Plasmons, collective excitations of electrons in solids, are associated with strongly confined electromagnetic fields, with wavelengths far below the wavelength of photons in free space. Such strong confinement nominally holds the potential to enable optoelectronic technologies that bridge the size difference between photonic and electronic devices. However, despite decades of research in plasmonics, many applications remain limited by plasmonic losses, thus motivating a search for new engineered plasmonic materials with lower losses. Among the promising candidates for low-loss plasmonic materials are solid-state lattices with flat and energetically isolated metallic bands—with commensurately small phase spaces for phonon-assisted optical losses, a major contributor to short plasmonic lifetimes. Such electronic band structures may be created by judiciously introducing an ordered lattice of defects in an insulating host material. Here, we explore this approach, presenting several low-loss, highly confined, and tunable plasmonic materials based on arrays of carbon substitutions in hexagonal boron nitride monolayers. From our first-principles calculations based on density functional theory, we find plasmonic structures with midinfrared plasmons featuring very high confinements (${\lambda }_{\text{vacuum}}/{\lambda }_{\text{plasmon}}$ exceeding 1400). In addition, we find that one of our materials exhibits a confinement of 700 while avoiding second-order—phonon-assisted—losses entirely (infinite quality factor at this order of perturbation theory). We provide a systematic explanation of how crystal structure, electronic bandwidth, local-field, and many-body effects inform the plasmonic dispersions and losses of these materials. The results are thus of relevance to low-loss plasmon engineering in other flat band systems.

We present a density functional theory study of the initial steps of chlorine deposition on the Mg(0001) surface. Such processes occur in chloride-ion batteries in which lithium and magnesium are used as anode materials. In addition, it is also of fundamental interest, as halide adsorption on metal electrodes is an important process in interfacial electrochemistry. We discuss the adsorption properties and determine the stable adsorption structures, both with respect to the free chlorine molecule but also as a function of the electrode potential. We find indications of the immediate formation of the ${\mathrm{MgCl}}_2$ surface salt structure upon exposure of Cl to a Mg surface. These findings are discussed with respect to the conversion of the Mg anode to a ${\mathrm{MgCl}}_2$ configuration which provides the thermodynamical driving force for the discharge of a Cl-ion battery.

We present first-principles calculations of the electronic properties of trigonal selenium with emphasis on photovoltaic applications. The band gap and optical absorption spectrum of pristine selenium is calculated from many-body perturbation theory yielding excellent agreement with experiments. We then investigate the role of intrinsic as well as extrinsic defects and estimate the equilibrium concentrations resulting from realistic synthesis conditions. The intrinsic defects are dominated by vacancies and we show that these do not result in significant nonradiative recombination. The charge balance remains dominated by vacancies when extrinsic defects are included, but these may give rise to sizable nonradiative recombination rates, which could severely limit the performance of selenium based solar cells. Our results thus imply that the pollution by external elements is a decisive factor for the photovoltaic efficiency, which will be of crucial importance when considering synthesis conditions for any type of device engineering.

InSe is a promising material for next-generation two-dimensional electronic and optical devices, the characteristics of which are largely determined by the type of band structure, direct or indirect. In general, atomic force microscopy and optical methods can be sensitive to peculiarities of the electronic structure, leading to different results. In this work we will focus on the luminescent properties of few-layer $\beta$-InSe with a thickness of 6–75 monolayers (MLs). Low-temperature micro-photoluminescence ($\mu$-PL) studies show a sharp increase in PL intensity in the range of thicknesses from 16 to 20 monolayers, where, in addition, there is a singularity in the dependence of the work function on the thickness. Time-resolved photoluminescence spectroscopy reveals three characteristic PL decay times that differ from each other by about an order of magnitude. We associate the processes underlying the two faster decays with the recombination of electrons and holes between the band extrema, either directly or through the interband relaxation of holes. Their contributions to the total PL intensity increase significantly in the same thickness range, 16–20 MLs. On the contrary, the slowest contribution, which we attribute mainly to the defect-assisted recombination, prevails at a smaller number of monolayers and then noticeably decreases. These results indicate the indirect-to-direct band-gap transition near 16–20 MLs, which determines the range of applicability of few-layer $\beta$-InSe for efficient light emitters.

