Electron core-hole processes at absorption edges sport several signatures, primarily in the form of bound states and/or excitonic enhancement. Through analysis of the resonant-Raman Auger effect for the two-dimensional transition-metal dichalcogenide $\mathrm{Mo}{\mathrm{S}}_2$, we demonstrate that these effects have significant manifestations at the S $K$ edge. We characterize two asymmetric excitonically enhanced edges: one at the fundamental band edge and one several electron volts above it following a second band gap that lies entirely within the unoccupied states, with the latter revealing an anomalous postcollision interaction effect. Our interpretation is supported by comparison of both the photoemission valence-band spectrum and the S $K$-edge x-ray-absorption spectrum with electronic structure calculations and an ab initio solution of the Bethe-Salpeter equation that accurately accounts for the electron-core-hole interaction.

The interplay between symmetry and topology in electronic band structures has been one of the central subjects in condensed-matter physics. Recently, it has been getting clear that a wide variety of useful information about the band topology can be extracted by focusing the symmetry representations of valence bands without computing Wilson loops. In this paper, we extend the previous studies on this subject to all ten Altland-Zirnbauer symmetry classes in each of 230 space groups. We derive various general statements that should be useful in the search for topological superconductors and topological semimetals.

The standard theoretical understanding of the lattice thermal conductivity ${\kappa }_{\ell }$ of semiconductor alloys assumes that mass disorder is the most important source of phonon scattering. In contrast, we find that for the random alloy ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$ the hitherto neglected contribution of interatomic force-constant (IFC) disorder is essential for the prediction of ${\kappa }_{\ell }$. We present an ab initio method based on special quasirandom structures and Green's functions which includes the role of IFC disorder and apply it in order to calculate ${\kappa }_{\ell }$ of ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$ and ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ alloys. We show that, while for ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$, phonon-alloy scattering is dominated by mass disorder, for ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$, the inclusion of IFC disorder is fundamental to reproduce the experimentally observed ${\kappa }_{\ell }$. We relate this to the underlying atomic-scale structural disorder in ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$. This feature is common to most III-V and II-VI random semiconductor alloys, and we expect the inclusion of IFC disorder in modeling lattice thermal conductivity to be important for a wide class of materials.

Gratings of infinite wires make, perhaps, the simplest class of metasurfaces, which, however, are utilized for a variety of different objectives in wave manipulation. Importantly, due to their analytical solvability, they can be fully optimized in a fast and direct way. In this study, a lattice of periodically corrugated cylindrical particles under oblique plane-wave excitation is considered and treated rigorously. Several cases of particle radii, distances between two consecutive cylinders, and angles of illumination are examined; as a result, very high selectivity of the metasurface response in terms of line-of-sight transmission and absorption is reported. That abrupt change in the device output becomes even more dramatic in the parametric vicinity of the emergence of new refractive orders, which makes the proposed metasurface exceptionally fitting for switching, filtering, and sensing applications.

We investigate the radiative heat transfer along a chain of nanoparticles using both a purely kinetic approach based on the solution of a Boltzmann transport equation and an exact method (Landauer's approach) based on fluctuational electrodynamics for temperatures close to room temperature. We show that the kinetic theory generally fails to predict properly the heat flux transported along the chain at both close (near-field regime) and large-separation (far-field regime) distances. We report a deviation of a factor of 2 between the heat fluxes predicted by the two approaches in the diffusive regime of heat transport, and we show that this difference becomes even greater than two orders of magnitude in the ballistic regime. In the case of metallic nanoparticles, we show that the kinetic approach completely fails in describing the magnetic contribution and gives dramatically wrong results for the electric one.

Hierarchical material nanostructuring is considered to be a very promising direction for high performance thermoelectric materials. In this work we investigate thermal transport in hierarchically nanostructured silicon. We consider the combined presence of nanocrystallinity and nanopores, arranged under both ordered and randomized positions and sizes, by solving the Boltzmann transport equation using the Monte Carlo method. We show that nanocrystalline boundaries degrade the thermal conductivity more drastically when the average grain size becomes smaller than the average phonon mean-free path. The introduction of pores degrades the thermal conductivity even further. Its effect, however, is significantly more severe when the pore sizes and positions are randomized, as randomization results in regions of higher porosity along the phonon transport direction, which introduce significant thermal resistance. We show that randomization acts as a large increase in the overall effective porosity. Using our simulations, we show that existing compact nanocrystalline and nanoporous theoretical models describe thermal conductivity accurately under uniform nanostructured conditions, but overestimate it in randomized geometries. We propose extensions to these models that accurately predict the thermal conductivity of randomized nanoporous materials based solely on a few geometrical features. Finally, we show that the new compact models introduced can be used within Matthiessen's rule to combine scattering from different geometrical features within $\sim 10\%$ accuracy.

