Controlling polarons and polaron-vacancy interactions (PVIs) in oxides is of vital importance for their physical properties and practical applications. Here, we leverage recent improvements in $\mathrm{DFT}+\mathrm{U}$ to investigate PVI energetics, atomic geometry, and electronic structure while varying two experimentally accessible control parameters. The industrially important $\mathrm{Pb}\left({\mathrm{Zr}}_x{\mathrm{Ti}}_{1-x}\right){\mathrm{O}}_3$ (PZT) system is studied to understand how composition affects static polaron properties, while epitaxial strain is introduced on the $x-y$ plane of the ${\mathrm{PbTiO}}_3$ (PTO) lattice, allowing the $z$ axis to relax. Upon doping PTO with Zr to form PZT, low Zr content results in preferential hybridization of $\mathrm{Zr}-4{\mathrm{d}}_{\mathrm{t}2\mathrm{g}}$ and $\mathrm{Ti}-3{\mathrm{d}}_{\mathrm{t}2\mathrm{g}}$ orbitals and unstable polaron-vacancy complexes (PVCs), despite the similarity between the PZT and PTO lattices. A crossover point occurs at $\sim 29\%$ Zr, where PVCs become stable as $\mathrm{Zr}-4{\mathrm{d}}_{\mathrm{t}2\mathrm{g}}$ and $\mathrm{Ti}-3{\mathrm{d}}_{\mathrm{eg}}$ states begin to separate rather than hybridize. In PTO, we find that increasing compressive strain generally increases polaron stability both in the bulk and in PVCs, while increasing tensile strain has little effect on polaron trapping. We show this is the result of competition between the strain energy and electronic transition energy: Under compressive strain, the electronic transition energy change outcompetes the strain energy, while tensile strain leaves both energies largely unchanged. Thereby, we demonstrate that significant changes to the electronic structure because of orbital hybridization with dopant states are the dominant effect on polaron stability, while the smaller effect of epitaxial strain may be used to more finely tune the polaron stability.

We performed the macroscopic electrical characterization of epitaxially grown, compressively strained ${\mathrm{K}}_{0.7}{\mathrm{Na}}_{0.3}{\mathrm{NbO}}_3$ thin films (70 nm) on ${\mathrm{SrRuO}}_3/\left(110\right){\mathrm{TbScO}}_3$ substrates, displaying monoclinic $\mathit{Mc}$ domains. Hysteresis measurements revealed ferroelectric behavior up to at least ${10}^7$ cycles at room temperature, with a polarization value ($2P$) of $25\mu \mathrm{C}/{\mathrm{cm}}^2$. A double-peak switching behavior was observed, attributed primarily to domain-wall pinning/depinning and back switching, with a wake-up effect analogous to that reported in doped hafnium oxide and lead zirconate titanate systems. Temperature-dependent measurements (100 K—300 K) confirm ferroelectricity at low temperatures, while the double-peak behavior becomes less pronounced due to suppression of thermally activated back switching. Capacitance-voltage measurements reveal an increased dielectric response at higher temperatures, consistent with enhanced domain wall mobility. The asymmetry in coercive voltages additionally points to internal bias fields likely arising from electrode interfaces. Our results provide a comprehensive overview of the switching dynamics of strained ${\mathrm{K}}_{0.7}{\mathrm{Na}}_{0.3}{\mathrm{NbO}}_3$ films and demonstrate ferroelectricity in the $\mathit{Mc}$ phase.

Altermagnetism, as an emerging magnetic phase that combines the strong spin-polarization characteristics of ferromagnets and the zero net magnetization advantage of antiferromagnets, holds significant potential in spintronics applications. CrSb, with its high transition temperature and large spin splitting, has recently garnered significant attention as an altermagnetic candidate. Our nuclear magnetic resonance and nuclear quadrupole resonance measurements reveal the absence of intrinsic magnetic fields at the Sb sites. At low temperature, the spin-lattice relaxation rate indicates there are no magnetic fluctuations from Cr moments. The perfect compensation of magnetic fields at Sb positions confirms that magnetic moments of Cr sublattices are symmetrically distributed. This finding offers crucial insights into the microscopic magnetic properties of CrSb.

