${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT) materials are a promising class of antiferromagnetic topological insulators whose films provide access to novel and technologically important topological phases, including quantum anomalous Hall states and axion insulators. MBT device behavior is expected to be sensitive to the various collinear and noncollinear magnetic phases that are accessible in applied magnetic fields. Here, we use classical Monte Carlo simulations and electronic structure models to calculate the ground state magnetic phase diagram as well as topological and optical properties for few-layer films with up to six septuple layers. Using magnetic interaction parameters appropriate for MBT, we find that it is possible to prepare a variety of different magnetic stacking sequences, some of which have sufficient symmetry to disallow nonreciprocal optical response and Hall transport coefficients. Other stacking arrangements do yield large Faraday and Kerr signals, even when the ground state Chern number vanishes.

Properties of soft materials are influenced by their anisotropic structuring under nonequilibrium fields. Although anisotropic structure-property relationships have been extensively explored theoretically, comparison to experiments requires determination of the microstructural orientation probability distribution function (OPDF) of microstructural elements. Small angle scattering (SAS) measurements encode information about the OPDF, but tools to navigate this connection are incomplete. Here, we develop and validate an explicit framework to link arbitrary OPDFs to SAS measurements. Specifically, we propose, validate, and apply a method, maximum a posteriori scattering inference (MAPSI), whereby the OPDF may be obtained from SAS measurements using a Bayesian estimation method. Using this method, we obtain estimates of the full 3D OPDF for two model semidilute fd-virus (rodlike) dispersions at concentrations that are approximately equal to and twice the overlap concentration. From the OPDF, we calculate its second and fourth moments and compare these to predictions for a dilute suspension of rigid rods and to a recent theory for semidilute suspensions. Finally, we use both the theoretical and measured moments to calculate the stress, both for dilute and semidilute suspensions. These predictions are not only compared to each other, but also to measured values of the shear stress, and point to new insights into the behavior of suspensions of highly elongated particles in the transition between dilute and semidilute behavior. We also use this new framework to provide perspective on the connection between scalar parameterizations of scattering and the OPDF that have frequently been used in the past. The new tools developed in this work provide an unprecedented path toward experimental validation of dynamical theories of rodlike colloids and polymers, and for measurement of nonequilibrium structures and stresses of other complex fluids and soft materials with SAS.

We determine the interaction between an $1/2\langle 110\rangle \left\{\overline{1}10\right\}$ edge dislocation and charged vacancies in MgO, using both molecular static simulations and the elasticity theory developed in the framework of the elastic dipole approach. In this study, the confrontation of these two methods highlights the specific role of the dislocation core structure on the interaction. We thus show that in MgO, the edge dislocation core, within a region across the glide planes that expands over several Burgers vector, is strongly attractive for vacancies, especially those of oxygen. However, the resulting pinning force on the dislocation remains weak and should not contribute to a significant hardening.

We measure spin-orbit torques (SOTs) in a model system of all-epitaxial ferrite/Pt bilayers to gain insights into charge-spin interconversion in Pt. With negligible electronic conduction in the insulating ferrite, the crystalline Pt film acts as the sole source of charge-to-spin conversion. A small fieldlike SOT independent of Pt thickness suggests a weak Rashba-Edelstein effect at the ferrite/Pt interface. By contrast, we observe a sizable dampinglike SOT that depends on the Pt thickness, from which we deduce the dominance of an extrinsic spin-Hall effect (skew scattering) and Dyakonov-Perel spin relaxation in the crystalline Pt film. Furthermore, our results point to a large internal spin-Hall ratio of $\approx 0.8$ in epitaxial Pt. Our experimental work takes an essential step towards understanding the mechanisms of charge-spin interconversion and SOTs in Pt-based heterostructures, which are crucial for power-efficient spintronic devices.

