Halide perovskites exhibit giant photostriction, that is, volume or shape changes upon illumination. However, the microscopic origin of this phenomenon remains unclear and there are experimental reports of both light-induced lattice expansion and contraction. In this Letter, we establish a general method, based on first-principles calculations and molecular orbital theory, which provides a microscopic picture of photostriction in insulators based on the orbital characters of their electronic bands near the Fermi level. For lead-halide perovskites, we find that different valence states have different bonding characters, leading to opposing strengthening or weakening of bonds depending on the photoexcitation energy. The overall trend is that light induces lattice contraction at low excitation energies, while giant lattice expansion occurs at high excitation energies, rationalizing experimental reports.

Lithium-rich garnets such as ${\mathrm{Li}}_7{\mathrm{La}}_3{\mathrm{Zr}}_2{\mathrm{O}}_{12}$ (LLZO) are promising solid electrolytes with potential application in all-solid-state batteries that use lithium-metal anodes. The practical use of garnet electrolytes is limited by pervasive lithium-dendrite growth, which leads to short-circuiting and cell failure. One proposed mechanism of lithium-dendrite growth is the direct reduction of lithium ions to lithium metal within the electrolyte, and lithium garnets have been suggested to be particularly susceptible to this dendrite-growth mechanism due to high electronic conductivities relative to other solid electrolytes. The electronic conductivities of LLZO and other lithium-garnet solid electrolytes, however, are not yet well characterized. Here, we present a general scheme for calculating the intrinsic electronic conductivity of a nominally insulating material under variable synthesis conditions from first principles, and apply this to the prototypical lithium-garnet LLZO. Our model predicts that under typical battery operating conditions, electron and hole mobilities are low ($<1{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$), and bulk electron and hole carrier concentrations are negligible, irrespective of initial synthesis conditions or dopant levels. These results suggest that the bulk electronic conductivity of LLZO is not sufficiently high to cause bulk lithium-dendrite growth during cell operation, and that any non-negligible electronic conductivity in lithium garnet samples is likely due to extended defects or surface contributions.

Ytterbium doping in all-inorganic lead-halide perovskites $\left[\mathrm{CsPb}{\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_3\right]$ generates interesting properties including quantum cutting and narrow line emission, making these materials attractive spectral down converters for solar photovoltaics. The relationship between this optical efficiency and the defect structure(s) associated with ${\mathrm{Yb}}^{3+}$ dopants within perovskites is not well understood. Various charge-neutral doping motifs have previously been proposed and studied computationally, including clusters involving two substitutional ${\mathrm{Yb}}^{3+}$ ions charge compensated by a single local ${\mathrm{Pb}}^{2+}$ vacancy. Near-band-edge defect states associated with such motifs are believed to play an important mechanistic role in quantum cutting itself. Here, we report the results of x-ray absorption and x-ray total-scattering measurements on ytterbium-doped $\mathrm{CsPb}{\mathrm{Cl}}_3$. XANES shows that the dopant oxidation state is exclusively ${\mathrm{Yb}}^{3+}$, and a combination of $\mathrm{Yb}{L}_3$ and $\mathrm{Pb}{L}_3$ extended x-ray absorption fine structure (EXAFS) shows that this ${\mathrm{Yb}}^{3+}$ substitutes exclusively at ${\mathrm{Pb}}^{2+}$ sites, where it adopts a pseudo-octahedral ${\left[\mathrm{Yb}{\mathrm{Cl}}_6\right]}^{3-}$ coordination environment. Shell-by-shell fits to the data show a short Yb-Cl bond distance of 2.58 Å compared to the Pb-Cl bond distance of 2.83 Å. We confirm this finding by x-ray pair distribution function analysis, which also shows evidence of additional ${\mathrm{Pb}}^{2+}$ vacancy formation induced by ${\mathrm{Yb}}^{3+}$ doping. We evaluate whether this is the primary mechanism of charge compensation using simulated EXAFS and pair distribution function data for several computed defect structures. Together, these results resolve the local dopant structures and charge-compensation mechanisms in lanthanide-doped all-inorganic lead-halide perovskites, and, as such, significantly advance the understanding of structure-function relationships in this important class of materials.