Triangular-lattice cobaltates with glaserite-type $X_{2}Y\mathrm{Co}{\left(T{\mathrm{O}}_4\right)}_2$ structure provide an ideal platform to investigate intriguing quantum magnetism. Here we report a comprehensive study of the structural and magnetic properties of a triangular-lattice cobalt vanadate ${\mathrm{Na}}_2\mathrm{SrCo}{\left({\mathrm{VO}}_4\right)}_2$. Room-temperature x-ray and neutron powder diffraction confirm that ${\mathrm{Na}}_2\mathrm{SrCo}{\left({\mathrm{VO}}_4\right)}_2$ crystallizes in the monoclinic $P{2}_1/c$ space group with slightly distorted triangular layers of ${\mathrm{Co}}^{2+}$ ions. Magnetization measurements reveal a ferromagnetic transition at $T_{\mathrm{C}}\approx 3.4\mathrm{K}$, where a sharp $\lambda$-type anomaly is observed in the specific heat. The magnetic entropy recovered up to 55 K approaches $90\%$ of $R\ln 2$, supporting an effective spin-1/2 state of ${\mathrm{Co}}^{2+}$ ions at low temperature. Neutron diffraction at 2.3 K (below $T_{\mathrm{C}}$) further confirms a long-range canted ferromagnetic order with the ${\mathrm{Co}}^{2+}$ moments aligned in the $ac$ plane and the ordered moment size of $\sim$ 2.6 ${\mu }_{\mathrm{B}}$. Comparing with its sister compounds with a trigonal symmetry, ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{VO}}_4\right)}_2$ with a collinear ferromagnetic structure and the recently discovered spin supersolid candidate ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$ with a distinct Y-like antiferromagnetic ground state, this study indicates the decisive role of the $T{\mathrm{O}}_4$ tetrahedra in tuning exchange interactions and contrasting magnetic behaviors of these glaserite-structure compounds.

$\mathrm{Co}{\mathrm{V}}_2{\mathrm{O}}_4$ (CVO) stands out among spinel vanadates for its ultrashort V–V distances, placing it at the brink of itinerant electron behavior—an ideal playground for strain engineering. In this work, we exploit this sensitivity by growing high-quality epitaxial CVO thin films on ${\mathrm{SrTiO}}_3$ (001) and MgO (001), inducing compressive and tensile strain, respectively. Using pulsed laser deposition under ultralow oxygen pressure, we achieve high crystalline quality and strain-controlled tetragonal distortions: $c>a$ under compression (STO) and $c<a$ under tension (MgO). Resonant elastic x-ray scattering confirms a normal spinel structure, with cobalt occupying tetrahedral sites and vanadium octahedral ones. Both strain types reduce charge transport, driving the system into a highly resistive state. Magnetic measurements reveal strain-driven anisotropy switching: STO films transition from out-of-plane to in-plane easy axis below 90 K, while MgO films flip from in-plane to out-of-plane below 45 K. These results highlight CVO's exceptional responsiveness to lattice strain, unlocking a path to finely tunable electronic and magnetic properties. With its strong spin-lattice coupling and potential in spin Hall magnetoresistance, strained CVO emerges as a compelling platform for next-generation low-power spintronic devices.

