Single crystals of ${\mathrm{CrSbSe}}_3$, a structurally pseudo-one-dimensional ferromagnetic semiconductor, were grown using a high-temperature solution growth technique and were characterized by x-ray diffraction, anisotropic temperature- and field-dependent magnetization, temperature-dependent resistivity, and optical absorption measurements. A band gap of 0.7 eV was determined from both resistivity and optical measurements. At high temperatures, ${\mathrm{CrSbSe}}_3$ is paramagnetic and isotropic, with a Curie-Weiss temperature of $\sim 145\mathrm{K}$ and an effective moment of $\sim 4.1{\mu }_B$/Cr. A ferromagnetic transition occurs at $T_c=71$ K. The $a$ axis, perpendicular to the chains in the structure, is the magnetic easy axis, while the chain axis direction, along $b$, is the hard axis. Magnetic isotherms measured around $T_c$ do not follow the behavior predicted by simple mean-field critical exponents for a second-order phase transition. A tentative set of critical exponents is estimated based on a modified Arrott plot analysis, giving $\beta \sim 0.25$, $\gamma \sim 1.38$, and $\delta \sim 6.6$.

Epitaxial layers of CoFeB were grown on MgO by means of laser molecular beam epitaxy using $\mathrm{C}{\mathrm{o}}_{40}\mathrm{F}{\mathrm{e}}_{40}{\mathrm{B}}_{20}$ target. The growth was combined with in situ structural characterization by three-dimensional reciprocal space mapping obtained from reflection high energy electron diffraction (RHEED) data. High-temperature single stage growth regime was adopted to fabricate CoFeB layers. As confirmed by the atomic force microscopy, the surface of CoFeB layers consists of closely spaced nanometer sized islands with dimensions dependent on the growth temperature. As shown by RHEED and XRD analysis, the CoFeB layers grown at high-temperature on MgO(001) possess body centered cubic (bcc) crystal structure with the lattice constant $a=2.87\AA$ close to that of the $\mathrm{C}{\mathrm{o}}_{75}\mathrm{F}{\mathrm{e}}_{25}$ alloy. It was further shown that following the same high-temperature growth technique the MgO/CoFeB/MgO(001) heterostructures can be fabricated with top and bottom MgO layers of the same crystallographic orientation. The CoFeB layers were also grown on the GaN(0001) substrates using MgO(111) as a buffer layer. In this case, the CoFeB layers crystallize in bcc crystal structure with the (111) axis perpendicular to the substrate surface. The magnetic properties of the CoFeB/MgO (001) heterostructures have been investigated by measuring magnetization curves with a vibrating sample magnetometer as well as by performing magneto-optical Kerr effect (MOKE) and ferromagnetic resonance (FMR) studies. FMR spectra were obtained for the variety of the magnetic field directions and typically consisted of a single relatively narrow resonance line. The magnetization orientations and the resonance conditions were calculated in the framework of a standard magnetic energy minimization procedure involving a single $K_{1\mathrm{c}}$ cubic term for the magnetocrystalline anisotropy. This allows a fairly accurate description of the angular dependences of the resonance fields—both in-plane and out-of-plane. It was shown that CoFeB layers exhibit in-plane fourth-order magnetic anisotropy. A two-step magnetization reversal model has been adopted for the CoFeB layers based on the VSM measurement analysis. Magnetization reversal studies performed by polar MOKE indicate that the magnetization lies in-plane in absence of magnetic field. Observed magnetic field dependences of reflected light ellipticity in geometry of longitudinal Kerr effect give convincing evidence for contribution of quadratic in magnetization terms in the dielectric tensor and clearly show the in-plane magnetization rotation.

Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a nondirect gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.

We investigated stability and the electronic structure of extended defects including antisite domain boundaries and stacking faults in the kesterite-structured semiconductors, ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ (CZTS) and ${\mathrm{Cu}}_2{\mathrm{ZnSnSe}}_4$ (CZTSe). Our hybrid density functional theory calculations show that stacking faults in CZTS and CZTSe induce a higher conduction band edge than the bulk counterparts, and thus the stacking faults act as electron barriers. Antisite domain boundaries, however, accumulate electrons as the conduction band edge is reduced in energy, having an opposite role. An Ising model was constructed to account for the stability of stacking faults, which shows the nearest-neighbor interaction is stronger in the case of the selenide.