We study the quantum nonlinear Hall effect in two-dimensional (2D) materials with time-reversal symmetry. When only one mirror line exists, a transverse charge current occurs in the second-order response to an external electric field, as a result of the Berry curvature dipole in momentum space. Candidate 2D materials to observe this effect are two-dimensional transition metal dichalcogenides (TMDCs). First, we use an ab initio based tight-binding approach to demonstrate that monolayer $T_d$-structure TMDCs exhibit a finite Berry curvature dipole. In the $1H$ and $1T^{\prime }$ phase of TMDCs, we show the emergence of a finite Berry curvature dipole with the application of strain and an electrical displacement field, respectively.

Interfacing solid-state emitters with photonic structures is a key strategy for developing highly efficient photonic quantum technologies. Such structures are often organized into two distinct categories: nanocavities and waveguides. However, any realistic nanocavity structure simultaneously has characteristics of both a cavity and waveguide, which is particularly pronounced when the cavity is constructed using low-reflectivity mirrors in a waveguide structure with good transverse light confinement. In this regime, standard cavity quantum optics theory breaks down, as the waveguide character of the underlying dielectric is only weakly suppressed by the cavity mirrors. By consistently treating the photonic density of states of the structure, we provide a microscopic description of an emitter including the effects of phonon scattering over the full transition range from waveguide to cavity. This generalized theory lets us identify an optimal regime of operation for single-photon sources in optical nanostructures, where cavity and waveguide effects are concurrently exploited.

We show how an accurate first-principles treatment of the antiferromagnetic ground state of ${\mathrm{La}}_2{\mathrm{CuO}}_4$ can be obtained without invoking any free parameters such as the Hubbard $U$. The magnitude and orientation of our theoretically predicted magnetic moment of $0.495{\mu }_B$ on Cu sites along the (100) direction are in excellent accord with experimental results. The computed values of the band gap (1.00 eV) and the exchange coupling ($-138$ meV) match the corresponding experimental values. We identify interesting band splittings below the Fermi energy, including an appreciable Hund's splitting of 1.25 eV. The magnetic form factor obtained from neutron scattering experiments is also well described by our calculations. Our study thus opens up a pathway for first-principles investigations of electronic and atomic structures and phase diagrams of cuprates and other complex materials.

We examine the Chern Kondo insulator proposed in a square optical lattice with staggered flux induced by $s\text{−}p$ orbital hybridization by revisiting its realization and taking into account the magnetic effects for the Kondo phases. The Ruderman-Kittel-Kasuya-Yoshida interaction is analyzed at the weak $s\text{−}p$ hybridization regime, with the anisotropic magnetic effects being discussed. Furthermore, the paramagnetic and magnetic phases coexisting with Kondo couplings are systematically investigated through the slave-boson theory, for which the rich effects on the Chern Kondo phase are obtained, including the antiferromagnetic, collinear antiferromagnetic Kondo insulator, and Kondo metal phases. The magnetic orders are shown to enhance the effective Kondo hybridization compared with the case without taking into account magnetic effects, and exhibit different influences on the bulk topology. In particular, the antiferromagnetic ordering always enhances the topological phase by increasing the bulk gap of the Chern Kondo phases. The results show the rich topological and magnetic effects obtained in the present Chern Kondo lattice model. We also investigate how to identify the topology and strong correlation effects through measuring the Hall conductance and double occupancy, which are achievable in ultracold atom experiments.

We review the magnetic and orbital ordered states in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ by performing resonant elastic x-ray scattering (REXS) at the Ru ${\mathrm{L}}_{2,3}$ edges. In principle, the point symmetry at Ru sites does not constrain the direction of the magnetic moment below $T_N$. However early measurements reported the ordered moment entirely along the $\vec{b}$ orthorhombic axis. Taking advantage of the large resonant enhancement of the magnetic scattering close to the Ru ${\mathrm{L}}_2$ and ${\mathrm{L}}_3$ absorption edges, we monitored the azimuthal, thermal, and energy dependence of the REXS intensity and find that a canting ($m_c\simeq 0.1m_b$) along the $\vec{c}$-orthorhombic axis is present. No signal was found for $m_a$ despite this component also being allowed by symmetry. Such findings are interpreted by a microscopic model Hamiltonian and pose new constraints on the parameters describing the model. Using the same technique we reviewed the accepted orbital ordering picture. We detected no symmetry breaking associated with the signal increase at the “so-called” orbital ordering temperature ($\simeq 260$ K). We did not find any changes of the orbital pattern even through the antiferromagnetic transition, suggesting that, if any, only a complex rearrangement of the orbitals, not directly measurable using linearly polarized light, can take place.