${\mathrm{Ce}}_2{\mathrm{Pd}}_2\mathrm{In}$ (${\mathrm{Mo}}_2\mathrm{Fe}{\mathrm{B}}_2$-type of crystal structure) with the ferromagnetic ground state based on $\text{Ce-}4f^1$ states exhibits two distinct phases of pressure-driven destruction of magnetic order. For pressures up to 3 GPa, a gradual buildup of Kondo interactions and of incommensurate spin-density wave are observed, while the ordering temperature remain in the range 4–5 K. A gradual but fast suppression of magnetism and formation of heavy fermion state (estimated Sommerfeld coefficient γ exceeding $400\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$) could be placed into the context of pressure evolution of crystal lattice, in which all nearest neighbors in the basal plane converged to the same value, forming a frustrated triangular lattice.

To elucidate the spin relaxation mechanism of $\mathrm{SrNb}{\mathrm{O}}_3$ (SNO) ultrathin films, the transport properties of a series of SNO films with various thicknesses $t$ were measured on both sides of the metal-insulator transition. The spin-orbit scattering time (${\tau }_{\mathrm{so}}$) was deduced from the analysis of the magnetoresistance with weak antilocalization theory, and it was found that ${\tau }_{\mathrm{so}}$ was inversely proportional to the momentum scattering time. This result was explained in terms of the D’Yakonov-Perel mechanism, indicative of the dominant Rashba effect. The values of the Rashba parameter, on the order of $1\times {10}^{-12}$ eV m were the largest in the values reported for other ultrathin films of metallic oxides.

In this study we analyze the dielectric properties of a recently published dataset to identify high-refractive-index and high-band-gap materials that are crucial for modern optoelectronic applications. We employ advanced crystal graph convolutional neural networks and density functional perturbation theory calculations to accelerate the discovery of such materials. Our analysis confirms the traditional inverse relationship between band gap and dielectric constant, which persists even in this large dataset. However, our study reveals several promising materials that possess competitive properties compared to current industry standards. Our findings provide valuable insights into the field of dielectric materials and demonstrate the potential of advanced machine learning and computational techniques for accelerating materials discovery.

There is a common belief that superconductivity and charge density wave (CDW) order accommodate homogenously in real space but compete with each other for the effective density of states in momentum space in CDW superconductors. By measuring resistivity along the $c$ axis in $\mathrm{Cs}{\left({\mathrm{V}}_{1-x}{\mathrm{Ta}}_x\right)}_3{\mathrm{Sb}}_5$, we observe strong superconducting fluctuation behavior coexisting with the CDW order in pristine ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, and the fluctuation region becomes narrowed when the Ta doping suppresses the CDW order. The onset transition temperature barely changes with the Ta doping. Therefore, the bulk superconductivity may be established by a doping-independent local pairing, and it can be suppressed in some regions by the spatially variable CDW order along the $c$ axis. Our results violate the above-mentioned belief about CDW superconductors and demonstrate the intricate interaction between superconductivity and CDW order in this kagome superconductor.

The spin-to-charge current interconversion in bilayers composed of ferromagnetic and nonmagnetic layers with strong spin-orbit coupling has garnered considerable attention due to its exceptional potential in advancing spintronics devices for data storage and logic applications. Platinum (Pt) stands out as one of the most effective materials for generating spin current. While the spin conversion efficiency is isotropic in polycrystalline Pt samples, an ongoing debate persists regarding its dependence on the crystalline direction in single crystalline samples. In this study, we aim to comprehensively evaluate the in-plane anisotropy of spin-charge interconversion using an array of complementary spin Hall and inverse spin Hall techniques with both incoherent and coherent excitation. Specifically, we investigate the spin-to-charge interconversion in epitaxial, (111)-oriented, Co/Pt bilayers with low surface roughness, as resulted from x-ray experiments. By varying the thickness of the Pt layer, we gain insights into the spin-charge interconversion in epitaxial Pt and highlight the effects of the interfaces. Our results demonstrate an isotropic behavior within the limits of our detection uncertainty. This finding significantly enhances our understanding of spin conversion in one of the most relevant systems in spintronics, and it paves the way for future research in this field.

