While anomalous diffusion coefficients with non-Arrhenius-like temperature dependence are observed in a number of metals, a conclusive comprehensive framework of explanation has not been brought forward to date. Here, we use first-principles calculations based on density functional theory to calculate self-diffusion coefficients in the bcc metals Mo and $\beta$-Ti by coupling quasiharmonic transition state theory and large-displacement phonon calculations and show that anharmonicity from thermal expansion is the major reason for the anomalous temperature dependence. We use a modified Debye approach to quantify the thermal expansion over the entire temperature range and introduce a method to relax the vacancy structure in a mechanically unstable crystal such as $\beta$-Ti. The effect of thermal expansion is found to be crucial for the nonlinear, non-Arrhenius “anomalous” self-diffusion in both bcc systems, with $\beta$-Ti showing a 60% larger relative nonlinearity parameter than Mo. Our results point to temperature dependence in the diffusion prefactor from thermal expansion as the major origin of anomalous self-diffusion. The methodology proposed for $\beta$-Ti is general and simple enough to be applicable to other mechanically unstable crystals.

Using artificial neural-network machine learning (ANN-ML) to generate interatomic potentials has been demonstrated to be a promising approach to address the longstanding challenge of accuracy vs efficiency in molecular dynamics (MD) simulations. Here, taking the Fe-Si-O system as a prototype, we show that accurate and transferable ANN-ML potentials can be developed for reliable MD simulations of materials at high-pressure and high-temperature conditions of the Earth's outer core. The ANN-ML potential for the Fe-Si-O system is trained by fitting the energies and forces of related binaries and ternary liquid structures at high pressures and temperatures obtained by first-principles calculations based on density functional theory (DFT). We show that the generated ANN-ML potential describes well the structure and dynamics of liquid phases of this complex system. In addition to binary systems (${\mathrm{Fe}}_{189}{\mathrm{Si}}_{61}, {\mathrm{Fe}}_{189}{\mathrm{O}}_{61}$, and ${\mathrm{Si}}_{80}{\mathrm{O}}_{160}$) and ternary systems (${\mathrm{Fe}}_{189}{\mathrm{Si}}_{38}{\mathrm{O}}_{23}$), whose snapshots are included in the training dataset, the reliability of the ANN-ML potential is validated in two other ternary systems (${\mathrm{Fe}}_{189}{\mathrm{Si}}_{23}{\mathrm{O}}_{38}$ and ${\mathrm{Fe}}_{158}{\mathrm{Si}}_{14}{\mathrm{O}}_{28}$), whose snapshots are not included in the training dataset. The efficient ANN-ML potential with DFT accuracy provides a promising scheme for accurate atomistic simulations of structures and dynamics of the complex Fe-Si-O system in the Earth's outer core.

Monolayer ${\mathrm{MoS}}_2$ is an attractive candidate for developing future electronics and exploring fundamental physics because of its unique quantum properties. High-performance ${\mathrm{MoS}}_2$ transistors with superior transport originating from spin-valley coupling have been reported. However, these transistors have used single-crystalline flakes. In most large-area polycrystalline ${\mathrm{MoS}}_2$, these excellent transport properties have not been realized because of the effects of grain boundaries, and their fundamental low-temperature transport has not been studied in detail. Here, we apply electrolyte-gating methods using chemically grown centimeter-scale polycrystalline monolayers. Owing to the high carrier density, the resistance is systematically tuned from insulating to metallic conduction. Importantly, we observed metallic transport at temperatures down to 1.9 K and a high mobility of $>100\mathrm{c}{\mathrm{m}}^2\mathrm{n}{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Moreover, the magnetotransport exhibits an electric-field-induced crossover from weak localization to weak antilocalization, indicating that high carrier density dramatically suppresses the grain boundary effects to enable intrinsic quantum transport. The results reveal that polycrystalline ${\mathrm{MoS}}_2$ monolayers have significant potential for use in large-area nanoelectronics.

Using phase-field simulations, we show how interfaces acting on the geometric-improper ferroelectric polarization of hexagonal manganite and ferrite thin films can be used to control the formation of charged domain walls. We modify the Landau expansion of the free energy valid in bulk to emulate interface effects known from previous cross-sectional experiments, and we verify our model by comparing our results with images obtained in these experiments. We then show how the interface affects the orientation of ferroelectric domain walls in the fully three-dimensional case. Furthermore, we demonstrate that interface effects combined with an external electric field enable us to specifically choose the dominant domain-wall type (head-to-head, tail-to-tail, or neutral). We also find that an electric field can stabilize a novel domain-wall type which only emerges in the improper ferroelectric order but not in the primary structural distortion. Since the domain walls have a conductivity that is different from the interior of the domains, the influence of the interfaces of a thin film on the type and distribution of the walls gives us the possibility to control the transport properties of a material by appropriate thin-film engineering.

