The PL6 color center in $4H$-SiC, known for its excellent ambient-temperature spin and optical properties, has an unresolved microscopic origin. In this first-principles study, we systematically investigate potential structures to clarify its nature. We first rigorously examine the DV-antisite hypothesis (a divacancy paired with a carbon antisite, ${\mathrm{C}}_{\mathrm{Si}}$), analyzing the energetic, electronic, and spin properties of various $V_{\mathrm{Si}}{V}_{\mathrm{C}}+{\mathrm{C}}_{\mathrm{Si}}$ configurations. Two ${\mathrm{C}}_{3\mathrm{v}}$-symmetric $\mathrm{kk}+{\mathrm{C}}_{\mathrm{Si}}$ complexes emerge as strong candidates within this framework. Subsequently, a critical comparison of hyperfine interaction signatures is performed between these candidates, the alternative OV model [specifically OV(hh) and OV(kk), an oxygen replacing C together with a Si vacancy], and experimental data. This analysis demonstrates that the OV(hh) structure more accurately reproduces PL6's hyperfine features. Furthermore, reevaluation of the proposed OV(hk) model for the related PL5 center reveals zero-field splitting parameter $E$ inconsistencies with experimental results, suggesting that PL5 and PL6 may have distinct origins. These findings provide crucial theoretical insights and motivate targeted experimental validation for these quantum defects.

Recent experiments highlight second-order nonlinear optics (NLO), including the bulk photovoltaic effect (BPVE) and second-harmonic generation (SHG), in weak interlayer coupling materials, as the photoelectric conversion is enhanced with layer numbers [Nature (London) 613, 53 (2023)]. Here, based on first-principles calculations, we investigate the BPVE and SHG in $\mathrm{Sn}{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$. The small interlayer coupling energy and minor band-gap variation with layer number indicate its weak interlayer coupling. The BPVE and SHG responses increase with layer number, regardless of the parity of the layer number. Moreover, the interlayer sliding has a negligible influence on the BPVE, which guarantees that the BPVE is robust against structural deformation. The parity-independent, sliding-insensitive BPVE, along with the accumulation effect with layer number of BPVE and SHG in $\mathrm{Sn}{\mathrm{P}}_2\mathrm{S}{\mathrm{e}}_6$, indicates its potential applications in NLO devices.

Electron-phonon coupling (EPC) plays an important role in FeSe, yet theoretical predictions have struggled to reconcile with experiments on doping- and pressure-dependent superconductivity. Here, we employ dynamical mean-field theory to investigate correlation-corrected EPC in FeSe under carrier doping and pressure. By incorporating correlation-corrected EPC strength and applying the Migdal-Eliashberg formalism for phonon-mediated superconductivity, we predict superconducting transition temperatures $T_c$ that closely match experiments and reproduce the observed doping- and pressure-dependent trends. We further find that the electronic correlation strength, characterized by the bandwidth near the Fermi level, shows an opposite trend to $T_c$, in contrast to EPC. Our work demonstrates the consistency of the correlation-promoted EPC and $T_c$ over a range of experimental conditions, which suggests that superconductivity in FeSe may originate primarily from correlation-promoted electron-phonon coupling.

We report on the characterization of $\mathrm{L}{\mathrm{i}}_4\mathrm{M}{\mathrm{n}}_2\mathrm{T}{\mathrm{e}}_2{\mathrm{O}}_{11}$, a previously unstudied layered oxide that exhibits ferrimagnetic ordering below 30 K and an exceptionally large coercive field exceeding 9 T at 2 K. The crystal structure consists of alternating ${\mathrm{Mn}}^{3+}$ triangular layers and zigzag chains, creating two inequivalent magnetic sublattices. Magnetic measurements reveal net in-plane magnetization and pronounced anisotropy, with strong hysteresis and domain pinning behaviors. The magnetization direction is controlled by the field-cooling history and remains locked under applied fields up to 9 T. Out-of-plane measurements show negligible net magnetization but exhibit a distinct high-field kink, likely attributed to spin reorientation. Heat capacity data show a broad magnetic anomaly that shifts and weakens under field, consistent with antiferromagnetic-dominated interactions. These results establish $\mathrm{L}{\mathrm{i}}_4\mathrm{M}{\mathrm{n}}_2\mathrm{T}{\mathrm{e}}_2{\mathrm{O}}_{11}$ as a rare ultrahard ferrimagnet with strong anisotropy, offering a promising platform for low-dimensional spintronic and domain engineering applications.