Modification of materials using ion beams has become a widespread route to improve or design materials for advanced applications, from ion doping for microelectronic devices to emulation of nuclear reactor environments. Yet, despite decades of studies, major issues regarding ion/solid interactions are not solved, one of them being the lattice-strain development process in irradiated crystals. In this work, we address this question using a consistent approach that combines x-ray diffraction (XRD) measurements with both molecular dynamics (MD) and rate equation cluster dynamics (RECD) simulations. We investigate four distinct materials that differ notably in terms of crystalline structure and nature of the atomic bonding. We demonstrate that these materials exhibit a common behavior with respect to the strain development process. In fact, a strain build-up followed by a strain relaxation is observed in the four investigated cases. The strain variation is unambiguously ascribed to a change in the defect configuration, as revealed by MD simulations. Strain development is due to the clustering of interstitial defects into dislocation loops, while the strain release is associated with the disappearance of these loops through their integration into a network of dislocation lines. RECD calculations of strain depth profiles, which are in agreement with experimental data, indicate that the driving force for the change in the defect nature is the defect clustering process. This study paves the way for quantitative predictions of the microstructure changes in irradiated materials.

Chemically relevant compositions (CRCs) and atomic arrangements of inorganic compounds have been collected as inorganic crystal structure databases. Machine learning is a unique approach to search for currently unknown CRCs from vast candidates. Herein we propose matrix- and tensor-based recommender system approaches to predict currently unknown CRCs from database entries of CRCs. Firstly, the performance of the recommender system approaches to discover currently unknown CRCs is examined. A Tucker decomposition recommender system shows the best discovery rate of CRCs as the majority of the top 100 recommended ternary and quaternary compositions correspond to CRCs. Secondly, systematic density functional theory (DFT) calculations are performed to investigate the phase stability of the recommended compositions. The phase stability of the 27 compositions reveals that 23 currently unknown compounds are newly found to be stable. These results indicate that the recommender system has great potential to accelerate the discovery of new compounds.

We study rectangular graphene flakes using mean field states as the basis for a configuration interaction calculation, which allows us to analyze the low lying electronic excited states including electron correlations beyond the mean field level. We find that the lowest energy transition is polarized along the long axis of the flake, but the charge distributions involved in these transitions are invariably localized on the zigzag edges. We also investigate the impact of both short and long range impurity potentials on the optical properties of these systems. We predict that even a weak impurity localized at a zigzag edge of the flake can have a significant—and often dramatic—effect on its optical properties. This is in contrast to impurities localized at armchair edges or central regions of the flake, for which we predict almost no change to the optical properties of the flake even with strong impurity potentials.

Perovskite metal-organic frameworks (MOFs) have recently emerged as potential candidates for multiferroicity. However, the compounds synthesized so far possess only weak ferromagnetism and low polarization. Additionally, the very low magnetic transition temperatures $\left(T_c\right)$ also pose a challenge to the application of the materials. We have computationally designed a mixed metal perovskite MOF—${\left[{\mathrm{C}(\mathrm{NH}}_2\right)}_3]\left[\left({\mathrm{Cu}}_{0.5}{\mathrm{Mn}}_{0.5}\right){\left(\mathrm{HCOO}\right)}_3\right]$—that is predicted to have magnetization two orders of magnitude larger than its parent $\left(\left[\mathrm{C}{\left({\mathrm{NH}}_2\right)}_3\right]\left[\mathrm{Cu}{\left(\mathrm{HCOO}\right)}_3\right]\right)$, a significantly larger polarization (9.$9\mu \mathrm{C}/{\mathrm{cm}}^2)$, and an enhanced $T_c$ of up to 56 K, unprecedented in perovskite MOFs. A detailed study of the magnetic interactions revealed a mechanism leading to the large moments as well as the increase in the $T_c$. Mixing a non-Jahn-Teller ion $\left({\mathrm{Mn}}^{2+}\right)$ into a Jahn-Teller host $\left({\mathrm{Cu}}^{2+}\right)$ leads to competing lattice distortions which are directly responsible for the enhanced polarization. The MOF is thermodynamically stable as evidenced by the computed enthalpy of formation and can likely be synthesized. Our work represents a first step towards rational design of multiferroic perovskite MOFs through the largely unexplored mixed metal approach.