With intricate lattice structures, kagome materials are an excellent platform to study various fascinating topological quantum states. In particular, kagome materials, revealing large responses to external stimuli such as pressure or magnetic field, are subject to special investigation. Here we study the kagome-net ${\mathrm{HoMn}}_6{\mathrm{Sn}}_6$ magnet that undergoes paramagnetic to ferrimagnetic transition (below 376 K) and reveals spin-reorientation transition below 200 K. In this compound, we observe the topological Hall effect and substantial contribution of anomalous Hall effect above 100 K. We unveil the pressure effects on magnetic ordering at a low magnetic field from the pressure tunable magnetization measurement. By utilizing high-resolution angle-resolved photoemission spectroscopy, Dirac-like dispersion at the high-symmetry point K is revealed in the vicinity of the Fermi level, which is well supported by the first-principles calculations. Our investigation will pave the way to understanding the magnetotransport and electronic properties of various rare-earth-based kagome magnets.

We report magnetic properties of a $4f$ honeycomb iodide $\mathrm{Sm}{\mathrm{I}}_3$ made up of edge-shared network of $\mathrm{Sm}{\mathrm{I}}_6$ octahedra. High temperature magnetic susceptibility indicates a ${\mathrm{\Gamma }}_7$ Kramers doublet ground state of ${\mathrm{Sm}}^{3+}\left(4f^5\right)$ ions stabilized by the spin-orbit coupling and octahedral crystal electric field, which interact with a Sm-I-Sm bond angle of nearly ${90}^{\circ }$. Magnetization measurements down to 0.1 K detected antiferromagnetic correlations and an anomaly in the magnetization curve before saturation without a sign of long-range order. Relevance between $\mathrm{Sm}{\mathrm{I}}_3$ and the antiferromagnetic Kitaev material proposed in the $4f\text{−}\mathrm{electron}$ system is discussed.

We report on an investigation of magnetic resonance phenomenon in ferromagnetic thin films with controlled nonuniform composition. An epitaxial Pd-Fe thin film with linear distribution of Fe content across the thickness is used as the model material. Anomalous perpendicular standing spin waves are observed and quantified using the collective dynamic equation. Numerical analysis yields the exchange stiffness constant for the diluted Pd-Fe alloy $D=2A/{\mu }_0{M}_s=15\mathrm{T}·{\mathrm{nm}}^2$ and the ratio of the effective magnetization to the saturation magnetization $M_{\text{eff}}/M_s=1.16$. It is demonstrated that, overall, the engineering of thin films with nonuniform composition across the thickness can be used for high-frequency or low-field magnonic operations using exchange spin waves.

We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator ${\mathrm{NiS}}_2$. Through temperature- and gate-voltage-dependent electronic transport measurements, a gating-induced three-dimensional metallic state is observed at positive gate bias on single-crystal surfaces. Based on transport, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an $\mathit{electrochemical}$ gating mechanism involving a substantial decrease in the S:Ni ratio over hundreds of nanometers, which is both nonvolatile and irreversible. Such findings are in striking contrast to the reversible, volatile, two-dimensional $\mathit{electrostatic}$ gate effect previously seen in pyrite ${\mathrm{FeS}}_2$. We attribute this stark difference in electrochemical vs electrostatic gating response in ${\mathrm{NiS}}_2$ and ${\mathrm{FeS}}_2$ to the much larger S diffusion coefficient in ${\mathrm{NiS}}_2$. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S, in contrast to the better understood oxide case, where electrolysis of atmospheric ${\mathrm{H}}_2\mathrm{O}$ provides an O reservoir. The present study of ${\mathrm{NiS}}_2$ thus provides insight into electrolyte gating mechanisms in functional materials, in a relatively unexplored limit.

