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.

We present a promising machine-learning model, which focuses on site magnetic properties for rapid screening in materials design. We perform high-throughput first-principles calculations to predict the magnetic anisotropy energies of a variety of iron-cobalt nitrides. We illustrate the efficacy of a spatial decomposition technique that divides the total magnetic anisotropy energy into contributions from individual sites in terms of spin-orbit coupling energies. The spatial decomposition scheme that we utilized works for a wide range of magnetic anisotropy energies. We also construct a machine-learning model by combining the site-specific spin-orbit coupling energies with structural details on each atomic site. We adopt the same approach to predicting the site-specific magnetic moments. We demonstrate the capability of our machine-learning model to accelerate computational screening of candidate materials that possess high magnetizations and large magnetic anisotropy energies.

Developing simple ways to control spin states in spintronic devices is a crucial step towards increasing their functionality. MnTe is a room-temperature antiferromagnet with promising spintronic properties, including for thermospintronics and magnon-based devices. Here, we show that, in MnTe, less than 1% Li is sufficient to produce a dramatic spin reorientation as observed by neutron diffraction. The behavior of the 0001 magnetic Bragg peak reveals a significant reorientation of the Mn spins from planar in the pure material to almost completely axial with minimal Li doping. Temperature dependence of the magnetic peaks in Li-doped samples indicates that axial spins shift back to planar suddenly upon approaching the Néel temperature ($T_N=307$ K). Density functional theory calculations support the idea that a shift in the Fermi level caused by doping is responsible for switching the material between two competing magnetic ground states. These results pave the way for developing easy switching of magnetic states in functional materials such as spintronic devices and topological insulators.

In the quest for materials for ferroelectrics-based spintronics with a large spin-orbit coupling, it is essential to carefully control the ferroelectric domains structure, their spatial organization, and the domain wall type. Here we perform the growth of GeTe thin films on Si by molecular beam epitaxy in a large thickness range. We show that the volume fraction along with the size of the ferroelectric nanodomains can be controlled by finely adjusting the deposition thickness and temperature. We evidence that the formation of ${71}^{\circ }$-type domain walls and in situ measurements during thermal cycling show the hysteretic appearance and decay of ferroelectric domains. In combination with a detailed analysis of the GeTe/Si interface, we demonstrate that the interfacial misfit dislocations formed during the growth play a key role in the stability of the ferroelectric nanodomains.

We identify all symmetry-enforced band crossings in nonmagnetic orthorhombic crystals with and without spin-orbit coupling and discuss their topological properties. We find that orthorhombic crystals can host a large number of different band degeneracies, including movable Weyl and Dirac points with hourglass dispersions, fourfold double Weyl points, Weyl and Dirac nodal lines, almost movable nodal lines, nodal chains, and topological nodal planes. Interestingly, spin-orbit coupled materials in space groups 18, 36, 44, 45, and 46 can have band pairs with only two Weyl points in the entire Brillouin zone. This results in simpler connectivity of the Fermi arcs and more pronounced topological responses than in materials with four or more Weyl points. In addition, we show that the symmetries of space groups 56, 61, and 62 enforce nontrivial weak ${\mathbb{Z}}_2$ topology in materials with strong spin-orbit coupling, leading to helical surface states. With these classification results in hand, we perform extensive database searches for orthorhombic materials crystallizing in the relevant space groups. We find that ${\mathrm{Sr}}_2{\mathrm{Bi}}_3$ and ${\mathrm{Ir}}_2\mathrm{Si}$ have bands crossing the Fermi energy with a symmetry-enforced nontrivial ${\mathbb{Z}}_2$ invariant, CuIrB possesses nodal chains near the Fermi energy, ${\mathrm{Pd}}_7{\mathrm{Se}}_4$ and ${\mathrm{Ag}}_2\mathrm{Se}$ exhibit fourfold double Weyl points, the latter one even in the absence of spin-orbit coupling, whereas the fourfold degeneracies in AuTlSb are made up from intersecting nodal lines. For each of these examples, we compute the ab initio band structures, discuss their topologies, and for some cases also calculate the surface states.

We study the flow behavior of a twist-bend nematic (${\mathrm{N}}_{\mathrm{TB}}$) liquid crystal. It shows three dynamic regimes in certain temperature and shear-rate range. In region I, $\sigma \sim \sqrt{\dot{\gamma }}$, in region II, the stress shows a near plateau, characterized by a power law $\sigma \sim {\dot{\gamma }}^{\alpha }$, where $\alpha \sim 0.1–0.4$ and in region III, $\sigma \sim \dot{\gamma }$. With increasing shear rate, $\sigma$ changes continuously from region I to II, whereas it changes discontinuously with a hysteresis from region II to III. In the near plateau (region II), we observe dynamic stress fluctuations, exhibiting regular, periodic, and quasiperiodic oscillations under the application of a steady shear. The observed spatiotemporal dynamics are close to those predicted theoretically in sheared nematogenic fluids.

