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.

We report on the results of magnetic susceptibility, electron paramagnetic resonance, and ${}^{207}\mathrm{Pb}$ nuclear magnetic resonance (NMR) studies of the magnetoelectric multiferroic $\mathrm{Pb}\left(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{S}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3$ (PFS) ceramic, as well as its solid solution with $\mathrm{Pb}\left(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3$ (PFN) of different degrees of the 1:1 ordering of magnetic $\mathrm{F}{\mathrm{e}}^{3+}$ and nonmagnetic $\mathrm{S}{\mathrm{b}}^{5+}$ ions. The ordering has been studied by x-ray diffraction (XRD) and NMR methods. In particular, two spectral lines, originating from the ordered and disordered regions, respectively, are resolved in the ${}^{207}\mathrm{Pb}$ NMR spectra. This demonstrates the presence of spatially heterogeneous ordering where ordered regions are embedded into a disordered matrix. Combining XRD and NMR data, we have determined both the long-range order parameter $s$ and the volume fraction of ordered regions $s^{\prime }$ for all investigated samples. The values vary in the range $s=0-0.93$ and $s^{\prime }=0-1$. We have found that the ${}^{207}\mathrm{Pb}$ Fermi contact interaction strongly depends on the disorder in the Fe/Sb positions: whereas it reaches 7.08 MHz in the ordered lattice, it is almost zero in the disordered environment. These results are further supported by the studies of PFS-PFN solid solutions. The analysis of experimental data in terms of density functional theory reveals a noticeably higher hybridization between Pb 6s and Fe 3d orbitals in the ordered case. The ordering of magnetic and nonmagnetic ions has a strong impact on the magnetic properties of PFS, leading to a transformation of the long-range ordered antiferromagnetic phase in chemically ordered samples to the spin glass state already in partially $(s=0.35)$ disordered specimens. In our opinion, the difference in the magnetic properties of PFN and PFS is related to the fact that PFN is completely disordered, in contrast to PFS, which is only partially disordered, with small ordered regions existing in the disordered matrix that prevent the percolation of the nearest-neighbor Fe−Fe exchange interaction across the lattice.

Nominal ${[\mathrm{Fe}\left(t\right)/\mathrm{Ir}\left(t^{\prime }\right)]}_n$ (M/M type), ${[\mathrm{FeOx}\left(t\right)/\mathrm{IrOx}\left(t^{\prime }\right)]}_n$ (O/O), and ${[\mathrm{Fe}\left(t\right)/\mathrm{IrOx}\left(t^{\prime }\right)]}_n$ (M/O) multilayers have been prepared by magnetron sputtering at room temperature. Composition, structure, and magnetic behavior have been analyzed. In the M/M samples, the Fe and Ir phases are identified as bcc and fcc, respectively. The magnetism evolves from bulklike iron to granular behavior as the thickness of the Fe layers decreases. An induced magnetic moment, ferromagnetically coupled to Fe, is observed on Ir by x-ray magnetic circular dichroism (XMCD). Besides, the presence of negative remanent magnetization is observed in the M/M samples. As for the M/O samples, the stronger affinity of iron for oxygen displaces the oxygen atoms giving rise to actual heterostructures that strongly differ from the nominal ones. For similar thickness of the two layers the Fe layer become oxidized while a mixture of metal and oxide phases is found in the Ir layer. The increase of the Fe thickness leads to a metallic Ir layer and a highly coercive ($\sim 4.4$ kOe) core-shell metal-oxide structure in the Fe layers.