In this paper, first principles computations are performed to investigate structural, electronic, magnetic, and thermodynamic properties of a novel two-dimensional ferromagnet/semiconductor junction, consisting of a monolayer of FeNi on a single monolayer of ${\mathrm{MoS}}_2$. After investigating more than 30 different arrangements of Fe and Ni atoms on ${\mathrm{MoS}}_2$, it is concluded that Ni-S bonding is more likely at the FeNi/${\mathrm{MoS}}_2$ interface, while highly spin polarized Fe atoms prefer to stay well above the ${\mathrm{MoS}}_2$ monolayer, forming a zigzag FeNi layer at the surface. Accurate investigation of the electronic structure of the most stable configuration of FeNi/${\mathrm{MoS}}_2$ junction evidences appearance of some majority and minority surface charge carriers with very different mobilities, predicting a high spin polarized conductivity in the junction. Tens of different spin orderings are applied to the FeNi monolayer in a $3\times 3$ supercell to achieve reliable Heisenberg exchange constants for parametrization of the magnetic energy of the FeNi/${\mathrm{MoS}}_2$ junction. The obtained exchange constants are then used for classical Monte Carlo simulation of magnetic thermodynamic properties of the system, including magnetic order parameter, heat capacity, and magnetic susceptibility. The predicted very high paramagnetic transition temperature ($>1300\text{K}$) along with the speculated high spin polarized conductivity evidences potential application of this junction in spintronics.

We study theoretically the differential conductance at a junction between a time-reversal symmetry-broken spin-orbit coupled system with a tunable band gap and a superconductor. We look for spin-dependent Andreev reflection (i.e., subgap transport) and show that when various mass terms compete in energy, there is substantial difference of Andreev reflection probability depending on the spin of the incident electron. We further analyze the origin of such spin dependence and show how the incident angle of the electrons controls the spin dependence of the transport.

Local structural heterogeneity is known to enhance the piezoelectric response of perovskite-based ferroelectrics. Here we investigate how this heterogeneity is affected by strong electric fields using dilute doped ${\mathrm{Eu}}^{+3}$ ions as local structural probes. We make use of the extreme sensitivity of the ${}^{5}D_0\to {}^{7}F_2$ hypersensitive transition of ${\mathrm{Eu}}^{+3}$ vis-à-vis the asymmetric distribution of the ligand field around the doped ion to demonstrate that a strong electric field (poling) increases the overall low-symmetry regions on the local length scale in a ${\mathrm{BaTiO}}_3$-based lead-free piezoceramic near the $P4mm\text{−}Amm2$ polymorphic phase boundary (PPB). Our study suggests that the mandatory treatment of poling given to a ceramic to make it piezoactive also contributes to the microscopic mechanisms that improve the system's piezo-response.

We use a time-dependent density functional theory (TDDFT) to analyze nonequilibrium dynamics of laser-excited electrons in transition metals (Ni, Cr, Cu) and shed light on the ultrafast thermalization process from a microscopical point of view. As a first result, after instant increase of the electron temperature up to 50 000 K, we observe that the dynamics of electron density of states is faster than a laser subcycle, on the attosecond timescale. This is related to an ultrafast rearrangement of excited electrons in space to accommodate strong changes in ion screening. Secondly, we show that the electron thermalization dynamics strongly depends on the electronic structure of a given metal. Excited by a 7-fs laser pulse, $d$-block transition metals exhibit two subsystems of electrons, each one achieving its own temperature. Due to a higher localization, electrons from $d$ block stay cold, while excited delocalized $sp$ electrons rapidly reach a high temperature. Electrons of each band of energy are mutually thermalized within the time of the laser pulse. Much more time is however needed to reach equilibrium of the whole electronic system. These results redraw the validity limits of current two-temperature models during the laser irradiation time, potentially impacting further material reaction.

We study the emergence and disappearance of defect states in the complex Su-Schrieffer-Heeger (cSSH) model, a non-Hermitian one-dimensional lattice model containing gain and loss on alternating sites. Previous studies of this model have focused on the existence of a non-Hermitian defect state that is localized to the interface between two cSSH domains, and is continuable to the topologically protected defect state of the Hermitian Su-Schrieffer-Heeger (SSH) model. For large gain/loss magnitudes, we find that these defect states can disappear into the continuum, or undergo pairwise spontaneous breaking of a composite sublattice/time-reversal symmetry. The symmetry-breaking transition gives rise to a pair of defect states continuable to nontopologically protected defect states of the SSH model. We discuss the phase diagram for the defect states, and its implications for non-Hermitian defect states.

