The electronic structure of epitaxial $\mathrm{LaV}{\mathrm{O}}_3$ (00l) thin films have been studied using resonant photoemission spectroscopy (RPES) and x-ray absorption near edge spectroscopy (XANES) measurements. The RPES study confirms the mixing of V $3d$ with O $2p$ states and the V $3d$ character at 6.9 eV and 1.5 eV binding energies, respectively. The resonant behavior of V $3d$ state suggests $3d^{n-1}$ final state, ascribed with an incoherent structure, and is attributed as a lower Hubbard band. The smaller value of crystal field splitting energy observed in O $K$-edge XANES spectrum and the hopping parameter obtained from simulations reveal a weaker hybridization between ligand O $2p$ and metal V $3d$ orbitals. Combined spectra of occupied and unoccupied states suggest the Mott-Hubbard type insulating nature of the epitaxial $\mathrm{LaV}{\mathrm{O}}_3$ thin films at room temperature, different from its bulk counterpart, which is placed intermediate between the charge transfer and Mott-Hubbard regime.

We study optical pumping of resident electron spins under resonant excitation of trions in $n$-type CdTe/(Cd,Mg)Te quantum wells subject to a transverse magnetic field. In contrast to the comprehensively used time-resolved pump-probe techniques with polarimetric detection, we exploit here a single beam configuration in which the time-integrated intensity of the excitation laser light transmitted through the quantum wells is detected. The transmitted intensity reflects the bleaching of light absorption due to optical pumping of the resident electron spins and can be used to evaluate the Larmor precession frequency of the optically oriented carriers and their spin relaxation time. Application of the magnetic field leads to depolarization of the electron spin ensemble so that the Hanle effect is observed. Excitation with a periodic sequence of laser pulses leads to optical pumping in the rotating frame if the Larmor precession frequency is synchronized with the pulse repetition rate. This is manifested by the appearance of Hanle curves every 3.36 or 44.2 mT for pulse repetition rates of 75.8 or 999 MHz, respectively. From the experimental data we evaluate the $g$ factor of $\left|g\right|=1.61$ and the spin relaxation time of 14 ns for the optically pumped resident electrons, in agreement with previous time-resolved pump-probe studies.

We present a scheme to generate an artificial gauge field for the system of neutral bosons, represented by polaritons in micropillars arranged into a square lattice. The splitting between the two polarizations of the micropillars breaks the time-reversal symmetry (TRS) and results in the effective phase-dependent hopping between cavities. This can allow for engineering a nonzero flux on the plaquette, corresponding to an artificial magnetic field. Changing the phase, we observe a characteristic Hofstadter's butterfly pattern and the appearance of chiral edge states for a finite-size structure. For long-lived polaritons, we show that the propagation of wave packets at the edge is robust against disorder. Moreover, given the inherent driven-dissipative nature of polariton lattices, we find that the system can exhibit topological lasing, recently discovered for active ring cavity arrays. The results point to a static way to realize artificial magnetic field in neutral spinful systems, avoiding the periodic modulation of the parameters or strong spin-orbit interaction. Ultimately, the described system can allow for high-power topological single-mode lasing which is robust to imperfections.

We consider a single-level quantum dot coupled to two leads which are ferromagnetic in general. Apart from tunneling processes conserving electron spin, we also include processes associated with spin flip of tunneling electrons, which appear due to Rashba spin-orbit coupling. Charge and heat currents are calculated within the nonequilibrium Green's function technique. When the electrodes are half metallic (fully spin polarized), the Rashba spin-orbit coupling leads to Fano-like interference effects, which result in an enhanced thermoelectric response. It is also shown that such a system can operate as a heat engine with a remarkable efficiency. Furthermore, the interplay of Rashba spin-orbit coupling and Zeeman splitting due to an external magnetic field is shown to allow controlling of such parameters of the heat engine as the power and efficiency.

Thermal and dynamical properties of optical and transport conductivities in doped buckled honeycomb lattices are studied for various doping densities and band gaps. At finite temperatures, a thermally convoluted polarization function is calculated by employing analytically derived temperature-dependent chemical potentials. With this finite-temperature polarization function, the optical conductivity, originating from an induced polarization current, is obtained in the long-wavelength limit, where both steps and negative peaks are shown as a function of frequency in its real and imaginary parts, respectively. Such spectral features can be used for analyzing plasmon dampings in silicene and ultrafast light modulations based on field-tunable band gaps. Additionally, in the presence of static screening, derived with the aid of the polarization function, for impurity elastic scattering, the transport conductivities are calculated for different doping densities and band gaps within the second-order Born approximation. The enhanced transport conductivity with a smaller band gap at intermediate temperatures will lead to high-mobility electron transistors for ultrafast electronics.