Magnetic quantum materials with a kagome-lattice arrangement of spins exhibit unique magnetic and electronic properties, which are closely linked to the topology of their electronic band structures. In this work we study the potential of magnetic quantum material, ${\mathrm{Fe}}_3\mathrm{Sn}$ with a kagome-lattice arrangement for terahertz spintronics. We investigate terahertz (THz) emission generated from Pt/c-plane ${\mathrm{Fe}}_3\mathrm{Sn}$ heterostructures, in which $c$ plane oriented epitaxial thin films of ${\mathrm{Fe}}_3\mathrm{Sn}$, grown on $c$-plane sapphire substrates with a Pt-seed layer, function as the ferromagnetic (FM) layer and spin source. Our measurements reveal a significant THz signal amplitude from the Pt/c-plane ${\mathrm{Fe}}_3\mathrm{Sn}$ heterostructure, which exceeds the amplitude produced by a standard 1-mm ZnTe crystal by 200%. We observe a reversal in THz signal polarity depending on the direction of pump excitation (front versus back) and the polarity of the magnetic field. These findings suggest that the mechanism behind THz emission is primarily driven by non-equilibrium spin to charge transport, consistent with mechanism of spin-to-charge conversion. Additionally, the THz emission is tunable with the angle of the external magnetic field, the thickness of the ${\mathrm{Fe}}_3\mathrm{Sn}$ layer, and the pump fluence. Unexpectedly, the THz emission from Pt/c-plane ${\mathrm{Fe}}_3\mathrm{Sn}$ remains independent of sample azimuth as well as the helicity and polarization of the incident light. This observation suggests the absence of any signature of unique quantum property of ${\mathrm{Fe}}_3\mathrm{Sn}$ such as Weyl nodes near the Fermi surface. We investigate the possible origins of this behavior and discuss its implications.

Ferroelectrics underpin a broad spectrum of technological applications due to their switchable ferroelectric polarization and the associated electro-mechanical responses under electrical, optical, thermal, and mechanical stimuli. Recent advances in membrane technology offer new opportunities to tune ferroelectric polarizations via mechanical strains at relatively large magnitudes and scales. However, its influence on the tunability of mechanical responses of the membrane remains underexplored. In this work, we developed a phase-field model for freestanding $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Ti}{\mathrm{O}}_3$ ferroelectric membranes with stress-free boundary conditions on top/bottom surfaces, and achieved strain-induced nonvolatile ferroelectric domain switching in the membrane. It is discovered that a tensile/compressive strain favors in-plane orthorhombic/out-of-plane tetragonal domain structures in $\mathrm{BaTi}{\mathrm{O}}_3$, which are stable after the strain is removed. This results in a difference in the elastic modulus of a $\mathrm{BaTi}{\mathrm{O}}_3$ membrane under different domain states, i.e., a large tunability of the elastic modulus. By controlling the temperature and Sr composition of the membrane, we found that the highest elastic tunability of ∼54% can be achieved at the paraelectric-ferroelectric (tetragonal) phase boundaries due to the energy degeneracy of the two phases that facilitates the ferroelectric-to-paraelectric phase transition. Local enhancement of elastic tunability is also realized at the phase boundaries between rhombohedral, orthorhombic, and tetragonal phases in the ferroelectric states. Our work elucidates the mechanism of nonvolatile domain switching and provides a design map to achieve tunable mechanical responses in the $\mathrm{B}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Ti}{\mathrm{O}}_3$ membrane, which is key to their potential applications in sensors, actuators, and nonvolatile ferroelectric memory devices.

AlN nitride is a large-band-gap polar material that has gained interest due to its ability to become ferroelectric and its compatibility with Si- and GaN-based technologies. This compound could, therefore, be a good candidate to design new artificial multiferroics if combined with a ferromagnetic electrode. We performed first-principles calculations to investigate the set up of the magnetoelectric coupling at the Co/AlN(0001) interface. Our results describe a complex interfacial atomic structure with variable local magnetic properties as a function of the atom alignment. We predict an average variation of interface spin magnetization, when the polarization is reversed, of 3.83 ${\mu }_{\text{B}} {\mathrm{nm}}^{-2}$, close to the values already reported in the literature at metal/oxide interfaces. This confirms the potential of this AlN-based ferroelectric compound to be used in future voltage-controlled spintronic devices.

