic properties of molecular semiconductor thin films are directly linked to their extrinsic nanoscale structural characteristics such as domain size and spatial distributions. In films prepared by common solution-phase deposition techniques such as spin casting and solvent-based printing, morphology is governed by a complex interrelated set of thermodynamic and kinetic factors that classical models fail to adequately capture, leaving them unable to provide much insight, let alone predictive design guidance for tailoring films with specific nanostructural characteristics. Here we introduce a comprehensive treatment of solution-based film formation enabling quantitative prediction of domain formation rates, coverage, and spacing statistics based on a small number of experimentally measureable parameters. The model combines a mean-field rate equation treatment of monomer aggregation kinetics with classical nucleation theory and a supersaturation-dependent critical nucleus size to solve for the quasi-two-dimensional temporally and spatially varying monomer concentration, nucleation rate, and other properties. Excellent agreement is observed with measured nucleation densities and interdomain radial distribution functions in polycrystalline tetracene films. Numerical solutions lead to a set of general design rules enabling predictive morphological control in solution-processed molecular crystalline films.