Deformations of amorphous polymer networks prepared with significant concentrations of liquid crystalline mesogens have been recently reported to undergo mechanotropic phase transitions. Here, we report that mechanotropic phase transitions are accompanied by an elastocaloric response ($\mathrm{\Delta }T=2.9$ K). Applied uniaxial strain to the elastomeric polymer network transitions the organization of the material from a disordered, amorphous state (order parameter $Q=0$) to the nematic phase ($Q=0.47$). Both the magnitude of the elastocaloric temperature change and the mechanically induced order parameter are dependent on the concentration of liquid crystal mesogens in the material.

Problems associated with large-scale growth of high-quality single-layer bismuthene constitute one of the main obstacles to the applications of bismuthene in electronic devices. Here we report the direct synthesis of high-quality bismuthene using molecular-beam epitaxy on dielectric copper oxide thin films. Scanning tunneling microscopy (STM) reveals the generation of dimer pair arrays and black-phosphorus-like bismuthene (BKP-Bi, α phase) at the initial stage on ${\mathrm{Cu}}_3{\mathrm{O}}_2/\mathrm{Cu}\left(111\right)$ surface. A phase transition from BKP-Bi to blue-phosphorus-like bismuthene (BLP-Bi, β phase) is characterized by high-resolution STM, complemented with first-principles calculations based on density-functional theory (DFT) showing a bond-breaking path. Employing the phase transition, we also demonstrate the construction of an atomically sharp in-plane homojunction along the phase boundary. DFT calculations reveal a one-dimensional edge state along the homojunction.

Polycrystalline thin-film solar cells are attractive for low-cost photovoltaics, but their efficiencies are hindered by material quality issues. State-of-the-art CdTe solar cells use ${\mathrm{CdCl}}_2$ annealing treatments whose effects are still being discovered at a fundamental level. Here, a series of CdTe samples with different annealing temperatures is investigated with high-resolution hyperspectral cathodoluminescence mapping measured at both room temperature and low temperature on the same microscopic areas. A statistical analysis over a large number of grains is combined with a local analysis at grain boundaries. The results elucidate the dynamic interplay between grain boundary and intragrain defect passivation and formation, in the midst of grain growth. The ${\mathrm{CdCl}}_2$ annealing initially contributes to an increase of the grain size and the passivation of both grain boundaries and grain interiors, increasing the overall luminescence and diffusion length. For higher annealing temperatures, a further increase of grain size is counterbalanced by the rise of bulk defects. The results illustrate the tradeoffs that lead to an optimal annealing temperature, as well as new methods for understanding defect passivation and creation in thin film solar cells.

The natural conductivity of as-grown ZnO is $n$-type. It has been challenging to produce stable $p$-type material, which has delayed possible technological applications. In this work, the conductivity transformation from $n$- to $p$-type ZnO films deposited by sputtering was followed as a function of copper and tellurium concentrations and of the substrate temperature during growth (room temperature, $150{}^{\circ }\mathrm{C}, 250{}^{\circ }\mathrm{C}$, and $350{}^{\circ }\mathrm{C}$). The nominal codopant concentrations ranged from 1 to 12 at %. For the minimum concentration, compensation effects yielded highly resistive ZnO. Nonetheless, by tailoring the concentration of Cu and Te, it was possible to vary the resistivity $({10}^3–{10}^{-2}\mathrm{\Omega }\mathrm{cm})$, mobility $(\sim {10}^{-2}–{10}^{\circ }\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s})$, and free-hole density $({10}^{15}–{10}^{20}\mathrm{c}{\mathrm{m}}^{-3})$ of $p$-type ZnO grown at $250{}^{\circ }\mathrm{C}$. Besides modifying the electrical properties, codoping changed the host band structure significantly, producing a band-gap renormalization from 3.2 eV (UV) to 1.8 eV (red). This control over the band gap is advantageous for applications where controllable photon absorption or emission are sought. Experimentally, films were stable for a period of at least six months. Band-gap engineered $p$-type ZnO:(Cu+Te) films open the possibility for the fabrication of all-ZnO optoelectronic devices such as homojunction solar cells and/or light-emitting diodes.

