Amorphous alumina is employed ubiquitously as a high-dielectric-constant material in electronics, and its thermal-transport properties are of key relevance for heat management in electronic chips and devices. Experiments show that the thermal conductivity of alumina depends significantly on the synthesis process, indicating the need for a theoretical study to elucidate the atomistic origin of these variations. Here we employ first-principles simulations to characterize the atomistic structure, vibrational properties, and thermal conductivity of alumina at densities ranging from 2.28 to $3.49 \mathrm{g}/{\mathrm{cm}}^3$. Moreover, using a machine-learned interatomic potential trained on first-principles data, we investigate how system size affects predictions of the thermal conductivity, showing that simulations containing 120 atoms can already reproduce the bulk limit of the conductivity. Finally, relying on the recently developed Wigner formulation of thermal transport, we shed light on the interplay between atomistic topological disorder and anharmonicity in the context of heat conduction, showing that the former dominates over the latter in determining the conductivity of alumina.

We study theoretically the nucleation and development of sequences of monolayers during the vapor-liquid-solid growth of semiconductor nanowires, in the case where all material originates from the catalyst nanodroplet. If at nucleation the droplet content is insufficient, a fractional monolayer forms quickly before propagating more slowly; the droplet then refills during a random waiting time until next nucleation. We compare the proper incomplete monolayer regime, where this occurs for each monolayer, with the mixed regime where full or fractional monolayers may form, depending on the nucleation event. We investigate in detail the most general case of the mixed regime at arbitrary temperature. Under simple assumptions of the dependence of nucleation and desorption rates upon liquid state, valid at least for III-V compounds (with low concentration of the volatile group V atoms in the liquid), we calculate semi-analytically the probability density of the concentration at nucleation and the statistics of the propagation, waiting and monolayer cycle times, without any growth simulation and duly accounting for the correlations between successive monolayers. We find that an effective incomplete monolayer regime, whereby a huge fraction of nucleations produce incomplete monolayers, may prevail over a wide range of nanowire-droplet geometry and growth conditions, with complete monolayers becoming frequent only at large nanowire radius, input rate, and temperature. We explain why, in this regime, growth tends to become quasideterministic, with a very narrow distribution of monolayer cycle times, which is beneficial for a precise control of nanowire ensembles and heterostructures. We investigate quantitatively the case of self-catalyzed GaAs nanowires and discuss the extension of our conclusions to other systems.

The recent discovery of ferroelectric behavior in few-layer materials, accompanied by the observation of antipolar domains in hexagonal boron nitride and transition metal dichalcogenide moiré bilayers, is paving the way for revolutionary advancements in the generation and manipulation of intrinsic electric dipoles through stacking. In addition, these cutting-edge quantum materials are reshaping our comprehension of phase transitions. Within the present paper, we unveil a size-dependent sliding behavior that marks a significant departure from conventional ferroelectrics. We also shed light on thermally induced spontaneous hyperlubric sliding within moiré bilayers, which can be used as a signal to distinguish topological phase transitions from an antipolar onto a ferroelectric bilayer. Our findings also suggest that the (topological) pinning of AA nodes in antipolar moiré homobilayers prevents the occurrence of an antipolar-to-paraelectric transformation.

For epitaxial ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ films grown by pulsed laser deposition, we report a combined structural and magnetotransport study of thin films and Hall bar devices patterned side-by-side on the same film. Structural properties of these films are investigated using x-ray diffraction and high-resolution transmission electron microscopy, and confirm that these films are epitaxially oriented and nearly phase pure. For magnetic fields applied along the $c$ axis, a positive magnetoresistance of 10% is measured for unpatterned ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ films, whereas for patterned Hall bar devices of channel widths of 10 and $5\mu \mathrm{m}$, magnetoresistance values of 40% and 140% are found, respectively. These films show switching behaviors from positive to negative magnetoresistance that are controlled by the direction of the applied magnetic field. The present results provide a promising route for achieving stable epitaxial synthesis of intermediate members of correlated layered strontium ruthenates, and for the exploration of device physics in thin films of these compounds.