We report the deposition and characterization of a set of high-quality ${\left({\mathrm{SrNiO}}_3\right)}_1/{\left({\mathrm{SrTiO}}_3\right)}_n$ superlattices using oxide molecular beam epitaxy. We demonstrate that one unit cell of ${\mathrm{SrNiO}}_3$ can be stabilized as a perovskite in these structures and that there is no charge transfer across the heterointerface. Detailed spectroscopy data analysis indicates that holes are confined to the ${\mathrm{SrNiO}}_3$ unit cells, resulting in a Ni valence between 3 and 4. Hole hopping conductivity is observed in the $n=1$ superlattice but is strongly suppressed along the $c$-axis direction for $n=3$ and 5, resulting in the insulating behavior in the latter. Our study will inspire the epitaxial synthesis of ${\left({\mathrm{SrNiO}}_3\right)}_1/{\left(R\mathrm{Ni}{\mathrm{O}}_3\right)}_n$ superlattices, as measurements of their properties could open a platform for studying nickelate-based superconductors.

The search for new ferromagnetic compounds is often targeted at known structure families, particularly those containing iron, cobalt, and manganese. We propose a method to expand this search to lesser-known structure types, using a database of experimental Curie and Néel temperatures. This study, in particular, illustrates the case of compounds containing the element cobalt, and we demonstrate how the use of such a database can lead to the discovery of ferromagnetic materials that had previously been overlooked. We report statistics from a literature survey of the magnetic properties of Co-based compounds with more than 33 at. % Co. We classify more than 13 000 compounds by structure type, cobalt content, and magnetic ground state. From these data, compounds $\mathrm{Ta}{\mathrm{Co}}_2\mathrm{Ga}$, ${\mathrm{La}}_6{\mathrm{Co}}_{13}\mathrm{Bi}$, and ${\mathrm{Nd}}_2{\mathrm{Co}}_3$ were identified as potential ferromagnets, and we confirm their ferromagnetic ordering theoretically via first-principles calculations and experimentally via synthesis and characterization measurements. In addition, the analysis is focused on the collection of data trends and discovery of ferromagnetic materials with easy-axis magnetic anisotropy. Both known ferromagnetic materials with unknown magnetic anisotropy, and unstudied compounds were considered. From the subset of known ferromagnets with unknown anisotropy, the compound ${\mathrm{Co}}_2\mathrm{Mg}$ was synthesized, characterized, and determined to have easy-axis magnetic anisotropy at room temperature.

Spintronic phenomena exhibiting a longitudinal resistance change under magnetization reversal are a quite novel feature in nanoscience, which has been intensively studied in hopes of realizing all-electrical magnetization direction detection devices, where no reference ferromagnetic layer is required. However, cryogenic temperatures and/or high magnetic fields have been required to achieve noticeable effects. Here, the high heat-to-charge conversion efficiency of the Heusler alloy Weyl semimetal ${\mathrm{Co}}_2\mathrm{MnGa}$ is exploited in single layer nanoscaled wires at room temperature to produce at least two orders of magnitude enhancement of the resistance change ratio, when compared with conventional ferromagnets. Such resistance change under magnetization reversal is consistently explained through temperature distribution simulations and direct thermoelectric measurements of the large anomalous Nernst effect (ANE) in this topologically nontrivial material. Although many reports consider ANE signals as perturbations or undesired artifacts, we demonstrate that they are dominant in this system and can be seized for nonvolatile memory readout, as shown in a prototype device. These results open up new horizons of using enhanced thermoelectric voltages in novel materials for magnetization direction detection in any system where significant temperature gradients exist.

In the field of van der Waals heterostructures, the twist angle between stacked two-dimensional layers has been identified to be of utmost importance for the properties of the heterostructures. In this context, we previously reported the growth of a single layer of unconventionally oriented epitaxial graphene that forms in a surfactant atmosphere [F. C. Bocquet et al., Phys. Rev. Lett. 125, 106102 (2020)]. The resulting G-$R{0}^{\circ }$ layer is aligned with the SiC lattice, and hence represents an important milestone towards high-quality twisted bilayer graphene, a frequently investigated model system in this field. Here, we focus on the surface structures obtained in the same surfactant atmosphere, but at lower preparation temperatures at which a boron nitride template layer forms on SiC(0001). In a comprehensive study based on complementary experimental and theoretical techniques, we find—in contrast to the literature—that this template layer is a hexagonal ${\mathrm{B}}_x{\mathrm{N}}_y$ layer, but not high-quality hBN. It is aligned with the SiC lattice and gradually replaced by low-quality graphene in the $0^{\circ }$ orientation of the ${\mathrm{B}}_x{\mathrm{N}}_y$ template layer upon annealing.