We successfully synthesized a verdazyl-based complex, ${\left(m\text{−}\mathrm{Py}\text{−}\mathrm{V}\right)}_3\left[\mathrm{Ni}{\left({\mathrm{NO}}_3\right)}_2\right]$, in which ${\mathrm{Ni}}^{2+}$ ions and verdazyl radicals form a one-dimensional, triangular spin necklace consisting of spin-1/2 and spin-1 units. Molecular orbital calculations reveal strong antiferromagnetic (AF) interactions between inversion-related radical pairs that form spin-1/2 singlet dimers. The remaining verdazyl and ${\mathrm{Ni}}^{2+}$ spins form frustrated triangular units, creating a distinctive spin network. Magnetic susceptibility and specific heat measurements identify a phase transition to an AF order. The application of magnetic fields suppresses the phase transition signal, suggesting field-induced decoupling of the spin-1 moments. Electron-spin resonance measurements are used to evaluate the easy-axis anisotropy of spin-1, which may promote the AF order. This work provides a rare example of a geometrically frustrated quantum spin chain realized via molecular design, thereby offering a platform for exploring frustration-driven quantum phases in low-dimensional materials.

The kagome-lattice compounds $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ ($R$ is a rare-earth element), where the Mn atoms form a kagome net in the basal plane, are currently attracting a great deal of attention as they have been shown to host complex magnetic textures and electronic topological states strongly sensitive to the choice of the $R$ atom. Among the magnetic $R$ atoms, ${\mathrm{TmMn}}_6{\mathrm{Sn}}_6$ orders with the easy-plane magnetization forming a complex magnetic spiral along the $c$ axis. Previous neutron studies carried out on polycrystalline samples found that Ga doping changes the magnetic anisotropy from easy-plane to easy-axis. Here we present magnetic and magnetotransport measurements on a single crystal and first-principles calculations in the doping series of ${\mathrm{TmMn}}_6{\mathrm{Sn}}_{6-x}{\mathrm{Ga}}_x$. We find that the magnetic properties are highly sensitive even to a small concentration of Ga. With minimal Ga substitution, the easy-plane anisotropy is maintained, which gradually changes to the easy-axis anisotropy with increasing Ga. We discuss these observations with respect to the effect of Ga doping on magnetocrystalline anisotropy and Tm crystal field.

Ab initio density-functional calculations show that orthorhombic $Pca{2}_1$ hafnia ${\mathrm{HfO}}_2$ mixed with vanadium at low concentration is a ferroelectric and ferromagnetic insulator. The multiorbital degeneracy of singly-occupied V states in the nominally $4+$ ionic state is broken by magnetism, reduced symmetry, and local distortion, causing a single one-electron majority state per V atom to be occupied. A gap of order 1 eV thus survives at all V concentrations, and intrinsic polarization is preserved at the level of 60–70% of the undoped-${\mathrm{HfO}}_2$ value. Magnetization is found to increase linearly with V content, with values of 30–40 $\mathrm{emu}/{\mathrm{cm}}^3$ at concentrations near the end of the stability range ($\sim 16\%$ V).

Rare-earth iron garnet thin films are widely used in magnonics and spintronics because of their insulating nature and unique magnetic properties, such as low Gilbert damping and a tunable magnetic anisotropy. However, these properties are mostly restricted to single-crystalline films grown on specific substrates like ${\mathrm{Gd}}_3{\mathrm{Sc}}_2{\mathrm{Ga}}_3{\mathrm{O}}_{12}$ (GSGG), limiting their applications. Lateral crystallization is a powerful technique enabling the design of new garnet heterostructures, with the potential to overcome these limitations. We demonstrate the crystallization of a thulium iron garnet film on top of a thin Pt layer, which is deposited on a GSGG substrate. Hereby, a hole pattern in the Pt layers acts as a crystallization seed, resulting in a single-crystalline garnet film extending onto the Pt layer. The hole pattern in combination with a lateral crystallization rate of about $18\mathrm{nm}/\min$ allows crystallization of large areas by post-deposition annealing at $700{}^{\circ }\mathrm{C}$ for a few hours. The resulting garnet film exhibits properties comparable to epitaxially grown films without an interlayer, with a Gilbert damping as low as 0.008. Therefore, lateral crystallization of garnet thin films can open up new possibilities for designing high-quality garnet/metal heterostructures, in particular for spintronic applications.