The ultimate limits of the carrier concentrations in ${\mathrm{In}}_2{\mathrm{O}}_3$ and Sn-doped ${\mathrm{In}}_2{\mathrm{O}}_3$ are derived from operando photoelectron spectroscopy of a solid oxide electrochemical cell with Y-doped ${\mathrm{ZrO}}_2$ as the oxygen electrolyte. It is demonstrated that the limits are determined by the transition of the oxygen vacancy to the neutral state and to the reduction of ${\mathrm{Sn}}^{4+}$ donors to ${\mathrm{Sn}}^{2+}$ electron traps, respectively. Maximum Fermi energies of 3.85 and $3.35\mathrm{eV}$ above the valence band maximum are identified for ITO and ${\mathrm{In}}_2{\mathrm{O}}_3$. The ultimate carrier concentrations achievable by Sn doping and by oxygen vacancies are estimated to be $1.8–1.9\times {10}^{21}{\mathrm{cm}}^{-3}$ and $6–7\times {10}^{20}{\mathrm{cm}}^{-3}$.

Nodal-line semimetals feature topologically protected band crossings between the bulk valence and conduction bands that extend along a finite dimension in the form of a line or a loop. While ZrSiS and similar materials have attracted extensive research as hosts for the nodal-line semimetallic phase, an alternative avenue has emerged in the form of isostructural rare-earth $\left(RE\right)$-based $RE\mathrm{SbTe}$ materials. Such systems possess intriguing potentialities for harboring elements of magnetic ordering and electronic correlations owing to the presence of $4f$ electrons intrinsic to the $RE$ elements. In this study, we have carried out angle-resolved photoemission spectroscopy (ARPES) and thermodynamic measurements in conjunction with first-principles computations on PrSbTe to elucidate its electronic structure and topological characteristics. Magnetic and thermal characterizations indicate the presence of well-localized $4f$ states with the absence of any discernible phase transition down to 2 K. The ARPES results reveal the presence of gapless Dirac crossings that correspond to a nodal-line along the $X\text{−}R$ direction in the three-dimensional Brillouin zone. Furthermore, Dirac crossing that makes up the nodal line, which forms a diamond-shaped nodal plane centered at the center of the Brillouin zone, is also identified within the experimental resolution. This study on the electronic structure of PrSbTe contributes to the understanding of the pivotal role played by spin-orbit coupling in the context of the $RE\mathrm{SbTe}$ family of materials.

Magnetic damping is one of the most important parameters governing the critical current density for current-induced magnetization switching in spin-torque devices. The anisotropic nature of magnetic damping, influenced by both intrinsic and extrinsic factors, has been demonstrated in both theoretical and experimental studies. Nevertheless, understanding the underlying mechanisms of magnetic damping anisotropy, particularly in the bilayer structure of a ferromagnet and a nonmagnetic metal, remains an open challenge. In this study, we investigate the crystallographic dependence of magnetic damping in an epitaxial Cr/Fe bilayer, which has negligible magnetocrystalline anisotropy. We experimentally extracted the total damping by conducting frequency-dependent ferromagnetic resonance measurement at different in-plane angles. The observed damping reveals a strong angular dependence with an anisotropy of ∼400% in the bilayer. All possible contributions to the total damping will be discussed carefully to clarify the origin of the profound damping anisotropy observed in our Cr/Fe bilayer.

Experimental methods capable of determining the electronic properties of the surfaces of materials suffer from severe limitations, including interference from bulk electronic states, insensitivity to unoccupied states, and the requirement that the material be conducting. In this work, we introduce inelastic H atom scattering as a tool to probe the electronic structures of surfaces, which can be applied to both conducting and nonconducting samples while exhibiting exceptional surface sensitivity. We illustrate the method for the example of Ge(111). The measurements show a semiconducting surface at low temperature that continuously becomes increasingly metallic at high temperature.

