The anharmonic lattice dynamics of oxide and halide perovskites play a crucial role in their mechanical and optical properties. Raman spectroscopy is one of the key methods used to study these structural dynamics. However, despite decades of research, existing interpretations cannot explain the temperature dependence of the observed Raman spectra. We demonstrate the nonmonotonic evolution with temperature of the scattering intensity and present a model for second-order Raman scattering that accounts for this unique trend. By invoking a low-frequency anharmonic feature, we are able to reproduce the Raman spectral line shapes and integrated intensity temperature dependence. Numerical simulations support our interpretation of this low-frequency mode as a transition between two minima of a double-well potential surface. The model can be applied to other dynamically disordered crystal phases, providing a better understanding of the structural dynamics, leading to favorable electronic, optical, and mechanical properties in functional materials.

Oxygen deficiency plays an essential role in tailoring the intriguing physical properties of complex oxides that can be harnessed in the cutting-edge energy and information technologies. A fine and continuous control of the oxygen content to modulate the rich physical properties of the complex oxides is highly desirable for both basic research and modern electronic technologies. Here, we report a recently developed ionic field-effect transistor (iFET) device with a solid oxygen ionic conductor (OIC) as the gate dielectric to finely and continuously control the oxygen content of the oxides. We realize a reversible structural transformation between brownmillerite $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ and perovskite $\mathrm{SrCo}{\mathrm{O}}_3$ by driving oxygen ions in and out of the samples with the OICFET at room temperature. With continuously changing the oxygen content, a comprehensive phase diagram of the $\mathrm{SrCo}{\mathrm{O}}_{2.5+x}$ $\left(0\le x\le 0.5\right)$ thin film from an antiferromagnetic insulator to a ferromagnetic metal is obtained in a single device. Our work provides a promising route to continuously and reversibly regulate the oxygen content and to explore unique physical states in the correlated electron oxides, and represents an important step towards the realization of a long-standing goal to develop complex oxide based devices that can be integrated with existing technologies.

For large-scale atomistic simulations of magnetic materials, the interplay of atomic and magnetic degrees of freedom needs to be described with high computational efficiency. Here we present an analytic bond-order potential (BOP) for iron-cobalt, an interatomic potential based on a coarse-grained description of the electronic structure. We fitted BOP parameters to magnetic and non-magnetic density functional theory (DFT) calculations of Fe, Co, and Fe-Co bulk phases. Our BOP captures the electronic structure of magnetic and nonmagnetic Fe-Co phases. It provides accurate predictions of structural stability, elastic constants, phonons, point and planar defects, and structural transformations. It also reproduces the DFT-predicted sequence of stable ordered phases peculiar to Fe-Co and the stabilization of B2 against disordered phases by magnetism. Our Fe-Co BOP is suitable for atomistic simulations with thousands and millions of atoms.

The interlayer exchange coupling confers specific properties to synthetic antiferromagnets that make them suitable for several applications of spintronics. The efficient use of this magnetic configuration requires an in-depth understanding of the magnetic properties and their correlation with the material structure. Here we establish a reliable procedure to quantify the interlayer exchange coupling and the intralayer exchange stiffness in synthetic antiferromagnets; we apply it to the ultrasmooth and amorphous ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ (5–40 nm)/Ru/ ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ material platform. The complex interplay between the two exchange interactions results in a gradient of the magnetization orientation across the thickness of the stack which alters the hysteresis and the spin wave eigenmodes of the stack in a nontrivial way. We measured the field dependence of the frequencies of the first four spin waves confined within the thickness of the stack. We modeled these frequencies and the corresponding thickness profiles of these spin waves using micromagnetic simulations. The comparison with the experimental results allows us to deduce the magnetic parameters that best account for the sample behavior. The exchange stiffness is established to be $16\pm 2$ pJ/m, independent of the ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ thickness. The interlayer exchange coupling starts from $-1.7$ $\mathrm{mJ/}{\mathrm{m}}^2$ for the thinnest layers, and it can be maintained above $-1.3$ $\mathrm{mJ/}{\mathrm{m}}^2$ for CoFeB layers as thick as 40 nm. The comparison of our method with earlier characterizations using the sole saturation fields argues for a need to revisit the tabulated values of interlayer exchange coupling in thick synthetic antiferromagnets.