As one of the promising candidates for developing next-generation structural materials, high-entropy alloys (HEAs) have recently attracted significant interest because of their unique mechanical properties, including their coexisting of high strength and ductility properties. Here, through atomic simulations, we demonstrate that the segregation of elements to grain boundaries (GBs) due to atomic-size differences, which is one of the most important characteristics of HEAs, contributes to the coexistence of high strength and high ductility in HEAs. To focus on only the effect of the size difference on the GB segregation, ignoring the difference in the chemical bonding energies among all the constituent elements, we employ two-dimensional virtual quinary HEA models. The HEAs are subjected to tensile and compressive load tests, and the stress required for dislocation emission from the GBs is measured. We demonstrate that the GB segregation in the HEAs increases the stress required for dislocation emission from the GBs, thereby increasing the strength of the HEAs. This is because the GB segregation in the HEAs stabilizes the GB structure by decreasing the GB free volume. Notably, the GB segregation also decreases the heterogeneity of the mechanical field between the grain interiors and the GBs, which is an intrinsic attribute of ordinary materials, and the homogenization of the mechanical field can improve the ductility of HEAs, preventing intergranular fracture. Our results can serve as a guide for designing HEAs with both high strength and high ductility through the effective utilization of GB segregation.

We present a first-principles study of the defect formation and electronic structure of erbium (Er)–doped yttria $\left({\mathrm{Y}}_2{\mathrm{O}}_3\right)$. This is an emerging material for spin-photon interfaces in quantum information science due to the narrow-linewidth optical emission from Er dopants at standard telecommunication wavelengths and their potential for quantum memories and transducers. We calculate formation energies of neutral and negatively and positively charged Er dopants and find the charge-neutral configuration to be the most stable, consistent with experiment. Of the two substitutional sites of Er for Y, the $C_2$ (more relevant for quantum memories) and $C_{3i}$ (more relevant for quantum transduction), we identify the former as possessing the lowest formation energy. The electronic properties are calculated using the Perdew-Burke-Ernzerhof functional along with the Hubbard $U$ parameter and spin-orbit coupling, which yields a $\sim 6{\mu }_B$ orbital and a $\sim 3{\mu }_B$ spin magnetic moment, and 11 electrons in the Er $4f$ shell, confirming the formation of charge-neutral ${\mathrm{Er}}^{3+}$. This standard density functional theory approach underestimates the band gap of the host and lacks a first-principles justification for $U$. To overcome these issues we performed screened hybrid functional calculations, including a negative $U$ for the $4f$ orbitals, with mixing $\left(\alpha \right)$ and screening $\left(w\right)$ parameters. These produced robust electronic features with slight modifications in the band gap and the $4f$ splittings depending on the choice of tuning parameters. We also computed the many-particle electronic excitation energies and compared them with experimental values from photoluminescence.

We investigate hydrogen (H) and mono and divacancy-hydrogen complex (${\mathrm{VH}}_x$ and ${\mathrm{V}}_2{\mathrm{H}}_x$) diffusion in body-centered cubic (bcc) iron using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo approach with on-the-fly event catalog building, to explore diffusion barriers and associated mechanisms for these defects. k-ART uncovers complex diffusion pathways for the bound complexes, with important barrier variations that depend on the geometrical relations between the position of the inserting Fe atom and that of the bound H. Since H is small and brings little lattice deformation around itself, these bound complexes are compact, and H is already fully unbound at the second-neighbor site. As more H is added, however, vacancies deform and affect the lattice over longer distances, contributing to increasing the ${\mathrm{VH}}_x$ complex diffusion barrier and its impact on its local environment. We find, moreover, that the importance of this trapping decreases when going from mono to divacancy complexes, although diffusion barriers for these complexes increase with the number of trapped H.