Benefiting from the facile fabrication and tunable optoelectronic properties, Se and ${\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ are promising candidates for optoelectronic applications, e.g., thin film solar cells and short-wavelength infrared detectors. However, the trap features of selenium based semiconductors are complicated mainly due to the low crystallinity induced dispersive nature. In particular, the underlying charge transport properties of ${\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ have not been systematically investigated, which is crucial for device optimization in real applications. In this work, we first introduce deep-level transient spectroscopy to characterize Se and Se-based semiconductors, and we also compare it with reverse-bias deep-level transient spectroscopy. It was found that the latter technique can result in much higher transient capacitance signal and better energy resolution. Based on the full analysis of the transient capacitance, we found the introduction of Te into Se can easily form two additional deep trap states around 0.55 eV and 0.75 eV, respectively, and it also explains why the device performance of ${\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ are still suffering from the large dark current and recombination losses.

Twist between neighboring layers and variation of interlayer distance are two extra ways to control the physical properties of stacked two-dimensional van der Waals materials without alteration of chemical compositions or application of external fields compared to their monolayer counterparts. In this work, we explored the dependence of the magnetic states of the untwisted and twisted bilayer 1T-${\mathrm{VX}}_2$ ($X$ = S, Se) on the interlayer distance by density functional theory calculations. We find that, while a magnetic phase transition occurs from interlayer ferromagnetism to interlayer antiferromagnetism either as a function of decreasing interlayer distance for the untwisted bilayer 1T-${\mathrm{VX}}_2$ or after twist, richer magnetic phase transitions consecutively take place for the twisted bilayer 1T-${\mathrm{VX}}_2$ as interlayer distance is gradually reduced. Besides, the critical pressures for the phase transition are greatly reduced in twisted bilayer 1T-${\mathrm{VX}}_2$ compared with the untwisted case. We derived the Heisenberg model with intralayer and interlayer exchange couplings to comprehend the emergence of various magnetic states. Our results point out an easy access toward tunable two-dimensional magnets.

$Abinitio$ calculations show the presence of a strong magnetoelectric interfacial coupling in a ${\mathrm{CaMnO}}_3$ ultrathin film grown on a strained ${\mathrm{BaTiO}}_3$ ferroelectric film. This heterostructure presents a polarization driven magnetic transition from a G-type to an A-type antiferromagnetic structure. Together with this magnetic transition we find a metallic behavior at the interface between these two insulators, where the charge character of the carriers can be tuned from electrons to holes by switching the electric polarization direction of the ferroelectric film. Besides, the electron gas is spin polarized while the hole gas is not.

Bulk ${\mathrm{V}}_2{\mathrm{O}}_3$ features concomitant metal-insulator (MIT) and structural (SPT) phase transitions at $T_{\mathrm{C}}\sim 160$ K. In thin films, where the substrate clamping can impose geometrical restrictions on the SPT, the epitaxial relation between the ${\mathrm{V}}_2{\mathrm{O}}_3$ film and substrate can have a profound effect on the MIT. Here, we present a detailed characterization of domain nucleation and growth across the MIT in (001)-oriented ${\mathrm{V}}_2{\mathrm{O}}_3$ films grown on sapphire. By combining scanning electron transmission microscopy and photoelectron emission microscopy (PEEM), we imaged the MIT with planar and vertical resolution. We observed that upon cooling, insulating domains nucleate at the top of the film, where strain is lowest, and expand downwards and laterally. This growth is arrested at a critical thickness of 50 nm from the substrate interface, leaving a persistent bottom metallic layer. As a result, the MIT cannot take place in the interior of films below this critical thickness. However, PEEM measurements revealed that insulating domains can still form on a very thin superficial layer at the top interface. Our results demonstrate the intricate spatial complexity of the MIT in clamped ${\mathrm{V}}_2{\mathrm{O}}_3$, especially the strain-induced large variations along the $c$ axis. Engineering the thickness-dependent MIT can provide an unconventional way to build out-of-plane geometry devices by using the persistent bottom metal layer as a native electrode.