Point defects can strongly suppress the thermal conductivity $\kappa$ of solid materials, which is crucial for a broad range of applications, such as thermal management of electronic devices and thermoelectrics. Understanding thermal transport in materials containing point defects often relies on atomistic simulations based on density functional theory (DFT) or empirical potentials (EPs). However, modeling thermal transport in defective materials using DFT is very computationally expensive or even prohibitive due to the breaking of crystal symmetry while EPs suffer from low accuracy. Recently, machine learning has been applied to the development of interatomic potentials, offering opportunities to model defective systems accurately and efficiently. Here, we present a Gaussian approximation potential (GAP) developed for crystalline cubic boron arsenide (c-BAs) with vacancies, which can achieve DFT-level accuracy in predicting its $\kappa$ and phonon transport properties at four orders of magnitude reduced computational cost, especially for phonon-vacancy and four-phonon scatterings. Particularly, we applied the GAP to investigate the effect of vacancies on the $\kappa$ of c-BAs by considering both three-phonon and four-phonon scattering. Special attention was paid to the competition between phonon-vacancy and four-phonon scattering, which tend to decrease and increase the temperature dependence of $\kappa$, respectively. Specifically, when the vacancy concentration is much lower than $0.07\%$ $(2.56\times {10}^{19}{\mathrm{cm}}^{-3})$, four-phonon scattering plays stronger roles in determining the temperature dependence of $\kappa$. As the vacancy concentration increases to $0.07\%$, the temperature dependence of $\kappa$ becomes close to that considering only three-phonon scattering, indicating the comparable effect of phonon-vacancy and four-phonon scattering. As the vacancy concentration further increases, the phonon-vacancy scattering becomes more dominant and pushes the $\kappa$ towards a temperature-independent behavior. Our work deepens the understanding of the phonon scattering landscape in c-BAs with vacancies and will be helpful for tailoring its thermal properties. Atomistic simulations combined with machine learning interatomic potentials are expected to be able to greatly advance the understanding of thermal transport in defective materials.

We present a study on the structural and magnetic properties of $\mathrm{Lu}\left({\mathrm{Fe}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Cr}}_{0.2}{\mathrm{Ni}}_{0.2}\right){\mathrm{O}}_3$ (Lu5BO) high-entropy oxide perovskite thin films. We use synchrotron-based x-ray absorption spectroscopy employing x-ray magnetic circular and linear dichroism (XMCD and XMLD) to perform an element-sensitive study on single-crystal epitaxial Lu5BO thin films. Together with XMCD magnetometry, the results reveal dominant antiferromagnetic order with a transition temperature around 100 K.

In the search for cost-effective ways to produce green hydrogen, harnessing the most abundant renewable energy source—sunlight—directly through photoelectrochemical water splitting is highly desirable. Cuprous oxide (${\mathrm{Cu}}_2\mathrm{O}$) has proven itself as a material with great potential for the hydrogen evolution reaction (HER) and stands out by its Earth abundance and low processing cost. In this photoelectron spectroscopy study we investigated the electron dynamics in a heterostructure comprised of a ${\mathrm{Cu}}_2\mathrm{O}$ surface oxide grown on Cu(111) underneath a hexagonal boron nitride ($h$-BN) film, which replaces a conventional capping layer for corrosion protection. Our results show that it can be a viable cuprous oxide-based photoelectrode material. The $h$-BN film stays intact during the oxidation, and the oxide layer forms an ordered structure with defined valence and conduction bands. The conduction band is at a suitable energy to drive the HER for water splitting, but the photoexcited electrons display short lifetimes. Electrons are mainly excited in the copper substrate and are then captured in long-lived defect states in the ${\mathrm{Cu}}_2\mathrm{O}$ layer. The water splitting efficiency of this photocathode may still be improved by reducing the defect density.