Crystalline $\mathrm{Si}{\mathrm{O}}_2$ quartz samples were irradiated with 1.64 GeV and 200 MeV Au ions and the irradiation-induced nanostructures originating from ion track formation were examined using transmission electron microscopy. The analysis revealed not only homogeneous amorphous ion tracks but also tracks with a concentric core-halo structure, characterized by a low-density core along the ion path surrounded by a damaged halo region. This type of track was found exclusively in sample regions thinner than 50 nm, suggesting that a thin sample is the prerequisite for the formation of such low-density track cores. Additionally, pitted tracks with a crater-like structure were identified at the sample edges. These findings are discussed in terms of surface phenomena, such as electronic sputtering as well as the role of sample thickness and edge effects, emphasizing the critical influence of sample geometry on the formation of these complex nanostructures.

The electrochromic phenomenon is described as the ability of a material to reversibly change its opacity and its color once an electric field is applied. Electrochromic oxides are suitable materials for optical applications due to the association of their wide temperature and voltage working range with their ability to change color once an electric field is applied. In this work, we explore the variation in the band gap of two-dimensional $\alpha \text{−}{\mathrm{MoO}}_3$ nanobelts after the localized application of intense electric fields using scanning tunneling microscopy and spectroscopy. A deterministic change of electronic band gap in ${\mathrm{MoO}}_3$ is observed for negative applied biases. These changes are fully reversible and robust with respect to successive cycles for a range of applied bias compatible with common applications. In our experiments the band-gap modifications are ascribed to changes in the surface stoichiometry due to the local electric field created by the tunneling tip. Our result points to the possibility of writing and reading surface electronic analogical information, with optical access to the material state, indicating a potential use in technological areas.

Motivated by theoretical predictions suggesting that ilmenite-type $\mathrm{MnGe}{\mathrm{O}}_3$ (i-MGO) may host a topological quantum state known as an antiferromagnetic Dirac semimetal, we have conducted a comprehensive investigation of its structural, magnetic, electronic, both linear and nonlinear optical properties, using both polycrystalline and single-crystalline samples obtained via high-pressure synthesis techniques. Single-crystal magnetization measurements have not only confirmed the antiferromagnetic transition at $T_{\mathrm{N}}=96\mathrm{K}$, previously observed in the polycrystalline samples, but also revealed a clear spin-flop transition at 60 kOe in an applied field along the $c$ axis. This bulk observation provides macroscopic evidence supporting the previously proposed antiferromagnetic structure, characterized by $\mathrm{M}{\mathrm{n}}^{2+}$ magnetic moments aligned along the $c$ axis with a propagation vector $\mathbf{k}=(0,0,0)$, which breaks both space-inversion ($\mathcal{P}$) and time-reversal ($\mathcal{T}$) symmetries. Furthermore, our powder neutron diffraction measurements independently confirm this magnetic structure. Electrical and optical measurements have shown that i-MGO is highly insulating and transmissive in the visible range, thereby excluding the theoretically predicted Dirac semimetal state. Our density functional theory calculations reveal that this discrepancy originates from the use of incorrect structural parameters in previous theoretical studies. As a clear manifestation of the broken $\mathcal{P}$ and $\mathcal{T}$ symmetries in the antiferromagnetic phase, we have observed a linear magnetoelectric effect and a pronounced optical second harmonic generation signal. These findings not only resolve the inconsistency with prior predictions but also establish i-MGO as a platform for studying symmetry-breaking-driven electromagnetic responses in ilmenite-type materials.