We report high-resolution angular magnetoresistance (AMR) experiments performed on crystals of ${\mathrm{ReO}}_3$ with high mobility $(>100000{\mathrm{cm}}^2/\mathrm{V}$ s at 2 K) and extremely low residual resistivity $(5–8 \mathrm{n}\mathrm{\Omega }\mathrm{cm})$. The Fermi surface, comprised of intersecting cylinders, supports open orbits. The resistivity ${\rho }_{xx}$ in a magnetic field $B=9$ T displays a singular pattern of behavior. With $\mathbf{E}\parallel \widehat{\mathbf{x}}$ and $\mathbf{B}$ initially $\parallel \widehat{\mathbf{z}}$, tilting $\mathbf{B}$ in the longitudinal $k_z\text{−}{k}_x$ plane leads to a steep decrease in ${\rho }_{xx}$ by a factor of 40. However, if $\mathbf{B}$ is tilted in the transverse $k_y\text{−}{k}_z$ plane, ${\rho }_{xx}$ increases steeply by a factor of 8. Using the Shockley-Chambers tube integral approach, we show that, in ${\mathrm{ReO}}_3$, the singular behavior results from the rapid conversion of closed to open orbits, resulting in opposite signs for AMR in orthogonal planes. The floor values of ${\rho }_{xx}$ in both AMR scans are identified with specific sets of open and closed orbits. Also, the “completion angle” ${\gamma }_c$ detected in the AMR is shown to be an intrinsic geometric feature that provides a new way to measure the Fermi radius $k_F$. However, additional sharp resonant features that appear at very small tilt angles in the longitudinal AMR scans are not explained by the tube integral approach.

Databases of density functional theory (DFT) calculations, such as the Open Quantum Materials Database (OQMD), have paved the way for accelerated materials discovery. DFT calculations require crystal structure information as input; however, due to inherent challenges in solving a compound's structure from powder diffraction data alone, there are thousands of experimentally synthesized compounds whose structures remain unsolved. We present a rapid DFT-based structure solution method capable of resolving numerous outstanding structure solution problems at low computational cost. The method involves (1) searching inorganic compound databases for all prototypes that match known structural characteristics of the compound, such as stoichiometry, space group, and number of atoms per cell, (2) performing DFT calculations of the target composition in each of the structural prototypes, and (3) evaluating these prototypes as candidates using a combination of DFT energy and match between calculated and experimental diffraction pattern. As this approach is straightforward and inexpensive, we employ it to solve 521 previously unsolved compounds from the Powder Diffraction File, resulting in a 1.4% expansion of the set of all experimental compounds in the OQMD. DFT calculations of these compounds could yield valuable properties.

Based on first-principles density functional calculations, we report the existence of both nontrivial band topology and superconductivity in three metallic Si allotropes, termed $Cmcm\text{−}{\mathrm{Si}}_4$, $Cmmm\text{−}{\mathrm{Si}}_4$, and $I4/mmm\text{−}{\mathrm{Si}}_4$. $Cmcm\text{−}{\mathrm{Si}}_4$ and $I4/mmm\text{−}{\mathrm{Si}}_4$ are predicted in this study, whereas $Cmmm\text{−}{\mathrm{Si}}_4$ was proposed in previous theoretical calculations. These metastable allotropes retain their crystalline structure at ambient pressure and exhibit superconductivity at the critical temperatures of 1.2–11.4 K. We investigate the topological characteristics of the electronic states and find the weak topological nature for all three allotropes. For $Cmcm\text{−}{\mathrm{Si}}_4$, in particular, the formation of the surface Dirac point is clearly identified. Our result provides a promising platform for realizing a topological superconducting state in all-Si systems at ambient pressure.

Understanding the interaction between polar and superconducting order parameters may hold the key to several classes of superconductors that remain poorly understood, including $\mathrm{SrTi}{\mathrm{O}}_3$ and several tellurides. Here we show that doped, strained $\mathrm{SrTi}{\mathrm{O}}_3$ films can exhibit both global or local polar order, respectively, depending on the amount of epitaxial mismatch strain, thereby providing a platform to understand how inversion symmetry breaking affects superconductivity. We find that the superconducting critical temperature correlates with the length scale of polar order. In particular, the transition temperature is enhanced when polar nanodomains are sufficiently large or, in the extreme limit, films are globally ferroelectric. In these cases, the Cooper pairs reside in a noncentrosymmetric environment. Conversely, low transition temperatures are found when the nanodomains are small. The findings point to the length scale of polar nanodomains and spin-orbit coupling as important parameters controlling the superconductivity of $\mathrm{SrTi}{\mathrm{O}}_3$. The ability to control the size of the polar domains opens up new opportunities to design and control the nature of superconductivity in a wide range of materials.