Molybdenum ditelluride $\left(\mathrm{Mo}{\mathrm{Te}}_2\right)$ has received considerable attention as a two-dimensional material due to its intriguing physical properties and potential for applications in a variety of devices. The optical properties of $2\text{H-}$ and $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoTe}}_2$ single crystals are investigated in this study using spectroscopic ellipsometry over the spectra range 0.73–6.42 eV and at temperatures of 4.2–500 K. According to the optical absorption spectra at 4.2 K, $2\text{H-MoT}{\mathrm{e}}_2$ has an indirect band gap of $1.06\pm 0.01$ eV, whereas $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoTe}}_2$ exhibits semimetal behavior. Furthermore, $2\text{H-MoT}{\mathrm{e}}_2$ has three distinct intralayer $A_{1\mathrm{s}},A_{2\mathrm{s}},\mathrm{and}{B}_{1\mathrm{s}}$ excitons, and one interlayer $A_{\mathrm{IL}}$ exciton at $1.136\pm 0.002, 1.181\pm 0.001, 1.469\pm 0.006$, and $1.21\pm 0.01$ eV, respectively. The spin-orbit coupling energy of $2\text{H-MoT}{\mathrm{e}}_2$ is 333 meV and is independent of temperature. By contrast, $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoTe}}_2$ has two low-energy optical transitions at $0.78\pm 0.01$ and $1.45\pm 0.01$ eV, which are redshifted and broadened upon an increase in temperature, indicating the increased electron-phonon interaction. We characterize the optical transitions of both phases of $\mathrm{Mo}{\mathrm{Te}}_2$ by comparing experimental data with the results of first-principles calculations.

To design materials for extreme applications, it is important to understand and predict phase transitions and their influence on material properties under high pressures and temperatures. Atomistic modeling can be a useful tool to assess these behaviors. However, this can be difficult due to the lack of fidelity of the interatomic potentials in reproducing this high pressure and temperature extreme behavior. Here, a hybrid EAM-R—which is the combination of embedded atom method (EAM) and rapid artificial neural network potential—for Tin (Sn) is described which is capable of accurately modeling the complex sequence of phase transitions between different metallic polymorphs as a function of pressure. This hybrid approach ensures that a basic empirical potential like EAM is used as a lower energy bound. By using the final activation function, the neural network contribution to energy must be positive, assuring stability over the whole configuration space. This implementation has the capacity to reproduce density functional theory results at 6 orders of magnitude slower than a pair potential for molecular dynamics simulation, including elastic and plastic characteristics and relative energies of each phase. Using calculations of the Gibbs free energy, it is demonstrated that the potential precisely predicts the experimentally observed phase changes at temperatures and pressures across the whole phase diagram. At 10.2 GPa, the present potential predicts a first-order phase transition between body-centered tetragonal (BCT) $\beta$-Sn and another polymorph of BCT-Sn. This structure transforms into body-centered cubic near the experimentally reported value at 33 GPa. Thus, the Sn potential developed in this paper can be used to study complex deformation mechanisms under extreme conditions of high pressure and strain rates unlike existing potentials. Moreover, the framework developed in this paper can be extended for different material systems with complex phase diagrams.

Magnetic skyrmions are topologically-protected spin textures that manifest in certain noncentrosymmetric ferromagnets under the right conditions of temperature and field. In thin-film skyrmion hosts, demagnetization effects combined with geometric confinement can result in two distinct types of spin textures: those with their axis of symmetry in the plane of the film (IP) and those with their axis pointing out-of-plane (OOP). Here we present Lorentz transmission electron microscopy evidence in conjunction with numerical modeling showing a flop transition between IP and OOP skyrmions in ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$ at room temperature. We show that this skyrmion flop transition is controllable via the angle of the external field relative to the film normal and we illustrate how this transition depends on thickness. Finally, we propose a skyrmion-writing device that utilizes the details of this transition.