A systematic study based on state-of-the-art first-principles calculations has been carried out to determine intrinsic thermoelectric properties in $n$- and $p$-type SiGe alloys. Both electronic and thermal transport coefficients have been evaluated using the Boltzmann transport equation, applied on the electronic and phonon band structure, respectively. The Seebeck coefficient, electrical conductivity, and thermal conductivity have been analyzed focusing on the effect of carrier concentration, temperature, and alloy composition. The resulting figure of merit is highest for heavy doping and at elevated temperatures ($>$1000 K) in the SiGe alloy with Ge content from 50% to 80%.

Assemblies of clusters have been sought for a long time to synthesize new materials with unprecedented physical phenomena or to integrate desired functionalities for technological applications. However, except for some carbon fullerenes and ligated clusters, little progress has been made in achieving assemblies of other clusters due to their tendency for agglomeration. Here we study interaction in dimers of 34 well-studied endohedrally doped clusters (i.e., superatoms) and propose the criteria for such superatoms to be potential building blocks in terms of surface coordination, charge state, distribution of the highest occupied molecular orbital, and electronic as well as atomic shell (double-shell) closure. From these results, the endohedrally doped $\mathrm{Ti}@{\mathrm{Ge}}_{16}$ cage cluster stands out as a suitable building block to assemble solids and nanostructures with outstanding stabilities and diverse physical properties. We report here the finding of antiferromagnetic Mott insulator in metal intercalated two-dimensional crystal of such cluster. Our study provides essential knowledge for achieving stable cluster assemblies of different dimensionalities with precisely tailorable electronic structure for device applications.

In this study, we present an approach (along with the needed computational strategies) for efficient and scalable feature engineering of the crystal structure in compounds of different chemical compositions. This approach utilizes a versatile and extensible framework for the quantification of a three-dimensional voxelized crystal structure in the form of 2-point spatial correlations of multiple atomic attributes and performs principal component analysis to extract the low-dimensional features that could be used to build surrogate models for material properties of interest. An application of the proposed feature engineering framework is demonstrated on a case study involving the prediction of the formation energies of crystalline compounds using two vastly different surrogate model building strategies; local Gaussian process regression and neural networks. Specifically, it is shown that the top 25 features (i.e., principal component scores) identified by the proposed framework serve as good regressors for the formation energy of the crystalline substance for both model building strategies.

We analyze the effects of chemical pressure induced by alkali-metal substitution and uniaxial strain on magnetism in the $A_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ ($A$=Na, K, Rb, and Cs) family of ternary arsenides with quasi-one-dimensional structure. Within the framework of the density functional theory, we predict that the nonmagnetic phase is very close to a three-dimensional collinear ferrimagnetic state, which realizes in the regime of moderate correlations, such tendency being common to all the members of the family with very small variations due to the different interchain ferromagnetic coupling. We uncover that the stability of such interchain ferromagnetic coupling has a nonmonotonic behavior with increasing the cation size, being critically related to the degree of structural distortions which is parametrized by the Cr-As-Cr bonding angles along the chain direction. In particular, we demonstrate that it is boosted in the case of the Rb, in agreement with recent experiments. We also show that uniaxial strain is a viable tool to tune the nonmagnetic phase towards an interchain ferromagnetic instability. The modification of the shape of the Cr triangles within the unit cell favors the formation of a net magnetization within the chain and of a ferromagnetic coupling among the chains. This study can provide relevant insights about the interplay between superconductivity and magnetism in this class of materials.

A complex interplay between magnetic domain structure and crystalline imperfections, here twins, is revealed in a rare-earth-free MnAl bulk magnet. The magnetic domains are observed to be in the nanometer range for a large part of the magnetic structure and to scale with the number density of twins formed during thermal processing. We explain this phenomenon by a reduction in domain-wall energy at the twinned regions as proven by ab initio calculations. In addition, our atomic-scale analysis reveals that the twin boundaries contain excess Mn atoms that reduce the local magnetization, serving as an obstacle for domain wall motion. These insights can help guide the strategic design of magnetic materials by controlling the initial phase distribution to tailor the twin density and hence, the distribution of domains.