Two-dimensional magnetic materials have recently attracted great interest due to their unique functions as the electric field control of a magnetic phase and the anomalous spin Hall effect. For such remarkable functions, a spin-orbit coupling (SOC) serves as an essential ingredient. Here we report a giant positive magnetoresistance in a layered magnetic semiconductor ${\mathrm{AgCrSe}}_2$, which is a manifestation of the subtle combination of the SOC and Zeeman-type spin splitting. When the carrier concentration, $n$, approaches the critical value of $2.5\times {10}^{18}\mathrm{c}{\mathrm{m}}^{\text{–}3}$, a sizable positive magnetoresistance of $\sim 400\%$ emerges upon the application of magnetic fields normal to the conducting layers. Based on the magneto-Seebeck effect and the first-principles calculations, the unconventional magnetoresistance is ascribable to the enhancement of effective carrier mass in the SOC-induced $J=3/2$ state, which is tuned to the Fermi level through the Zeeman splitting enhanced by the $p\text{−}d$ coupling. This study demonstrates an aspect of the SOC-derived magnetotransport in two-dimensional magnetic semiconductors, paving the way to spintronic functions.

Dielectric capacitors hold an enormous advantage for energy storage that requires a fast charging/discharging rate; but relatively low energy capacity is a key limitation for conventional dielectric materials. Recently, design strategies by tuning the ferroelectric ${\mathrm{BiFeO}}_3$ (BFO) to an antiferroelectric or relaxor have shown great promise, especially owing to the large polarization at high electric field. Here, using a first-principles-based method, it is predicted that rare-earth substitution of varied elements and composition can systematically tune the stability of the antiferroelectric phase, leading to further enhanced energy densities and high efficiencies, for instance, reaching 239 J ${\mathrm{cm}}^{-3}$ in thulium substituted BFO and generally beyond 80%, respectively. The storage performance is further interpreted based on a simple model, pointing to the importance of transition fields, polarization of the ferroelectric state, and dielectric constant.

We reveal numerically and experimentally that dielectric resonances can enhance the absorbance and emission of monolayer ${\mathrm{MoS}}_2$. By quantifying the absorbance of the Si disk resonators and the monolayer ${\mathrm{MoS}}_2$ separately, a model taking into account the absorbance as well as quantum efficiency modifications by the dielectric disk resonators successfully explains the observed emission enhancement under normal light incidence. It is demonstrated that the experimentally observed emission enhancement at different pump wavelength results from the absorbance enhancement, which compensates the emission quenching by the disk resonators. In order to further maximize the absorbance value of monolayer ${\mathrm{MoS}}_2$, a perfect absorbing structure is proposed. By placing a Au mirror beneath the Si nanodisks, the incident electromagnetic power is fully absorbed by the hybrid monolayer-${\mathrm{MoS}}_2$–disk system. It is demonstrated that the electromagnetic power is redistributed within the hybrid structure and 53% of the total power is absorbed by the monolayer ${\mathrm{MoS}}_2$ at the perfect absorbing wavelength.

The generation of single-photon emitters in hexagonal boron nitride around 2 eV emission is experimentally well recognized; however, the atomic nature of these emitters is unknown. In this Letter, we use first-principles calculations to demonstrate that carbon trimer substitutional defect (${\mathrm{C}}_2{\mathrm{C}}_{\mathrm{N}}$) is a possible source of 2 eV single-photon emitter in hBN. We showcase the calculations of a complete set of static and dynamical properties related to quantum defects, including exciton-defect couplings and electron-phonon interactions, from both density functional theory and many-body perturbation theory. In particular, we show that it is critical to consider both radiative and nonradiative processes when comparing with experimental lifetime for known 2 eV emitters. We find that ${\mathrm{C}}_2{\mathrm{C}}_{\mathrm{N}}$ has several key physical properties matching the ones of experimentally observed single-photon emitters. These include the zero-phonon line (2.13 eV), Huang-Rhys factor (1.35), photoluminescence lifetime (2.19 ns), phonon-sideband energy (180 meV), and photoluminescence spectrum. The identification of defect candidates for 2 eV emission paves the way for controllable single-photon emission generation.

Unexpected physical phenomena resulting from the seemingly inconsequential substitutions of chemically similar lanthanide elements in the ${\mathrm{Pr}}_{1-x}{\mathrm{Gd}}_x\mathrm{ScGe}$ system are exploited to further the understanding of rare-earth magnetism and inform materials design. By directly probing magnetic moments of crystallographically indistinguishable Pr and Gd we solve the puzzles of how an unusual magnetic memory and strong exchange bias emerge at specific, easily predictable chemistries. Both effects are rooted in a robust antiparallel arrangement of large $4f$ magnetic moments of light and heavy lanthanides. This enables precise control of nearly zero net magnetization either opposed to, or aligned with, the external magnetic field that persists over a wide range of temperatures and fields. Further, spontaneous perturbations in the random distribution of lanthanide ions makes strong exchange bias possible in bulk single-phase compounds bordering magnetic compensation, consequently expanding the materials base beyond artificial magnetic multilayers and broadening the range of potential applications of the phenomenon.