${\mathrm{BiVO}}_4$ is a promising photocatalyst for efficient water oxidation, with surface reactivity determined by the structure of active catalytic sites. Surface oxidation in the presence of oxygen vacancies induces electron localization, suggesting an atomistic route to improve the charge transfer efficiency within the catalytic cycle. In this paper, we study the effect of oxygen vacancies on the electronic and optical properties at ${\mathrm{BiVO}}_4$ surfaces upon water oxidation. We use density functional theory and many-body perturbation theory to explore the change in the electronic and quasiparticle energy levels and to evaluate the electron-hole coupling as a function of the underlying structure. We show that while the presence of defects alters the atomic structure and largely modifies the wave-function nature, leading to defect-localized states at the quasiparticle gap region, the optical excitations remain largely unchanged due to the substantial hybridization of defect and nondefect electron-hole transitions. Our findings suggest that defect-induced surface oxidation supports improved electron transport, both through bound and tunable electronic states and via a mixed nature of the optical transitions, expected to reduce electron-hole defect trapping.

The size and shape of the primary dendrite tips determine the principal length scale of the microstructure evolving during solidification of alloys. $In\text{−}situ$ x-ray measurements of the tip shape in metals have been unsuccessful so far due to insufficient spatial resolution or high image noise. To overcome these limitations, high-resolution synchrotron radiography and advanced image processing techniques are applied to a thin sample of a solidifying Ga-35wt.%In alloy. Quantitative $in\text{−}situ$ measurements are performed of the growth of dendrite tips during the fast initial transient and the subsequent steady growth period, with tip velocities ranging over almost two orders of magnitude. The value of the dendrite tip shape selection parameter is found to be ${\sigma }^{*}=0.0768$, which suggests an interface energy anisotropy of ${\varepsilon }_4=0.015$ for the present Ga-In alloy. The nonaxisymmetric dendrite tip shape amplitude coefficient is measured to be $A_4\approx 0.004$, which is in excellent agreement with the universal value previously established for dendrites.

We present an optical study of high crystalline quality $\gamma \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ films grown epitaxially on (0001)-oriented sapphire. Well-defined structures are detected at 2.147, 2.240, 2.359, and 2.508 eV in the optical absorption at $T=10$ K. On the basis of the selection rules, we associate them to direct optical transitions with excitons built from the upper and spin-orbit splitoff valence bands and the first conduction band at the $\mathrm{\Gamma }$ point. The first two lowest excitonic peaks vanish from the absorption spectrum above $T=120$ K at a critical temperature for which the photoluminescence emission fades out as well. It leads us to an estimate of the exciton binding energy of the order of 10 meV in $\gamma \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$. Last, we interpret additional absorption peaks to LO-phonon replicas of the free exciton with a phonon energy of $\sim 14$ meV.

We perform a density functional theory study of the effects of oxygen adsorption on the structural and electronic properties of Gr/Co(0001) and Gr/Co/Ir(111) interfaces. In both interfaces, the graphene-Co distance increases with increasing O concentration. The oxygen intercalation effectively decreases the electronic interaction, preventing the hybridization of graphene states with Co $d$ orbitals, hence (partly) restoring the typical Dirac cone of pristine graphene. In the case of graphene/Co 1ML/Ir(111), which presents a moiré pattern, the interplay between the O distribution and the continuous change of the graphene-Co registry can be used to tune graphene corrugation and electronic properties. The computed electronic properties are in very good agreement with previously reported angle-resolved photoemission spectroscopy and photoemission electron microscopy measurements for Gr/Co(0001).

An oxide interface may become conducting when one oxide is polar, such as ${\mathrm{LaAlO}}_3$ (LAO) grown on ${\mathrm{SrTiO}}_3$ (001). We anticipate that the functional use of a solid electrolyte ${\mathrm{LiNbO}}_3$ (LiNO) in LAO/LiNO allows one to tune the quasi-two-dimensional electron gas (q2DEG) which emerges there. This phenomenon can be achieved by charging and discharging lithium niobate. In this paper, on the basis of ab initio calculations of the hydrogen-doped LAO/LiNO interface, we demonstrate how the q2DEG and its spin polarization appear. The out-of-plane electric polarization of LAO/LiNO forces the H dopants to accommodate at the Al-terminated interface. This results in the formation of the q2DEG, whose spatial extent is about 1.5 nm along the stacking direction. Since the spin-orbit coupling stimulates the spin textures in q2DEG, we believe that the charge current via the q2DEG may generate a variety of spin-orbitronic effects.