Combining first-principles calculations with recently developed statistical physics tools, we determine carbon diffusion mechanisms and the resulting diffusion coefficients in pure-Fe and weakly alloyed $M_3\mathrm{C}$ cementites. The predicted coefficients in ${\mathrm{Fe}}_3\mathrm{C}$ are in good agreement with experimental measurements of carburization rate in ferritic steels. In our proposed diffusion mechanisms, C migrates by jumps between interstitial sites rather than via the C Frenkel pair mechanism, as proposed by previous studies based on semiempirical simulations. In the alloyed cementites, the C diffusion can be slowed down due to the presence of Mn solutes up to 500 K, while it is mostly unaffected by the addition of Mo or Cr solutes.

Boron carbide, a lightweight, high temperature material, has various applications as a structural material and as a neutron absorber. The large solubility range of carbon in boron, between $\approx 9\%$ and 20%, has been theoretically explained by some of us by the thermodynamical stability of three icosahedral phases at low temperature, with respective carbon atomic concentrations: 8.7% (${\mathrm{B}}_{10.5}\mathrm{C}$, named ${\mathrm{OPO}}_1$), 13.0% (${\mathrm{B}}_{6.7}\mathrm{C}$, named ${\mathrm{OPO}}_2$), whose theoretical Raman spectra are still unknown, and 20% (${\mathrm{B}}_4\mathrm{C}$), from which the nature of some of the Raman peaks are still debated. We report theoretical and experimental results of the first-order, nonresonant, Raman spectrum of boron carbide. Density functional perturbation theory enables us to obtain the Raman spectra of the ${\mathrm{OPO}}_1$ and ${\mathrm{OPO}}_2$ phases, which are perfectly ordered structures with however a complex crystalline motif of 414 atoms, due to charge compensation effects. Moreover, for the carbon-rich ${\mathrm{B}}_4\mathrm{C}$, with a simpler 15-atom unit cell, we study the influence of the low energy point defects and of their concentrations on the Raman spectrum, in connection with experiments, thus providing insights into the sensitivity of experimental spectra to sample preparation, experimental conditions, and setup. In particular, this enables us to propose a new structure at 19.2% atomic carbon concentration, ${\mathrm{B}}_{4.2}\mathrm{C}$, that, within the local density approximation of density functional theory (DFT-LDA), lies very close to the convex hull of boron carbide, on the carbon-rich side. This new phase, derived from what we name the “3+1” defect complex, helps in reconciling the experimentally observed Raman spectrum with the theory around 1000 ${\mathrm{cm}}^{-1}$. Finally, we predict the intensity variations induced by the experimental geometry and quantitatively assess the localization of bulk and defect vibrational modes and their character, with an analysis of “chain” and “icosahedral” modes.

High-performance, benchmark organic semiconductor rubrene is investigated by modulated-photocurrent (MPC) spectroscopy. In this technique, periodically modulated and steady-state photocarrier populations are simultaneously generated in the sample by two independent light beams, with the resultant photocurrent measured by a lock-in amplifier. The technique allows identifying a bandlike carrier motion (as opposed to incoherent hopping) as the dominant charge transport mechanism in this highly crystalline molecular semiconductor, in agreement with prior studies, yet without the need for fabrication of complex transistor devices. Moreover, MPC spectroscopy is used to determine the important parameters, such as the density of states for traps, their carrier capture coefficient, and attempt to escape frequency, as well as the quantum efficiency of photocarrier generation and the photocarrier mobility. The MPC spectroscopy is also shown to be able to discriminate charge transport at the surface of the crystal from that occurring in the bulk. The wealth of the thus obtained information is essential for our better understanding of the microscopic mechanisms governing photoconductivity in this class of materials.