${\mathrm{W}}_{\text{0.75}}{\mathrm{Re}}_{\text{0.25}}$, in its bulk form, is an interesting superconducting material due to its multiple crystalline phases, each exhibiting distinct superconducting characteristics. However, little is known about how these phases manifest in thin-film form, where deposition conditions and dimensionality are critical aspects. Here, we investigate superconducting ${\mathrm{W}}_{0.75}{\mathrm{Re}}_{0.25}$ thin films deposited via UHV dc magnetron sputtering. In order to tune the crystalline phase of the films, we further explored the effect of incorporating ${\mathrm{N}}_2$ during the deposition. The superconducting and normal-state properties as a function of deposition conditions were investigated, revealing the role of the crystal phase on the film transport properties.

We carried out high-resolution (0.02 $\mathrm{c}{\mathrm{m}}^{-1}$) measurements of the photoluminescence (PL) spectra of $\mathrm{CaW}{\mathrm{O}}_4$:${\mathrm{Ho}}^{3+}$, including in a magnetic field, and performed the spectral modeling. Well-resolved hyperfine structure was observed in the luminescence spectra of $\mathrm{CaW}{\mathrm{O}}_4$:${\mathrm{Ho}}^{3+}$ for the first time. Energies of several crystal-field levels were specified, and $g$ factors were determined. We show that the PL spectra are much more sensitive to lattice strains in $\mathrm{CaW}{\mathrm{O}}_4$:${\mathrm{Ho}}^{3+}$ than the absorption spectra. Zeeman spectroscopy offers an additional sensitive tool for detecting random lattice deformations. The spectra were simulated using refined crystal-field parameters and taking into account both hyperfine and deformation interactions. Here, we used the previously obtained distribution function of random strains induced by point lattice defects in an elastically anisotropic $\mathrm{CaW}{\mathrm{O}}_4$ crystal. Luminescence cryothermometry and magnetometry based on $\mathrm{CaW}{\mathrm{O}}_4$:${\mathrm{Ho}}^{3+}$ are discussed.

The interplay of charge-density wave (CDW) and superconductivity is a key issue in low-dimensional materials, whereas it has yet to be clarified even in simple two-dimensional materials such as monolayer transition-metal dichalcogenide. We fabricated a monolayer 1H-${\mathrm{NbS}}_2$ film on bilayer graphene/SiC(0001) by combining the molecular-beam epitaxy and topotactic chemical reaction, and investigated the electronic structure by angle-resolved photoemission spectroscopy in collaboration with first-principles calculation. We found that although the Fermi surface of monolayer 1H-${\mathrm{NbS}}_2$ consists of large hole pockets centered at the $\mathrm{\Gamma }$ and K points similarly to isostructural monolayer 1H-${\mathrm{NbSe}}_2$, the shape of the pocket at the K point is more triangular shaped in ${\mathrm{NbS}}_2$. Scanning tunneling microscopy and transport measurements at low temperature show no evidence for CDW or superconductivity in ${\mathrm{NbS}}_2$ in stark contrast to ${\mathrm{NbSe}}_2$. The degradation of CDW in ${\mathrm{NbS}}_2$ is likely caused by the suppression of quasiparticle scattering between the hot spots connected by the $2/3\mathrm{\Gamma }\mathrm{M}$ nesting vector associated with the change in the shape of the K-centered pocket. The present results suggest the highly susceptible nature of CDW and superconductivity to the subtle change of the fermiology in Nb dichalcogenides.

We have performed a materials investigation into the properties of antimony telluride films, grown by molecular beam epitaxy (MBE) as the temperature of an antimony cracking zone was varied. Through x-ray diffraction, magnetotransport studies, and examination using transmission electron microscopy, we find that as the cracker temperature is increased, the formation of defects is suppressed, due to the higher surface mobilities of the smaller antimony molecular species. This work highlights the importance of using a cracker cell and its optimization for the growth of high quality MBE grown films of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$.