Modern superconducting qubits based on two-dimensional (2D) transmons typically involve the growth of Nb thin films on high-resistivity Si substrates. Since imperfections at the Nb-Si heterointerface have been implicated as a source of two-level systems that limit quantum coherence times, detailed characterization and understanding of niobium silicide interfacial layers are critical to improving superconducting qubit technology. While bulk binary intermetallic niobium silicide phases are well understood, the thermodynamic phase stability and properties of ultrathin niobium silicides, such as those found at the Nb-Si heterointerface in 2D transmons, have not yet been explored. Here, we report finite-sized effects for ultrathin niobium silicide films using density functional theory calculations and predict nanoscale stabilization of ${\mathrm{Nb}}_6{\mathrm{Si}}_5$ over the bulk $\alpha \text{−}{\mathrm{Nb}}_5{\mathrm{Si}}_3$ phase. This result is consistent with our experimental observations of a niobium silicide interfacial layer between a sputtered Nb thin film and the underlying Si substrate. Furthermore, our calculations show that ${\mathrm{Nb}}_6{\mathrm{Si}}_5$ nanofilms are nonmagnetic, making them superior to nanofilms of $\alpha \text{−}{\mathrm{Nb}}_5{\mathrm{Si}}_3$ that exhibit antiferromagnetic correlations detrimental to long coherence times in superconducting qubits. By providing atomic-scale insight into niobium silicide nanofilms, this paper can help guide ongoing efforts to optimize Nb-Si heterointerfaces for long coherence times in superconducting qubits.

The orthorhombic TiNiSi structure-type compounds show interesting electronic structures comprising in most cases a pseudogap in the density of states and several small electron and hole pockets at the Fermi energy. These features are promising and can be exploited to test their potential as thermoelectric materials for waste heat conversion. Here, we investigate the effect of electron doping in the semimetallic member $\mathrm{ZrNiSi}$ of this family. We show that by doping with Sb for Si in $\mathrm{ZrNiSi}, S$ and $\sigma$ can both be increased simultaneously for initial Sb doping defying the oppositely directed trend commonly observed in most materials. In the doped samples, $\sigma$ at $300\mathrm{K}$ increases from $1000\mathrm{S}{\mathrm{cm}}^{-1}$ to as high as $2500\mathrm{S}{\mathrm{cm}}^{-1}$; at the same time, the peak value of $S$, which is $-20\mu \mathrm{V}{\mathrm{K}}^{-1}$ in $\mathrm{ZrNiSi}$, increases by more than a factor of two. The simultaneous enhancement of $\sigma$ and $S$ has been explained using the first-principles density functional theory based band structure calculations. The as-cast (i.e., unannealed) ${\mathrm{ZrNiSi}}_{1-x}{\mathrm{Sb}}_x$ samples show phase segregation due to a spinodal-type decomposition with two coexisting TiNiSi structure-type phases with different Sb/Si ratios. The thermal conductivity ($\kappa$) in the doped samples drops significantly from $12\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ ($x=0$) to nearly $2\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ ($x=0.2$) at 300 K. As a result, the peak thermoelectric figure of merit ($zT$) increases from 0.005 $(x=0)$ to 0.023 $(x=0.2)$. Further enhancement in $zT$ is obtained by codoping of Hf (Zr site) and Sb (Si site), which improves the phase stability and chemical homogeneity while keeping the thermal conductivity still very low due to Zr-Hf point mass fluctuation, resulting in a peak $zT$ value of 0.055, i.e., almost an order of magnitude higher value than for the pristine $\mathrm{ZrNiSi}$. We show that the thermoelectric properties of TiNiSi structure-type semimetals can be enhanced by aliovalent doping. This principle can be employed on other members of the TiNiSi to improve the $zT$ in this family of compounds further.

Recent experiments have validated prior theories of unusual high thermal conductivity $\left(\kappa \right)$ in boron arsenide (BAs) and revealed large $\kappa$ variation associated with extended and point defects in the samples. The peak $\kappa$ provides valuable insights into the competition between intrinsic phonon-phonon scattering processes and extrinsic boundary and defect scattering processes in BAs. However, prior measurement methods have not been able to measure the peak $\kappa$ because of fundamental and technical limitations. Here, we report peak $\kappa$ measurements of BAs crystals synthesized under different conditions with source materials of different purities via a vapor transport method. For three representative samples, the measured $\kappa$ peak appears at temperatures between 120 and 150 K and varies from $410\pm 60\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ to $830\pm 100\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. The measured thermal conductivities agree with theoretical calculations across the full temperature range. The similar calculated and measured peak temperatures helps to narrow down the boundary scattering mean free path to be around 4 μm in two samples and 5 μm in another, while the variation of the peak magnitude reveals a one-order-of-magnitude difference in the strength of point defect scattering. The phonon-defect scattering behavior correlates well with the measured electronic Raman scattering background, the impurity concentrations revealed by secondary ion mass spectroscopy, and the Hall concentration and mobility of the $p$-type samples except for an anomalously high hole concentration that appears in one sample, which indicates nonuniform impurity distribution. The observed correlation clarifies the origins of extrinsic phonon scattering mechanisms in BAs crystals.