Molecular magnets based on ${\left[{\mathrm{Mo}}_3\right]}^{11+}$ units with one unpaired electron per trimer have attracted recent interest due to the identification of quantum spin liquid candidacy in some family members. Here, we present comprehensive measurements on polycrystalline samples of ${\mathrm{ZnScMo}}_3{\mathrm{O}}_8$, ${\mathrm{MgScMo}}_3{\mathrm{O}}_8$, and ${\mathrm{Na}}_3{\mathrm{Sc}}_2{\mathrm{Mo}}_5{\mathrm{O}}_{16}$ with the same ${\mathrm{Mo}}_3{\mathrm{O}}_{13}$ magnetic building blocks. The crystal structures are characterized with x-ray or neutron powder diffraction and the magnetic ground states are determined by performing ac and dc susceptibility, specific heat, neutron powder diffraction, and muon spin relaxation measurements. Our work indicates that ${\mathrm{ZnScMo}}_3{\mathrm{O}}_8$ and ${\mathrm{MgScMo}}_3{\mathrm{O}}_8$ have ferromagnetic Curie-Weiss temperatures of 18.5 and 11.9 K, ordered ground states with net moments (low-moment ferromagnetism or canted antiferromagnetism), and zero field ordering temperatures of $T_c=$ 6 K and $T_c<$ 2 K, respectively. On the other hand, ${\mathrm{Na}}_3{\mathrm{Sc}}_2{\mathrm{Mo}}_5{\mathrm{O}}_{16}$ hosts a dynamical magnetic ground state with no evidence for magnetic ordering or spin freezing down to 20 mK despite an antiferromagnetic Curie-Weiss temperature of $-36.2$ K, and therefore is a candidate for quantum spin liquid behavior. By comparing the present results to past work on the same family of materials, we construct a phase diagram which illustrates that the magnetic ground states of these Mo-based molecular magnets are very sensitive to small changes in the nearest-neighbor Mo-Mo distance.

The emergence of long-range magnetic order in noncentrosymmetric compounds has stimulated interest in the possibility of exotic spin transport phenomena and topologically protected spin textures for applications in next-generation spintronics. Polar magnets, with broken symmetries of spatial inversion and time reversal, usually host chiral spin textures. This work reports on a wurtzite-structure polar magnetic metal, identified as $A{A}^{\prime }$-stacked ${\left({\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}\right)}_5\mathrm{Ge}{\mathrm{Te}}_2$, which exhibits a Néel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temperature. Atomic resolution imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the emergence of the polar phase at 50% Co. This discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies, and it ushers in a promising new framework for spintronics.

The recently reported $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\text{K}$, Rb, Cs) family of kagome metals are candidates for unconventional superconductivity and chiral charge density wave (CDW) order; both potentially arise from nested saddle points in their band structures close to the Fermi energy. Here, we use chemical substitution to introduce holes into ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and unveil an unconventional coupling of the CDW and superconducting states. Specifically, we generate a phase diagram for ${\mathrm{CsV}}_3{\mathrm{Sb}}_{5-x}{\mathrm{Sn}}_x$ that illustrates the impact of hole doping the system and lifting the nearest van Hove singularity toward and above $E_F$. Superconductivity exhibits a nonmonotonic evolution with the introduction of holes, resulting in two “domes” peaked at 3.6 and 4.1 K and the rapid suppression of three-dimensional CDW order. The evolution of CDW and superconducting order is compared with the evolution of the electronic band structure of ${\mathrm{CsV}}_3{\mathrm{Sb}}_{5-x}{\mathrm{Sn}}_x$, where the complete suppression of superconductivity seemingly coincides with an electronlike band comprised of Sb $p_z$ orbitals pushed above $E_F$.

Atomic-level modeling performed at large scales enables the investigation of mesoscale materials properties with atom-by-atom resolution. The spatial complexity of such cross-scale simulations renders them unsuitable for simple human visual inspection. Instead, specialized structure characterization techniques are required to aid interpretation. These have historically been challenging to construct, requiring significant intuition and effort. Here we propose an alternative framework for a fundamental structural characterization task: classifying atoms according to the crystal structure to which they belong. Our approach is data-centric and favors the employment of Machine Learning over heuristic rules of classification. A group of data-science tools and simple local descriptors of atomic structure are employed together with an efficient synthetic training set. We also introduce the first standard and publicly available benchmark data set for evaluation of algorithms for crystal-structure classification. It is demonstrated that our data-centric framework outperforms all of the most popular heuristic methods—especially at high temperatures when lattices are the most distorted—while introducing a systematic route for generalization to new crystal structures. Moreover, through the use of outlier detection algorithms our approach is capable of discerning between amorphous atomic motifs (i.e., noncrystalline phases) and unknown crystal structures, making it uniquely suited for exploratory materials synthesis simulations.