The advent of multiprincipal element alloys (MPEAs) has sparked significant interest in physical metallurgy. These alloys, distinguished by their high configurational entropy aimed at minimizing free energy, have raised intriguing questions regarding the interplay of chemical disorder and chemical short-range order (CSRO) in kinetics of MPEAs. Particularly, the widely assumed phenomenon of sluggish diffusion has come under scrutiny. Here we employ atomistic simulations to quantitatively evaluate the impact of CSRO on diffusion in a representative medium-entropy alloy. We introduce a physical order parameter to establish a quantitative correlation between CSRO and diffusivity. Surprisingly, our simulations suggest that CSRO, rather than chemical disorder, serves to suppress and localize defect diffusion. The observed link between diffusion activation energy and CSRO provides a rationale for the observed deceleration in diffusion due to chemical ordering. A linear relationship between diffusion activation energy and activation entropy is unveiled, aligning with the Meyer-Neldel rule, and replicating experimental results. These insights help clarify the intricate role of chemical disorder in plastic deformation of chemically disordered materials.

We present a theoretical study of relative stability, as well as of the magnetic and electronic properties, of ${\mathrm{AgF}}_2$ and ${\mathrm{CuF}}_2$, in two related structural forms: orthorhombic (ambient pressure form of ${\mathrm{AgF}}_2$) and monoclinic (ambient pressure form of ${\mathrm{CuF}}_2$), using density functional theory. We demonstrate that the $P{2}_1/c\to Pbca$ structural transition is associated with a weakening of the intrasheet magnetic superexchange ($|{\mathrm{J}}_{2\mathrm{D}}|$). This finding aligns with the flattening of two-dimensional (2D) layers, a smaller charge-transfer energy, and a stronger admixing of ${\mathrm{Ag}}_{\mathrm{d}}/{\mathrm{Cu}}_{\mathrm{d}}\text{−}{\mathrm{F}}_{\mathrm{p}}$ states in the monoclinic structure, compared to the orthorhombic form. Consequently, monoclinic ${\mathrm{AgF}}_2$ should be targeted in experiments as it is likely to exhibit stronger magnetic coupling than its orthorhombic counterpart. The dynamically stable $P{2}_1/c$ form of ${\mathrm{AgF}}_2$ could be achieved through two alternative paths: by applying negative strain or by rapidly quenching silver(II) difluoride from temperatures above 480 K to low temperatures.

Stiffness and ductility can be achieved by reducing the grain size of ceramics to the nanometer scale. However, the deformation mechanism of nanoceramics is still controversial due to technical limitations. In this paper, via synchrotron radial x-ray diffraction technology combined with a diamond anvil cell, the deformation behavior of nanosized titanium nitride (TiN) has been studied in situ under high-pressure conditions up to 41.3 GPa. Results show that the nano-TiN exhibits a much higher bulk modulus and yield strength than its bulk counterpart. Nano-TiN starts to yield at around 17.3 GPa and the maximum yield strength is approximately 8.7 GPa at 41.3 GPa. The presence of 110 and 001 texture in nanosized TiN was confirmed under pressure, indicating a strong activity of dislocation. This paper suggests that the reduction in grain size to the nanoscale improves both the stiffness and ductility of TiN via a dislocation-mediated mechanism.

We have developed a machine learned interatomic potential for the prototypical austenitic steel ${\mathrm{Fe}}_7{\mathrm{Cr}}_2\mathrm{Ni}$, using the Gaussian approximation potential (GAP) framework. This GAP can model the alloy's properties with close to density functional theory (DFT) accuracy, while at the same time allowing us to access larger length and time scales than expensive first-principles methods. We also extended the GAP input descriptors to approximate the effects of collinear spins (spin GAP), and demonstrate how this extended model successfully predicts structural distortions due to antiferromagnetic and paramagnetic spin states. We demonstrate the application of the spin GAP model for bulk properties and vacancies and validate against DFT. These results are a step towards modeling the atomistic origins of ageing in austenitic steels with higher accuracy.