Universal force fields generalizable across the periodic table represent a new trend in computational materials science. However, the applications of universal force fields in material simulations are limited by their slow inference speed and the lack of first-principles accuracy. Instead of building a single model simultaneously satisfying these characteristics, a strategy that quickly generates material-specific models from the universal model may be more feasible. Here, we propose a new workflow pattern, PFD (Pre-training, Fine-tuning, and Distillation), which automatically generates machine-learning force fields for specific materials from a pre-trained universal model through fine-tuning and distillation. By fine-tuning the pre-trained model, our PFD workflow generates force fields with first-principles accuracy while requiring one to two orders of magnitude less training data compared to traditional methods. The inference speed of the generated force field is further improved through distillation, meeting the requirements of large-scale molecular simulations. Comprehensive testing across diverse materials including complex systems, such as amorphous carbon, interface, etc., reveals marked enhancements in training efficiency, which suggests the PFD workflow a practical and reliable approach for force field generation in computational material sciences.

Materials have various degrees of freedom in their composition and structure, resulting in a large chemical space characterized by these combinations. To accelerate the discovery of promising, unreported materials, methods for identifying favorable compositions and their stable structures efficiently are required. In this study, we propose a crystal structure prediction method that combines low computational cost with high accuracy. This approach integrates symmetry-constrained random structure generation, a universal machine-learning interatomic potential, and first-principles calculations. Reproducibility tests conducted on the crystal structures of eight reported compositions demonstrate the efficacy of our method. To identify promising compositions from the vast chemical space strategically, we developed a machine learning model that estimates the existence probability of unreported compositions using information from reported compositions. Validation involved treating the reported compositions as unreported, achieving an 88% reproducibility rate. As an application of our approach, we explored novel ternary oxides having polar structures by incorporating an additional machine learning model to predict their polarity based on compositional information. Crystal structure prediction was carried out for 18 unreported compositions, each predicted by machine learning to have an existence probability exceeding 0.9 and polar structures. Our exploration yielded three candidates for novel polar oxides that are thermodynamically and dynamically stable. Further, each candidate has an unreported structure, underscoring the potential of our approach to expand chemical space.

Quasi-elastic scattering (QES) serves as a sensitive probe of slow spin fluctuations in low-dimensional magnetic systems, multiferroics, and frustrated quantum magnets. Here, we investigate spin and lattice dynamics in anion-substituted $\mathrm{NiP}{\mathrm{S}}_{3-x}\mathrm{S}{\mathrm{e}}_x$ using temperature- and polarization-dependent Raman spectroscopy. Above the Néel temperature, the Raman spectra in the cocircular polarization configuration show a QES response near zero frequency, which is described by a Lorentzian profile. This QES is linked to paramagnetic spin fluctuations and the associated energy density of spins, which are slightly suppressed with increasing Se content. Notably, even inside the magnetically ordered state (below 50 K), we observe a strong enhancement of this QES signal, indicative of persistent fluctuations of the ordered magnetic moments. Concurrently, spin-phonon coupling manifests as an anomalous renormalization of phonon self-energy in certain phonon modes that couple strongly to the magnetic ions, with their frequencies shifting systematically as Se concentration increases. These results suggest that spin fluctuations and lattice vibrations remain strongly intertwined in $\mathrm{NiP}{\mathrm{S}}_{3-x}\mathrm{S}{\mathrm{e}}_x$, and that chemical substitution provides a means to tune this spin-phonon coupling in van der Waals magnets.

Employing temperature ($T$)-dependent soft x-ray magnetic circular dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS) at the Co, Cr, and Fe $2p$ absorption edges, we have investigated the electronic structures of ${\mathrm{CoCr}}_{2-x}{\mathrm{Fe}}_x{\mathrm{O}}_4$ ($0\le x\le 0.2$) spinel ferrites, which were reported to reveal the negative magnetization phenomenon. We have found that Co ions are divalent and occupy the tetrahedral ($T_d$) sites, while both Fe and Cr ions are trivalent and occupy the octahedral ($O_h$) sites with a little mixture of $T_d$-site Fe ions. XMCD measurements demonstrate that the magnetic moments of Co ions are antiparallel to those of Fe/Cr ions and that all the Co, Cr, and Fe ions are in the high-spin (HS) states. With increasing $x$, the XMCD signals of all elements exhibit the sign changes for $x>\sim 0.15$, demonstrating the magnetization flip due to the change of the majority spins from the Co spins to the Fe/Cr spins for $x>\sim 0.15$. With increasing $T$, all the magnetic moments vanish above $T\sim 140\mathrm{K}$, except for $x$ = 0.15 that exhibits nonvanishing moments up to $T\sim 200\mathrm{K}$. This work provides direct evidence for the magnetization flip in ${\mathrm{CoCr}}_{2-x}{\mathrm{Fe}}_x{\mathrm{O}}_4$ at $x\ge 0.2$. The magnitudes of the spin ($m_{\mathrm{spin}}$) and the orbital ($m_{\mathrm{orb}}$) magnetic moments are determined from the sum-rule analysis of their low-$T$ XMCD spectra. Based on the systematic XMCD/XAS study, we have provided a model for the electronic and spin configurations of Co, Cr, and Fe ions in ${\mathrm{CoCr}}_{2-x}{\mathrm{Fe}}_x{\mathrm{O}}_4$ in the low-$T$ regime.