${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{45}{\mathrm{Sn}}_{05}$ heated above 600 K decomposes into ferromagnetic ${\mathrm{Ni}}_2\mathrm{MnSn}$ precipitates in an antiferromagnetic NiMn matrix. If an external magnetic field is applied during annealing, magnetic hysteresis curves with high coercive fields of up to 5 T can be achieved. The origin of this hysteresis has been attributed to the coupling of the antiferromagnetic matrix with the ferromagnetic precipitates, whose location and morphology were not known. To close this knowledge gap, four samples with varying annealing treatments were investigated using switching magnetization magnetic force microscopy. One sample was additionally analyzed with transmission electron microscopy and atom probe tomography. The decomposition type is identified to be a cellular precipitation starting at grain boundaries and growing into the grains. This leads to a multilayer thin film like lamellar structure with a lamella thickness in the nm range. Our results provide a basis for understanding the magnetic interactions, which lead to the magnetic hysteresis with ultra high coercivity.

The condensation of paired fermions into superfluid states changes progressively depending on the coupling strength. At the midpoint of the crossover between Bardeen-Cooper-Schrieffer (BCS) weak-coupling and Bose-Einstein condensate (BEC) strong-coupling limits, paired fermions condensate most robustly, thereby leading to the emergence of a pseudogap due to enhanced pairing fluctuations. In the case of electrons in solids, excessively strong interactions often induce competing electronic orders instead of strong-coupling superconductivity, and experimental comprehension of the pseudogap remains incomplete. In this study, we provide experimental evidence demonstrating the opening of a pseudogap, marking the incipient stage of the BCS-BEC crossover in the organic system $\kappa \text{−}(\mathrm{BEDT}\text{−}{\mathrm{TTF})}_{2}X$. By controlling electron correlations, we investigate the thermodynamic properties of the BCS-BEC crossover and pseudogap phase. Since the superconductivity of $\kappa \text{−}{\left(\text{BEDT}\text{−}\mathrm{TTF}\right)}_{2}X$ arises from a simple Fermi liquid that does not exhibit any other electronic orders, our study sheds light on the inherent nature of the BCS-BEC crossover.

High-quality low-bulk-carrier-concentration 3D topological insulator crystals and films are required for the majority of their potential applications. Creating a p-n transition using composition grading is one of the ways to obtain compensated regions in the bulk of 3D topological insulator crystals. Eventual formation of a p-n junction in 3D topological insulator surface states is expected to enhance the surface-transport-related spin filtering and charge-to-spin conversion. Here we report a detailed study of the transport and surface electronic structure of composition-graded ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_{1.34}{\mathrm{Sb}}_{0.66}{\mathrm{Te}}_3$ single crystals with built-in lateral p-n transition. The defect compensation naturally achieved at the p-n interface results in a strong reduction of the bulk carrier concentrations in both crystals. In the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ crystal a high-mobility n-type conductivity region is formed with electron Hall mobility of 70 000 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and Hall concentration of $2\times {10}^{18}{\mathrm{cm}}^{-3}$ at 4.2 K. In the ${\mathrm{Bi}}_{1.34}{\mathrm{Sb}}_{0.66}{\mathrm{Te}}_3$ crystal the region of intrinsic conductivity with the lowest observed hole Hall concentration of $6\times {10}^{17}{\mathrm{cm}}^{-3}$ and hole Hall mobility of 10 000 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ is formed in the vicinity of the p-n junction. The downward band bending was observed on the surface of the p-type conductivity region of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_{1.34}{\mathrm{Sb}}_{0.66}{\mathrm{Te}}_3$, providing an almost barrierless topological surface state electron channel with no topological p-n junction formed. The composition grading can be used as a reliable method of obtaining high-quality single crystals with relatively large areas of low bulk carrier concentrations and enhanced charge carrier mobility, which can be used in further nanoscale topological insulator device fabrication.

Perovskite rare-earth nickelates exhibit intriguing electrical and optical properties, such as metal-to-insulator transitions, resulting from the strong interplay between charge disproportionation and electron correlation. Of these materials, lanthanum nickelate, ${\mathrm{LaNiO}}_3$, is the only exception where the metallic phase is robust at all temperatures. Recently, significant efforts have been made to induce an insulating phase in ${\mathrm{LaNiO}}_3$ by dimensionality or stoichiometry control. Electrolyte gating can be an efficient alternative to manipulate such electronic behavior reversibly and controllably. In this work, we performed systematic ionic liquid gating studies with electric double-layer transistor devices to control the electronic properties of ${\mathrm{LaNiO}}_3$. The electrolyte gating in ${\mathrm{LaNiO}}_3$ leads to an insulating phase transition with an increased film resistivity by over six orders of magnitude. The electrolyte gating behaviors are found to be dependent on not only gating voltage and duration, but also the atmospheric environment and temperature. X-ray photoelectron spectroscopy analysis reveals that the ionic liquid gating changes the O vacancy concentration and Ni valence state with varying gating times. The phase transition is attributed to enhanced electron correlation as well as opening of the charge transfer gap due to the reduced overlap between Ni and O bands. Intriguingly the filling of carriers into the Mott-Hubbard gap vs charge transfer gap is controlled by the gate voltage. These results suggest that electrolyte gating devices can be useful for manipulating electron-electron correlation, boosting materials research to realize exotic physics in correlated systems.