Experimental studies of high-purity kagome-lattice antiferromagnets (KAFMs) are of great importance in attempting to better understand the predicted enigmatic quantum spin-liquid ground state of the KAFM model. However, realizations of this model can rarely evade magnetic ordering at low temperatures due to various perturbations to its dominant isotropic exchange interactions. Such a situation is encountered, for example, due to sizable Dzyaloshinskii-Moriya magnetic anisotropy in ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_3$, which stands out from other KAFM materials due to its perfect crystal structure. We also find evidence of magnetic ordering in the distorted sibling compound ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{18}[{\mathrm{Cl}}_8$(OH)], which has recently been proposed to feature a spin-liquid ground state arising from a spatially anisotropic kagome lattice. Our findings are based on a combination of bulk susceptibility, specific heat, and magnetic torque measurements that disclose a Néel transition temperature of $T_N=11$ K in this material, which might feature a coexistence of magnetic order and persistent spin dynamics as previously found in ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_3$. Contrary to previous studies of single crystals and powders containing impurity inclusions, we use high-purity single crystals of ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{18}[{\mathrm{Cl}}_8$(OH)] grown via an optimized hydrothermal synthesis route that minimizes such inclusions. This study thus demonstrates that the lack of magnetic ordering in less pure samples of the investigated compound does not originate from the reduced symmetry of the spin lattice, but is instead of extrinsic origin.

We investigate the process by which a water molecule diffuses on the surface of metal electrodes under constant bias voltage by first-principles density functional theory. In this study, we present the constant electron chemical potential (constant-${\mu }_{\mathrm{e}}$) methods combined with the nudged elastic band method. The water diffusion on the Al(111) was calculated using the minimum energy paths (MEPs) for understanding the difference between the constant-${\mu }_{\mathrm{e}}$ and conventional methods. The simulation shows that the MEP of the water molecule, its adsorption site, and the activation barrier strongly depend on the applied bias voltage. Comparing to the constant total number of electrons (constant-$N_{\mathrm{e}}$), we found the larger change in the tilted angle of the water dipole in the MEP by the constant-${\mu }_{\mathrm{e}}$ method. For the comparison between the theoretical results and the previous experiment, we simulate the MEP for a single water diffusion on the Pt(111) surface using constant-${\mu }_{\mathrm{e}}$ method. When we applied positive bias voltage to the Pt electrode, the result of the activation barrier for a water molecule decreases with increasing the bias voltage, which is consistent with the previous scanning tunneling microscopy (STM) experiment. The proposed constant-${\mu }_{\mathrm{e}}$ method plays a significant role in understanding the interaction between the electric field and the surface of the material, and is a reliable tool for the simulation of reactions under bias voltage not only using STM but also at the electrochemical interface.

Understanding how properties of materials change due to nuclear transmutations is a major challenge for the design of structural components for a fusion power plant. In this study, by combining a first-principles matrix Hamiltonian approach with thermodynamic integration we investigate quasi-steady-state configurations of multicomponent alloys, containing defects, over a broad range of temperature and composition. The model enables simulating transmutation-induced segregation effects in materials, including tungsten where the phenomenon is strongly pronounced. Finite-temperature analysis shows that voids are decorated by Re and Os, but there is no decoration by tantalum (Ta). The difference between the elements is correlated with the sign of the short-range order (SRO) parameter between impurity and vacancy species, in agreement with atom probe tomography (APT) observations of irradiated W-Re, W-Os, W-Ta alloys in the solid solution limit. Statistical analyses of Re and Os impurities in vacancy-rich tungsten show that the SRO effects involving the two solutes are highly sensitive to the background concentration of the solute species. In quaternary W-Re-Os-Vac alloys containing $1.5\%$ Re and $0.1\%$ Os, the SRO Re-Os parameter is negative at 1200 K, driving the formation of concentrated Re and Os precipitates. Comparison with experimental transmission electron microscopy and APT data on W samples irradiated at the High-Flux Reactor shows that the model explains the origin of anomalous segregation of transmutation products (Re,Os) to vacancy clusters and voids in the high-temperature limit pertinent to the operating conditions of a fusion power plant.