Excitons localized around point defects in semiconductors are promising candidates for long-lived and photon-addressable qubits. However, their microscopic origin is difficult to characterize due to the computational complexity of studying large systems with defects. Here we study the quasiparticle and optical absorption spectrum of the divacancy defect in 3C-SiC, a prototypical defect for quantum information applications, by means of large-scale $GW$ and $GW$ plus Bethe-Salpeter equation calculations. Despite the presence of localized unoccupied quasiparticle states in the gap, we find that the low-energy excitonic states are made primarily of transitions from occupied defect states to continuum conduction states from SiC, especially from the $X$ point of the Brillouin zone. The mixed character of defect states and bulk states of these low-energy exciton states is in contrast with the ${\mathrm{NV}}^{-}$ center in diamond and the divacancy in 4H-SiC, where the deep defect levels are well separated from bulk states. Our calculations provide a quantitative prediction of the defect quasiparticle energy levels and a physical understanding of the zero-phonon absorption. They highlight the important role of frontier conduction bands in the optical properties and formation of low-energy excitons in 3C-SiC divacancy.

The superconducting materials family of doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ remains intensively studied in the field of condensed matter physics due to strong experimental evidence for topologically nontrivial superconductivity in the bulk. However, at the surface of these materials, even the observation of superconductivity itself is still controversial. We use scanning tunneling microscopy (STM) down to 0.4 K to show that on the surface of bulk superconducting ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, no gap in the density of states is observed around the Fermi energy as long as clean metallic probe tips are used. Nevertheless, using scanning electron microscopy and energy-dispersive x-ray analysis, we find that micrometer-sized flakes of ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ are easily transferred from the sample onto the STM probe tip and that such flakes consistently show a superconducting gap in the density of states. We argue that the superconductivity in ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals does not extend to the surface when the topological surface state (TSS) is intact, but in microflakes, the TSS has been destroyed due to strain and allows the superconductivity to extend to the surface. To understand this phenomenon, we propose that the local electric field, always found in electron-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in the presence of the TSS due to an intrinsic upward band bending, works against superconductivity at the surface.

We experimentally investigate the range of the Dzyaloshinskii-Moriya interaction (DMI) occurring at magnetic interfaces within metallic heterostructures. To this aim we perform Brillouin light scattering spectroscopy on a set of Co-based, asymmetric metallic heterostructures, incorporating atomically thin continuous films obtained by room-temperature sputtering, and of identical orientation and quality. We thus get access to the intrinsic dependence of the interfacial DMI and other magnetic interactions on the thickness of the nonmagnetic layer adjacent to Co, which is chosen among Pt, Ru, and Au. Notably, we observe that a robust DMI is already generated by as few as two atomic planes of Pt and that interfacial DMI can be efficiently suppressed by a dusting of Ru equivalent to a single atomic plane coverage. These results point directly towards a mechanism where DMI is generated within the two first atomic planes away from the interface, in agreement with first-principles calculations. This locally generated DMI is, however, likely to be modulated by more distant atoms in the case of strain effects. The short-range aspect of the interfacial DMI opens up the synthesis of dense magnetic multilayers, allowing for a strong interfacial DMI even with very thin layers, which can be further tuned by strain engineering.

Manipulating electronic interlayer coupling in layered van der Waals (vdW) materials is essential for designing optoelectronic devices. Here, we control vibrational and electronic interlayer coupling in bi- and trilayer $2\mathrm{H}\text{−}{\mathrm{MoS}}_2$ using large external electric fields in a microcapacitor device. The electric field lifts Raman selection rules and activates phonon modes in excellent agreement with ab initio calculations. Through polarization-resolved photoluminescence spectroscopy in the same device, we observe a strongly tunable valley dichroism with maximum circular polarization degree of $\sim 60\%$ in bilayer and $\sim$ 35% in trilayer ${\mathrm{MoS}}_2$ that is fully consistent with a rate equation model which includes input from electronic band structure calculations. We identify the highly delocalized electron wave function between the layers close to the high-symmetry $Q$ points as the origin of the tunable circular dichroism. Our results demonstrate the possibility of electric-field-tunable interlayer coupling for controlling emergent spin-valley physics and hybridization-driven effects in vdW materials and their heterostructures.