The electronic structures of the UT and CT phases of both ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ and the counterpart ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ are studied by density functional theory and tight-binding models. We find the 4p states of As contribute substantially to the relative instability between $(\pi ,\pi ,\pi )$ and $(0,0,\pi )$ of Pauli susceptibility ${\chi }_0$, which may result in fluctuations mediating the pairing between electrons. This means that considering only the 3d states of Fe is not enough to capture the superconducting properties of iron-based superconductors. This finding is supported by the even higher relative instability at $(\pi ,\pi ,\pi )$ of interorbital Pauli susceptibility between the 3d orbital of Fe and 4p orbital of As compared with the total intraorbital and interorbital Pauli susceptibility from the 3d orbital of Fe, indicating that the 4p states of As affect ${\chi }_0$ considerably. Moreover, while the 3d states of Ca are found to contribute positively to the relative instability at $(\pi ,\pi ,\pi )$, the 5d states of La contribute negatively to the relative instability at $(\pi ,\pi ,\pi )$, illustrating the the 3d states of Ca and 5d states of La play contrary contributions to superconductivity. Our results provide qualitative criteria for the superconducting properties of ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CaFe}}_2{\mathrm{As}}_2$, and they reveal that both the 4p states of As and interlayer cation states should not be neglected in further studies on superconductivity of iron-based superconductors.

Layered semimetallic van der Waals material $\mathrm{Ti}{\mathrm{Se}}_2$ has attracted a lot of attention because of interplay of a charge density wave (CDW) state and superconductivity. Its sister compound ${\mathrm{TiS}}_2$, being isovalent to $\mathrm{Ti}{\mathrm{Se}}_2$ and having the same crystal structure, shows a semiconducting behavior. The natural question rises—what happens at the transition point in $\mathrm{Ti}{\mathrm{Se}}_{2-x}{\mathrm{S}}_x$, which is expected for x close to one. Here we report the growth and characterization of TiSeS single crystals and the study of the electronic structure using density functional theory (DFT) and angle-resolved photoemission (ARPES). We show that TiSeS single crystals have the same morphology as $\mathrm{Ti}{\mathrm{Se}}_2$. Transport measurements reveal a metallic state; no evidence of CDW was found. DFT calculations suggest that the electronic band structure in TiSeS is similar to that of $\mathrm{Ti}{\mathrm{Se}}_2$, but the electron and hole pockets in TiSeS are much smaller. The ARPES results are in good agreement with the calculations.

Although the concept of high entropy has flourished in the development of engineeringly important multiprincipal elemental crystalline materials, the role of entropy is controversial in driving possible disorder-order transition. Here we provide a thermodynamic perspective on this transition based on absolute free energy calculations of a list of equilibrated CoCrNi configurations extracted from the annealing history. A set of new physical quantities associated with the degree of anharmonicity, chemical short-range order, and Shannon-entropy-informed disorder temperatures are proposed to signify the disorder-order transition. The interrelationships between these thermodynamic quantities consistently suggest a disorder-order transition that is supported by experimental observation. The analysis further recognizes the critical role of the anharmonic effect in driving the random solid solution to a chemically short-range ordered phase. The free energy insights help us to understand the formation mechanism of locally ordered structures emerged from the solid solution of ideal mixing.

Alloying indium into the topological insulator ${\mathrm{SnBi}}_2{\mathrm{Te}}_4$ induces bulk superconductivity with critical temperatures $T_c$ up to 1.85 K and upper critical fields up to about 14 kOe. This is confirmed by electrical resistivity, heat capacity, and magnetic susceptibility measurements. The heat capacity shows a discontinuity at $T_c$ and temperature dependence below $T_c$ consistent with weak coupling BCS theory, and suggests a superconducting gap near 0.25 meV. The superconductivity is type-II and the topological surface states have been verified by photoemission. A simple picture suggests analogies with the isostructural magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, in which a natural heterostructure hosts complementary properties on different sublattices, and motivates new interest in this large family of compounds. The existence of both topological surface states and superconductivity in ${\mathrm{Sn}}_{1-\mathrm{x}}{\mathrm{In}}_{\mathrm{x}}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ identifies these materials as promising candidates for the study of topological superconductivity.