We present a first-principles investigation of the combined effects of chemical doping and nanostructuring on the thermoelectric performance of the double halide perovskite ${\mathrm{Cs}}_2{\mathrm{NaYbCl}}_6$. Using density functional theory and Boltzmann transport calculations, we explicitly include all relevant scattering mechanisms (namely, electron–phonon, phonon–phonon, Coulomb impurity, phonon–impurity, and grain boundary scattering) to evaluate electrical and thermal transport coefficients. Our results show that Coulomb scattering from dopants is strongly screened and negligible compared to dominant electron–phonon interactions. Thus, both $n$- and $p$-type doping enhance electrical conductivity while only moderately reducing the Seebeck coefficient, leading to a significant increase in power factor. Phonon–impurity scattering is found to be minimal, while grain boundary scattering effectively reduces lattice thermal conductivity without strongly affecting carrier mobility. Combining optimal $n$-type doping (${10}^{19}{\mathrm{cm}}^{-3}$) with nanoscale grains (10 nm), the figure of merit $ZT$ increases from $\sim {10}^{-8}$ in the pristine crystal to $\sim 0.12$. These findings demonstrate a viable pathway for improving thermoelectric efficiency in wide-band-gap, lead-free perovskites through controlled extrinsic modifications.

Adding a few atomic percent of Bi to III–V semiconductors leads to significant changes in their electronic structure and optical properties. Bismuth substitution on the pnictogen site leads to a large increase in spin-orbit splitting ${\mathrm{\Delta }}_{\mathrm{SO}}$ at the top of the valence band (${\mathrm{\Gamma }}_{8v}-{\mathrm{\Gamma }}_{7v}$) and a large reduction in the band gap, creating unique opportunities in semiconductor device applications. Quantifying these changes is key to the design and simulation of electronic and optoelectronic devices. Using hybrid functional calculations, we predict the band gap of III–Vs (III = Al, Ga, In and V = As, Sb) with low concentrations of Bi (3.125% and 6.25%), the effects of adding Bi on the valence- and conduction-band edges, and the band offset between these dilute alloys and their III–V parent compounds. As expected, adding Bi raises the valence-band maximum (VBM). However, contrary to previous assumptions, the conduction-band minimum (CBM) is also significantly lowered, and both effects contribute to the sizable band-gap reduction. Changes in band gap and ${\mathrm{\Delta }}_{\mathrm{SO}}$ are notably larger in the arsenides than in the antimonides. We also predict cases of band-gap inversion (${\mathrm{\Gamma }}_{6c}$ below ${\mathrm{\Gamma }}_{8v}$), and ${\mathrm{\Delta }}_{\mathrm{SO}}$ larger than the band gap, which are key parameters for designing topological materials and for minimizing losses due to Auger recombination in infrared lasers.

Silver released from TRISO fuel particles can migrate through the SiC layer and deposit on reactor components, posing radiation hazards and operational challenges. Despite numerous proposed mechanisms, the precise pathway of silver transport through intact 3C-SiC remains unresolved. We present a physics-informed model for estimating the effective diffusivity of silver in polycrystalline 3C–SiC. Molecular dynamics (MD) simulations yield diffusivities for $\mathrm{\Sigma }3$ and $\mathrm{\Sigma }9$ grain boundaries (GBs), while literature values are used for other GB types and the bulk. These are combined using a bounds-averaging approach accounting for distinct GB transport properties. Bayesian inference of experimental data provides credible intervals for effective Arrhenius parameters and reveals a correlation between activation energy and pre-exponential factor. Although the homogenized model captures GB-mediated transport mechanisms, it overpredicts silver diffusivity relative to experiments. To resolve this, a multiplicative correction based on reversible trapping at nanopores is introduced. It is derived from first principles and is shown to reproduce observed transport behavior. Sensitivity analysis identified trap desorption energy and $\mathrm{\Sigma }9$ GB diffusivity as dominant factors influencing Ag transport. The resulting framework provides a mechanistic description of Ag transport suitable for integration into higher-scale fuel performance models.