We studied a low-field giant magnetostrictive spin-flop transition in a colossal magnetoresistance manganite ${\mathrm{La}}_{1.4}{\mathrm{Sr}}_{1.6}{\mathrm{Mn}}_2{\mathrm{O}}_7$ using resonant soft x-ray diffraction and soft x-ray absorption spectroscopy at the Mn $L_{2,3}$ edge. The spin-flop transition is induced by an instability of magnetocrystalline anisotropy near a critical $e_g$ orbital configuration with a balanced occupation in $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ states, which contribute in-plane and out-of-plane orbital angular momenta, respectively. The magnetic field drives a certain change in the orbital occupation with lattice distortion to switch the magnetic anisotropy, resulting in the spin-flop transition. These results provide a comprehensive mechanism of interplay between spin, orbital, and lattice degrees of freedom to realize a low-field giant magnetoelasticity.

Crystal structure, thermal properties, and magnetic properties were studied systematically in ${\mathrm{Y}}_{1-x}\mathrm{P}{\mathrm{r}}_x\mathrm{Cr}{\mathrm{O}}_3$ with $0.0\le x\le 0.3$ compositions. Magnetic susceptibility and specific-heat measurements show an increase in the antiferromagnetic transition temperature $\left(T_N\right)$ as Pr is substituted in the Y sites and notable magnetic features are observed below $T_N$. Strong coupling between magnetic and crystalline parameters is observed in a small range of Pr compositions. A small perturbation in the lattice parameters by a Pr ion is sufficient to induce a spin-reorientation transition followed by magnetization reversal to finally induce the exchange-bias effect. The spin-reorientation temperature $\left(T_{\mathrm{SR}}\right)$ is increased from 35 to 149 K for $0.025\le x\le 0.1$ compositions. It is found that the Cr spin sublattice rotates continuously from $T_{\mathrm{SR}}$ to a new spin configuration at lower temperature. In addition, magnetization reversal is observed at $T^{*}\sim 35\mathrm{K}$ for $x=0.05$ up to $T^{*}\sim 63\mathrm{K}$ for $x=0.20$ composition. The $M\text{−}H$ curves show a negative exchange-bias effect induced by Pr ions, which are observed below 100 K and are more intense at 5 K. At 10 K, the magnetic contribution of the specific heat as well as the ZFC magnetization show the rise of a peak with increasing Pr content. The magnetic anomaly could be associated with the freezing of the Pr magnetic moment randomly distributed at the $4c$ crystallographic site. A clear correspondence between spin reorientation, magnetization reversal, and exchange-bias anisotropy with the tilting and octahedral distortion is also discussed.

Ferroelectric materials are an alternative to semiconductor-based photovoltaics and offer the advantage of above bandgap photovoltage generation. However, there are few known compounds, and photovoltaic efficiencies remain low. Here, we report the discovery of a photovoltaic effect in undoped lead magnesium niobate–lead titanate crystal and a significant improvement in the photovoltaic response under suitable electric fields and temperatures. The photovoltaic effect is maximum near the electric-field-driven ferroelectric dipole reorientation, and increases threefold near the Curie temperature $\left(T_c\right)$. Moreover, at ferroelectric saturation, the photovoltaic response exhibits clear remanent and transient effects. The transient-remanent combinations together with electric and thermal tuning possibilities indicate photoferroelectric crystals as emerging elements for photovoltaics and optoelectronics, relevant to all-optical information storage and beyond.