In the present work, we have succeeded in controlling association-dissociation process of thermally activated In impurities introduced in polycrystalline ZnO by a multistep doping method. The thermal behavior and eventual residential sites of In ions were monitored on an atomic scale by means of time-differential perturbed angular correlation spectroscopy with the ${}^{111}\mathrm{In}(\to {}^{111}\mathrm{Cd})$ probe. The present doping method of combined heat treatments in air and in vacuum enabled the introduction of a great fraction of 0.5 at. % In donors into defect-free substitutional Zn sites. It was demonstrated that the In-containing ZnO samples show a clear positive correlation between the electrical conductivity and the concentration of In donors at substitutional Zn sites, achieving a conductivity as high as five orders of magnitude compared with undoped ZnO.

Co-substituted ${\mathrm{BiFeO}}_3$ systems exhibit both ferroelectric and antiferromagnetic (AFM) orders with a considerably high electric polarization $\mathbf{P}$ and a modestly canted magnetization $\mathbf{M}$ and a cross coupling between them at room temperature. In the present study, employing first-principles calculations and Monte Carlo simulations, we unravel the microscopic interactions that contribute to stabilize this particular canted AFM order with $\mathbf{M}$. Our results propose a correlation between the spin-state configuration of the trivalent ${\mathrm{Co}}^{3+}$ ions and the energetics associated with the magnetic orders. Because of the complex interplay between the Hund's coupling $\left(J_H\right)$ and crystal field splitting $\left({\mathrm{\Delta }}_{\mathrm{CF}}\right)$, the trivalent ${\mathrm{Co}}^{3+}$ ions are known to exhibit three spin $\left(S\right)$ configurations, namely low $(t_{2g}^6\to S=0)$ spin (LS), intermediate $(t_{2g}^5{e}_g^1\to S=1)$ spin (IS), and high $(t_{2g}^4{e}_g^2\to S=2)$ spin (HS). We observe electron correlation induced LS $\to$ HS state transition, which implies a close competition among these spin states in the $R3c$ polar structure. Moreover, our detailed analysis points towards the formation of heterogeneous spin-state configuration at room temperature. While the LS state configuration was not observed to exhibit any particular tendency to stabilize the canted AFM phase, the HS state tends to strongly favor the formation of this desired magnetic order. On the other hand, interestingly, the formation of finite fraction of the IS state can contribute to enhance the magnitude of $\mathbf{M}$. Our investigation is expected to initiate further quest for appropriate magnetic substitutes to ensure enhanced functionalities of the system.

We have investigated magnetic Co atoms self-assembled on ultraflat stanene on Cu(111) substrate by utilizing scanning tunneling microscopy/spectroscopy (STM/STS) in conjunction with density functional theory (DFT). By means of depositing Co onto the stanene/Cu(111) held at 80 K, Co atoms have developed into monomer, dimer, and trimer structures containing one, two, and three Co atoms, respectively. As per atomically resolved topographic images and bias-dependent apparent heights, the atomic structure models based on Sn atoms substituted by Co atoms have been deduced, which are in agreement with both self-consistent DFT calculations and STM simulations. Apart from that, the projected density of states has revealed a maximum at around $-0.5$ eV from the Co-3$d_{3z^2-r^2}$ minority band, which contributes predominately to the peak feature at about $-0.3$ eV in tunneling conductance $(dI/dU)$ spectra taken at the Co atomic sites. As a result of the exchange splitting between the $\mathrm{Co}\text{−}3d$ majority and minority bands, there are nonzero magnetic moments, including about $0.60{\mu }_B$ in monomer, $0.56{\mu }_B$ in dimer, and $0.29{\mu }_B$ in trimer of the Co-atom assembly on the stanene. Such a magnetic Co-atom assembly, therefore, could provide the vital building blocks to stabilize the local magnetism on two-dimensional stanene with nontrivial topological properties.