The dislocation microstructure developing during plastic deformation strongly influences the stress-strain properties of crystalline materials. Resent theoretical investigations based on the 2D continuum theory of straight parallel edge dislocations were able to predict a periodic dislocation microstructure. The results obtained, however, can only be considered as a very first step toward the understanding of the origin of dislocation patterning. One of the most challenging problems is the modeling of the formation of the fractal like dislocation microstructure. So, it is crucial to determine the statistical properties of such a structure developing at ideal multiple slip orientation. In the paper, by x-ray line profile analysis and the method of high resolution electron backscatter diffraction (HR-EBSD) a complex experimental characterization of dislocation microstructure developing in uniaxially compressed Cu single crystals is presented. With these methods, the maps of the internal stress, the Nye tensor, and the geometrically necessary dislocation (GND) density were determined at different load levels. It is found from the fractal analysis of the GND maps that the fractal dimension of the cell structure is decreasing with increasing average spatial dislocation density fluctuation. Moreover, it is shown that the evolution of different types of dislocations can be successfully monitored with the HR-EBSD-based technique.

Short-range order (SRO) is an important structural characteristic in multiprincipal element alloys (MPEAs) that plays a crucial role in their superior mechanical and physical properties. The development of SRO is usually tangled with structural evolution associated with diffusion-induced atomic transport, making it challenging to accurately determine the diffusion coefficients in MPEAs based on conventional modeling techniques. In this study, we combine machine learning with kinetic Monte Carlo to compute the self-diffusion coefficients in NiCoCr and NiFeCr MPEAs with distinct SRO tendencies under long-time structural evolution. Our results show that the long-term diffusion process governed by thermodynamic and kinetic factors can promote SRO formation due to chemically biased exchange between vacancy and specific components in the alloy. We further demonstrate that accurate diffusion coefficients in MPEAs can only be obtained through long-term simulations allowing for sufficient structural evolution. This study underlines the shortcomings of the presently widely employed techniques to study diffusion in MPEAs and highlights the intricate coupling between SRO and diffusion.

Hall effect measurements in doped polymer semiconductors are widely reported but are difficult to interpret due to screening of Hall voltages by carriers undergoing incoherent transport. Here, we propose a refined analysis for such Hall measurements, based on measuring the Hall coefficient as a function of temperature, and modeling carriers as existing in a regime of variable “deflectability” (i.e., how strongly they “feel” the magnetic part of the Lorentz force). By linearly interpolating each carrier between the extremes of no deflection and full deflection, we demonstrate that it is possible to extract the (time-averaged) concentration of deflectable charge carriers, $\langle n_d\rangle$, the average, temperature-dependent mobility of those carriers, $\langle {\mu }_d\rangle \left(T\right)$, as well as the ratio of conductivity that comes from such deflectable transport, $d\left(T\right)$. Our method was enabled by the construction of an improved AC Hall measurement system, as well as an improved data extraction method. We measured Hall bar devices of ion-exchange doped films of PBTTT-${\mathrm{C}}_{14}$ from 10–300 K. Our analysis provides evidence for the proportion of conductivity arising from deflectable transport, $d\left(T\right)$, increasing with doping level, ranging between 15.4% and 16.4% at room temperature. When compared to total charge-carrier-density estimates from independent methods, the values of $\langle n_d\rangle$ extracted suggest that carriers spend $\sim 37\%$ of their time of flight being deflectable in the most highly doped of the devices measured here. The extracted values of $d\left(T\right)$ being less than half this value thus suggest that the limiting factor for conductivity in such highly doped devices is carrier mobility, rather than concentration.

The off-normal ion irradiation of semiconductor materials is seen to induce nanopatterning effects. Different theories are proposed to explain the mechanisms that drive self-reorganization of amorphizable surfaces. One of the prominent hypothesis associates formation of nanopatterning with the changes of sputtering characteristics caused by changes in surface morphology. At ultralow energy, when sputtering is negligible, the Si surface has still been seen to reorganize forming surface ripples with the wave vector is either aligned with the ion beam direction or perpendicular to it. In this work, we investigate the formation of ripples using molecular dynamics in all the three regimes of ripple formation: low angles where no ripples form, intermediate regime where the ripple wave vectors are parallel to the beam, and high angles where they are perpendicular to it. We obtain atom-level insight on how the ion-beam driven atomic dynamics at the surface contributes to organization, or lack of it, in all the different regimes. Results of our simulations agree well with experimental observations in the same range of ultralow energy of ion irradiation.