The inverse Faraday effect is an optomagnetic phenomenon that describes the ability of circularly polarized light to induce magnetism in solids. The capability of light to control magnetic order in solid state materials and devices is of interest for a variety of applications, such as magnetic recording, quantum computation, and spintronic technologies. However, significant gaps in understanding about the effect persist, such as what material properties govern the magnitude of the effect in metals. In this work, we report time-resolved measurements of the specular inverse Faraday effect in nonmagnetic metals, i.e., the magneto-optic Kerr effect induced by circularly polarized light. We measure this specular inverse Faraday effect in Cu, Pd, Pt, W, Ta, and Au at a laser wavelength of 783 nm. For Ta and W, we investigate both α and β phases. We observe that excitation of these metals with circularly polarized light induces significant circular dichroism. This nonlinear magneto-optical response to circularly polarized light is an order of magnitude larger in α-W than other metals, e.g., Pt or Au, and is greater than nearly all other reported values for the inverse Faraday effect in other materials. Our results benchmark the range of the inverse Faraday effect that can be observed in nonmagnetic metals and provide insight into what material properties govern the inverse Faraday effect in metals.

The computational study of energy storage and conversion processes calls for simulation techniques that can reproduce the electronic response of metal electrodes under electric fields. Despite recent advancements in machine-learning methods applied to electronic-structure properties, predicting the nonlocal behavior of the charge density in electronic conductors remains a major open challenge. We combine long-range and equivariant kernel methods to predict the Kohn-Sham electron density of metal electrodes in response to various kinds of electric field perturbations. By taking slabs of gold as an example, we first show how the nonlocal electronic polarization generated by the interaction with an ionic species can be accurately reproduced in electrodes of arbitrary thickness. A finite-field extension of the method is then derived, which allows us to predict the charge transfer and the electrostatic potential drop induced by the application of a homogeneous and constant electric field. Finally, we demonstrate the capability of the method to reproduce the charge density response in a gold/electrolyte capacitor under an applied voltage, predicting the system polarization with a greater accuracy than state-of-the-art classical atomic-charge models.

${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ with an incommensurate Nowotny chimney-ladder (NCL) structure is a promising material for thermoelectric applications due to its low thermal conductivity. Previous experimental studies on ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ have mainly focused on polycrystalline samples, resulting in a limited understanding of the material's intrinsic thermoelectric properties and the underlying causes of its low thermal conductivity. Here we report the synthesis and thermoelectric properties of single crystalline ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$. Millimeter-sized ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ single crystals grown by the chemical vapor transport method enable the study of the intrinsic thermoelectric properties. The Seebeck coefficient of ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ is negative and its magnitude increases linearly with temperature, showing a degenerate $n$-type semiconductor behavior. Analysis of the electrical resistivity and specific heat data indicates the existence of an Einstein mode with a characteristic temperature of about 60 K, suggesting the presence of low-energy optical phonons. The thermal conductivity of ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ along the $c$ axis is as low as $1.9\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K and exhibits a nearly temperature-independent characteristic, which is distinct from the previous theoretical calculations with a stronger temperature dependence. The low thermal conductivity may be attributed to the scattering of acoustic phonons by low-energy optical modes and the presence of non-extended diffuson modes, as reported in another NCL compound, $\mathrm{Mn}{\mathrm{Si}}_{1.74}$. This study provides valuable insights into the electrical and thermal properties of ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$, which can open up possibilities for future advances in thermoelectric applications.