The cuprate high-temperature superconductors are known to host a wide array of effects due to interactions and disorder. In this paper, we look at some of the consequences of these effects which can be visualized by scanning tunneling spectroscopy in order to provide a guide for future experiments. These interaction and disorder effects can be incorporated into a mean-field description by means of a self-energy appearing in the Green's function. We first examine the quasiparticle scattering interference (QPI) spectra in the superconducting state at optimal doping as temperature is increased. Assuming agreement with angle-resolved photoemission experiments which suggest that the scattering rate depends on temperature, resulting in the filling of the $d$-wave gap, we find that the peaks predicted by the octet model become progressively smeared as temperature is increased. When the scattering rate is of the same order of magnitude as the superconducting gap, the spectral function shows Fermi-arc-like patterns, while the power spectrum of the local density of states shows the destruction of the octet-model peaks. We next consider the normal state properties of the optimally-doped cuprates. We model this by adding a marginal Fermi liquid (MFL) self-energy to the normal-state propagator and consider the dependence of the QPI spectra on frequency, temperature, and doping. We demonstrate that the MFL self-energy leads to a smearing of the caustics appearing in the normal-state QPI power spectrum as either temperature or frequency is increased at fixed doping. The smearing is found to be more prominent in the MFL case than in an ordinary Fermi liquid. We also consider the case of a marginal Fermi liquid with a strongly momentum-dependent self-energy which gives rise to a visible “nodal-antinodal” dichotomy at the normal state and discuss how the spectra as seen in ARPES and STS differ from both an isotropic metal and a broadened $d$-wave superconductor. Finally, we discuss how these results become modified in the presence of weak distributed disorder and finite-temperature smearing.

It is widely assumed that disordered auxetic structures (i.e., structures with a negative Poisson's ratio) must contain reentrant polygons in two dimensions ($2\mathrm{D}$) and reentrant polyhedra in $3\mathrm{D}$. Here, we show how to design disordered networks in $2\mathrm{D}$ with only convex polygons. The design principles used allow for any Poisson's ratio $-1<\nu <1/3$ to be obtained with a prescriptive algorithm. By starting from a Delaunay triangulation with a mean coordination $\langle z\rangle \simeq 6$ and $\nu \simeq 0.33$ and removing those edges that decrease the shear modulus the least, without creating any reentrant polygons, the system evolves monotonically towards the isostatic point with $\langle z\rangle \simeq 4$ and $\nu \simeq -1$.

The orbital contribution to the magnetic moment of the transition-metal ion in the isostructural weak ferromagnets $A{\mathrm{CO}}_3$ ($A=$ Mn,Co,Ni) and ${\mathrm{FeBO}}_3$ was investigated by a combination of first-principles calculations, nonresonant x-ray magnetic scattering, and x-ray magnetic circular dichroism. A nontrivial evolution of the orbital moment as a function of the $3d$ orbitals filling is revealed, with a particularly large value found in the Co member of the family. Here, the coupling between magnetic and lattice degrees of freedom produced by the spin-orbit interaction results in a large single-ion anisotropy and a peculiar magnetic-moment-induced electron cloud distortion, evidenced by the appearance of a subtle scattering amplitude at space-group-forbidden reflections and significant magnetostrictive effects. Our results, which complement a previous investigation on the sign of the Dzyaloshinskii-Moriya interaction across the series, highlight the importance of spin-orbit coupling in the physics of weak ferromagnets and prove the ability of modern first-principles calculations to predict the properties of materials where the Dzyaloshinskii-Moriya interaction is a fundamental ingredient of the magnetic Hamiltonian.

The XY pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, with pseudospin-1/2 at the ${\mathrm{Yb}}^{3+}$ site, has been celebrated as a potential host for the quantum spin ice state. The substitution of nonmagnetic Ti with Pt gives ${\mathrm{Yb}}_2{\mathrm{Pt}}_2{\mathrm{O}}_7$, a system with remarkably similar magnetic properties. The large nuclear gyromagnetic ratio (${\gamma }_n/2\pi =9.15$ MHz/T) of ${}^{195}\mathrm{Pt}$ makes ${\mathrm{Yb}}_2{\mathrm{Pt}}_2{\mathrm{O}}_7$ an ideal material for NMR investigation of its unconventional magnetic properties. Based on the ${}^{195}\mathrm{Pt}$ nuclear spin-lattice relaxation rate $1/T_1$ and the magnetic specific heat $C_p$ measured in a broad range of magnetic field $B_{\mathrm{ext}}$, we demonstrate that the field-induced magnon gap linearly decreases with $B_{\mathrm{ext}}$ but an additional low-energy mode of spin excitations emerges below $\sim 0.5$ T.