Ferroelectricity was recently reported in thin films with several compositions in the $\mathrm{Hf}{\mathrm{O}}_2\text{−}\mathrm{Zr}{\mathrm{O}}_2$ system with orthorhombic crystal structure. In the present paper we study the growth by pulsed laser deposition and the structural characterization of strained epitaxial $\mathrm{H}{\mathrm{f}}_{0.5}\mathrm{Z}{\mathrm{r}}_{0.5}{\mathrm{O}}_2$ films on $\left(001\right)$-oriented yttria-stabilized zirconia (YSZ) substrates. We have determined the conditions for the coherent growth and correlated the deposition parameters with the films structure and microstructure studied through a combination of x-ray diffraction, electron backscatter diffraction, and scanning transmission electron microscopy. In the range of experimental parameters explored, all the films show monoclinic structure with distorted lattice parameters relative to bulk.

We investigate the evolution of the cross-plane thermal conductivity $\kappa$ of superlattices (SLs) as interfaces change from perfectly abrupt to totally intermixed, by using nonequilibrium molecular dynamics simulations in combination with the spectral heat current calculations. We highlight the role of surface-interdiffusion-driven intermixing by calculating the $\kappa$ of SLs with changing interface roughness, whose tuning allows for $\kappa$ values much lower than the “alloy limit” and the abrupt interface limit in same cases. The interplay between alloy and interface scattering in different frequency ranges provides a physical basis to predict a minimum of thermal conductivity. More specifically, we also explore how the interface roughness affects the thermal conductivities for SL materials with a broad span of atomic mass and bond strength. In particular, we find that (i) only when the “spacer” thickness of SLs increases up to a critical value, $\kappa$ of rough SLs can break the corresponding “alloy limit,” since SLs with different “spacer” thickness have different characteristic length of phonon transport, which is influenced by surface-interdiffusion-driven intermixing to different extend. (ii) Whether $\kappa$ changes monotonically with interface roughness strongly depends on the period length and intrinsic behavior of phonon transport for SL materials. Especially, for the SL with large period length, there exists an optimal interface roughness that can minimize the thermal conductivity. (iii) Surface-interdiffusion-driven intermixing is more effective in achieving a low $\kappa$ below the alloy limit for SL materials with large mass mismatch than with small one. (iv) It is possible for SL materials with large lattice mismatch (i.e., bond strength) to design an ideally abrupt interface structure with $\kappa$ much below the alloy limit. These results have clear implications for optimization of thermal transport for heat management and for the development of thermoelectric materials.

A wide range of technological applications of lithium tantalate (LT) is closely related to the defect chemistry. In literature, several intrinsic defect models have been proposed. Here, using a combinational approach based on DFT and solid-state NMR, we demonstrate that distribution of electric field gradients (EFGs) can be employed as a fingerprint of a specific defect configuration. Analyzing the distribution of ${}^7\mathrm{Li}$ EFGs, the FT-IR and electron spin resonance (ESR) spectra, and the ${}^7\mathrm{Li}$ spin-lattice relaxation behavior, we have found that the congruent LT samples provided by two manufacturers show rather different defect concentrations and distributions although both were grown by the Czochralski method. After thermal treatment hydrogen out-diffusion and homogeneous distribution of other defects have been observed by ESR, NMR, and FT-IR. The defect structure in one of two congruent LT crystals after annealing has been identified and proved by defect formation energy considerations, whereas the more complex defect configuration, including the presence of extrinsic defects, has been suggested for the other LT sample. The approach of searching the EFG fingerprints from DFT calculations in NMR spectra can be applied for identifying the defect clusters in other complex oxides.