In this work, we investigated the magnetotransport and magnetic properties of GdGaSi, having a noncentrosymmetric tetragonal structure, with space group $I{4}_{1}md$. Our theoretical results are supported by experimental studies. First-principles calculations reveal that GdGaSi is an antiferromagnetic semimetallic system, characterized by dominant electron-type charge carriers. In addition, the possible nontriviality of the crossing at the Fermi energy is consistent with isostructural LaPtSi-structured materials. The compound shows robust antiferromagnetic (AFM) ordering with a Néel temperature of 19 K, and a spin-reorientation signature below $T_N$. The semimetallic nature with positive magnetoresistance ($\simeq$ 2% at 2 K and 8 T) is observed from the magnetotransport data, having electrons as the majority charge carrier, established from the Hall measurements. The strong correlation in magnetism and transport properties is supported by various observations, like (1) concordant transitions in $M\left(T\right)$ and $\rho \left(T\right)$ data, (2) change in the concentration and mobility of electrons below $T_N$, and (3) splitting of Kohler's plots in the two branches across the long-range magnetic transition. Thus, our findings establish GdGaSi as a material with intertwined magnetic and transport degrees of freedom, within a noncentrosymmetric lattice.

Electric-field-induced phase transitions are the most important characteristics of antiferroelectric materials, furnishing them with rich functional properties. While they are actively studied for their potential applications such as high-strain transducers and electrocaloric devices, the applied electric field needed to reach the polar phase makes structural studies throughout the full transition of utmost importance. Here, the evolution of both structure and strain in antiferroelectric $\mathrm{PbZr}{\mathrm{O}}_3$ thin films was investigated by in situ synchrotron x-ray diffraction during electrical actuation up to 700 kV/cm applied DC electric field. The ferroelectric phase, characterized by its polar order and resulting piezoelectric activity, was found to nucleate at an electric field of 200 kV/cm and to disappear below 160 kV/cm, showing a hysteretic behavior. The variation of the different Bragg peak widths of antiferroelectric and ferroelectric phases revealed the variation of strain distributions in the thin film where antiferroelectric and polar regions coexist during the phase transformation. In addition, the effective longitudinal piezoelectric coefficient of the ferroelectric phase itself, which remains inaccessible by classical macroscopic interferometric measurements, was determined for the first time and reached a value of 67 pm/V, which is quite significant for a thin film clamped on a substrate. These findings provide a clearer understanding of the dynamics of the electric-field-induced phase transition in antiferroelectric $\mathrm{PbZr}{\mathrm{O}}_3$ thin films.

We investigate elastic, vibrational, and thermal properties of hexagonal (lonsdaleite) ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloys in the framework of density functional theory. We calculate elastic stiffness constants and phonon frequencies across the full composition range and analyze their dependence on chemical composition and atomic arrangement. While the elastic moduli show an almost linear composition dependence, phonon energies and dispersion are significantly affected by the specific arrangement of Si and Ge atoms on the hexagonal lattice sites. From the elastic properties, we determine the propagation of sound waves and the critical layer thickness for epitaxial growth. We further analyze global thermal quantities including heat capacities and thermal expansion coefficients as functions of composition and temperature, revealing negative thermal expansion at low temperatures. All calculated properties are compared with theoretical and experimental results for cubic ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloys, revealing remarkable quantitative similarities, attributed to the identical tetrahedral coordination of both structures.

We present a detailed study of common intrinsic defects in ${\mathrm{CuInS}}_2$ and ${\mathrm{CuGaS}}_2$ using the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional scheme. The impact of the two HSE parameters $\alpha$ and $\omega$ on the band gap and compliance with the generalized Koopmans' theorem is investigated. Using the formation energy formalism and calculated thermodynamic charge-transition levels, we assess the electronic properties of the defects and explore the connection of charge-transition levels with optical-transition levels. Calculated Franck-Condon shifts for emission highlight the importance of lattice relaxation for the attribution of defects to luminescence peaks. Our results show that once these effects are included, predictions become closer to photoluminescence measurements available in the literature.