We observed a colossal dielectric response of small (5–10 nm) oxygen-deficient $\mathrm{H}{\mathrm{f}}_x\mathrm{Z}{\mathrm{r}}_{1-x}{\mathrm{O}}_2$ nanoparticles ($x=1–0.4$), prepared by solid-state organonitrate synthesis. The effective dielectric permittivity of the pressed $\mathrm{H}{\mathrm{f}}_x\mathrm{Z}{\mathrm{r}}_{1-x}{\mathrm{O}}_2$ nanopowders has a pronounced maximum at $38–88{}^{\circ }\mathrm{C}$, the shape of which can be fitted by the Curie-Weiss type dependence modified for the diffuse ferroelectric-paraelectric phase transition. The maximal value of the dielectric permittivity increases from $1.5\times {10}^3$ (for $x=1$) to $1.5\times {10}^5$ (for $x=0.4$) at low frequencies ($\sim 4$ Hz), being much smaller, namely, changing from 7 (for $x=1$) to 20 (for $x=0.4$) at high frequencies ($\sim 500$ kHz). The frequency dispersion of the dielectric permittivity maximum position is almost absent; meanwhile, the shape and width of the maximum changes in a complex way with increase in frequency. The temperature dependencies of the dielectric permittivity and resistivity are almost mirrorlike turned over with respect to each other, which means that all their features, such as position and shape of maxima, plateau, minima, and inflexions, almost coincide after the mirror reflection with respect to the temperature axis. These correlations of resistivity and dielectric permittivity are well described in the Heywang barrier model applied together with the variable range hopping conduction model in semiconducting ferroelectrics. Ferroelectriclike behavior of the small oxygen-deficient $\mathrm{H}{\mathrm{f}}_x\mathrm{Z}{\mathrm{r}}_{1-x}{\mathrm{O}}_2$ nanoparticles is expected from the Landau-Ginzburg-Devonshire approach and density functional theory calculations. Obtained results may be useful for developing silicon-compatible functional nanomaterials based on $\mathrm{H}{\mathrm{f}}_x\mathrm{Z}{\mathrm{r}}_{1–x}{\mathrm{O}}_2$ nanoparticles.

Recent reports on a family of kagome metals of the form $Ln{\mathrm{Ti}}_3{\mathrm{Bi}}_4$ ($Ln=\mathrm{Lanthanide}$) have stoked interest due to the combination of highly anisotropic magnetism and a rich electronic structure. The electronic structure near the Fermi level is proposed to exhibit Dirac points and van Hove singularities (VHSs). In this manuscript, we use angle-resolved photoemission spectroscopy measurements in combination with density functional theory calculations to investigate the electronic structure of an interesting kagome metal ${\mathrm{LaTi}}_3{\mathrm{Bi}}_4$. Our results reveal multiple VHSs with one VHS located in the vicinity of the Fermi level. We clearly observe two flat bands, which originate from the destructive interference of wave functions within the Ti kagome motif. These flat bands and VHSs originate from Ti $d$ orbitals and are very responsive to the polarization of the incident beam. We notice a significant anisotropy in the electronic structure, resulting from the breaking of sixfold rotational symmetry in this material. Our findings demonstrate this member of Ti based kagome materials as a promising platform to explore novel emerging phenomena in the wider $Ln{\mathrm{Ti}}_3{\mathrm{Bi}}_4$ ($Ln=$ lanthanide) family.