It has been shown that W in its resistive form possesses the largest spin-Hall ratio among all heavy transition metals, which makes it a good candidate for generating efficient dampinglike spin-orbit torque (DL-SOT) acting upon adjacent ferromagnetic or ferrimagnetic (FM) layer. Here we provide a systematic study on the spin transport properties of W/FM magnetic heterostructures with the FM layer being ferromagnetic $\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ or ferrimagnetic $\mathrm{C}{\mathrm{o}}_{63}\mathrm{T}{\mathrm{b}}_{37}$ with perpendicular magnetic anisotropy. The DL-SOT efficiency $|{\xi }_{\mathrm{DL}}|$, which is characterized by a current-induced hysteresis loop shift method, is found to be correlated to the microstructure of W buffer layer in both W/$\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ and W/$\mathrm{C}{\mathrm{o}}_{63}\mathrm{T}{\mathrm{b}}_{37}$ systems. Maximum values of $|{\xi }_{\mathrm{DL}}|\approx 0.144$ and $|{\xi }_{\mathrm{DL}}|\approx 0.116$ are achieved when the W layer is partially amorphous in the W/$\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ and W/$\mathrm{C}{\mathrm{o}}_{63}\mathrm{T}{\mathrm{b}}_{37}$ heterostructures, respectively. Our results suggest that the spin-Hall effect from resistive phase of W can be utilized to effectively control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism.

Goldstone modes are massless particles resulting from spontaneous symmetry breaking. Although such modes are found in elementary particle physics as well as in condensed-matter systems like superfluid helium, superconductors, and magnons, structural Goldstone modes are rare. Epitaxial strain in thin films can induce structures and properties not accessible in bulk and has been intensively studied for (001)-oriented perovskite oxides. Here we predict Goldstone-like phonon modes in (111)-strained $\mathrm{SrMn}{\mathrm{O}}_3$ by first-principles calculations. Under compressive strain the coupling between two in-plane rotational instabilities gives rise to a Mexican hat-shaped energy surface characteristic of a Goldstone mode. Conversely, large tensile strain induces in-plane polar instabilities with no directional preference, giving rise to a continuous polar ground state. Such phonon modes with $U$(1) symmetry could emulate structural condensed-matter Higgs modes. The mass of this Higgs boson, given by the shape of the Mexican hat energy surface, can be tuned by strain through proper choice of substrate.

We report measurements of electron transport phenomena in the intrinsic ferromagnetic semiconductor GdN doped with $1.3\times {10}^{21}{\mathrm{cm}}^{-3}$ electrons. The conductivity, carrier concentration, and thermoelectric power are compared with expectations based on an LSDA+$U$ band structure. In the ferromagnetic state the carriers fill the majority-spin conduction band pockets to the bottom of the minority-spin band. The resistance implies an electron mobility of 18 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at zero temperature, and in turn a mean-free path of 10–30 nm. Spin disorder scattering rapidly reduces the mobility near the 70 K Curie temperature ($T_{\text{C}}$). The thermoelectric power is negative in the paramagnetic phase, as expected for a $n$-type conductor, with a magnitude that is in agreement with the Fermi energy implied by the band structure. The thermopower reverses sign to be positive in the ferromagnetic phase, which correlates with a strongly temperature-dependent electron diffusion from spin-disorder scattering that increases rapidly as the temperature rises toward $T_{\text{C}}$.

The intertwined and competing energy scales of various interactions in rare-earth nickelates $R\mathrm{Ni}{\mathrm{O}}_3$ ($R=\mathrm{La}$ to Lu) hold potential for a wide range of exotic ground states realized upon structural modulation. Using terahertz (THz) spectroscopy, the low-energy dynamics of a novel non-Fermi liquid (NFL) metallic phase induced in compressive $\mathrm{PrNi}{\mathrm{O}}_3$ thin film was studied by evaluating the quasiparticle scattering rate in the light of two distinct models over a wide temperature range. First, evaluating THz conductivity in the framework of extended Drude model, the frequency-dependent scattering rate is found to deviate from the Landau Fermi liquid (LFL) behavior, thus, suggesting NFL-like phase at THz frequencies. Second, fitting THz conductivity to the multiband Drude-Lorentz model reveals the band-dependent scattering rates and provides microscopic interpretation of the carriers contributing to the Drude modes. This is first evidence of NFL-like behavior in nickelates at THz frequencies consistent with dc conductivity, which also suggests that THz technology is indispensable in understanding emerging electronic phases and associated phenomena. We further demonstrate that the metal-insulator transition in nickelates has the potential to design efficient THz modulators.