We demonstrate that an aperiodic array of certain quantum networks comprising magnetic and nonmagnetic atoms can act as perfect spin filters for particles with arbitrary spin state. This can be achieved by introducing minimal quasi-one dimensionality in the basic structural units building up the array, along with an appropriate tuning of the potential of the nonmagnetic atoms, the tunnel hopping integral between the nonmagnetic atoms, and the backbone, and, in some cases, by tuning an external magnetic field. This latter result opens up the interesting possibility of designing a flux-controlled spin demultiplexer using quantum networks. The proposed networks have close resemblance with a family of recently developed photonic lattices, and the scheme for spin filtering can thus be linked, in principle, to the possibility of suppressing any one of the two states of polarization of a single photon, almost at will. We use transfer matrices and a real space renormalization group scheme to unravel the conditions under which any aperiodic arrangement of such topologically different structures will filter out any given spin projection. The filtering turns out to be engineered by an energy-independent commutation of the basic transfer matrices, which results out of a unique set of correlation between the system parameters and/or the external flux. The commutation generates absolutely continuous subbands populated by extended, Bloch-like eigenstates in the densities of states, even for such aperiodic systems, thus defying localization and creating unattenuated transport over a continuous range of energy eigenvalues. This is an example which goes well beyond the previous studies on disordered systems, where delocalization of single particle excitations could be achieved by resonance, but only for a finite set of energy eigenvalues of the system. Our results are analytically exact, and corroborated by extensive numerical calculations of the spin-polarized transmission and the density of states of such systems.

Two-dimensional semiconducting transition metal dichalcogenides (TMDs) exhibit an intrinsic Ising spin-orbit coupling (SOC) along with a valley contrasting Berry curvature, which can generate a purely anomalous spin and valley Nernst signal driven by a thermal gradient. We show that a small Bychkov-Rashba coupling, which is present in gated TMDs, can enhance the valley Nernst signal by at least 1–2 orders of magnitude. We find that the Nernst signal in these materials is dominated by the anomalous geometrical contribution, and the conventional contribution is much weaker. Importantly, the Nernst signal is also highly tunable by external gating. Although the total Nernst signal vanishes due to time reversal (TR) symmetry, a small magnetic coupling lifts the valley degeneracy and generates an amplified Nernst response. Additionally, we also discuss the Nernst response of bilayer TMDs, and show a similar enhancement and modulation of the Nernst signal due to Rashba SOC. Our predictions are highly pertinent to ongoing experimental studies in TMDs. The generated large anomalous Nernst signal can directly probe the presence of a large Berry curvature in these materials, and may serve as a promising tunable platform for caloritronics applications.

While ${\mathrm{CaFeO}}_3$ exhibits upon cooling a metal-insulator transition linked to charge ordering, ${\mathrm{SrFeO}}_3$ and ${\mathrm{BaFeO}}_3$ keep metallic behaviors down to very low temperatures. Moreover, alkaline-earth ferrites do not seem prone to orbital ordering in spite of the $d^4$ formal occupancy of ${\mathrm{Fe}}^{4+}$. Here, from first-principles simulations, we show that the metal-insulator transition of ${\mathrm{CaFeO}}_3$ is structurally triggered by oxygen rotation motions as in rare-earth nickelates. This not only further clarifies why ${\mathrm{SrFeO}}_3$ and ${\mathrm{BaFeO}}_3$ remain metallic but allows us to predict that an insulating charge-ordered phase can be induced in ${\mathrm{SrFeO}}_3$ from an appropriate engineering of oxygen rotation motions. Going further, we unveil the possibility to switch from the usual charge-ordered to an orbital-ordered insulating ground state under moderate tensile strain in ${\mathrm{CaFeO}}_3$ thin films. We rationalize the competition between charge and orbital orderings, highlighting alternative possible strategies to produce such a change of ground state, also relevant to manganite and nickelate compounds.

We provide the bulk topological invariant for chiral higher-order topological insulators in (i) fourfold rotoinversion invariant bulk crystals, and (ii) inversion-symmetric systems with or without an additional threefold rotation symmetry. These states of matter are characterized by a nontrivial ${\mathbb{Z}}_2$ index, which we define in terms of symmetric hybrid Wannier functions of the filled bands, and can be readily calculated from the knowledge of the crystalline symmetry labels of the bulk band structure. The topological invariant determines the generic presence or absence of protected chiral gapless one-dimensional modes localized at the hinges between conventional gapped surfaces.

${\text{In}}_x{\text{Ga}}_{1-x}\text{N}$ disks in GaN nanowires (DINWs) have emerged as a viable technology for on-chip tunable visible spectrum emission without the use of a phosphor. Here, we present a study of the optical emission and absorption dynamics in DINWs that incorporates the important role of background disorder states. The optical emission in the system is dominated by quantum-confined excitons, however, we show here that the excitons are coupled to a large density of background disorder states. Rapid nonradiative decay (compared to other decay rates such as spontaneous emission) from disorder states into excitons is observed after optical excitation of our sample that dominates the nonlinear absorption dynamics. Because disorder states are ubiquitous in InGaN layers, we believe that this result reveals an important decay channel that should be incorporated in future modeling and engineering of InGaN-based optical devices in general.

We have developed an exchange-correlation kernel in the framework of time-dependent density-functional theory that remarkably accounts for the electron-hole interaction and the optical properties of semiconductors. This kernel, which we name JGM-G, generalizes the jellium-with-gap kernel of Trevisanutto et al. [Phys. Rev. B 87, 205143 (2013)] by considering the gradient of the density as a new ingredient. We have tested it on various materials, from low-band-gap to wide-band-gap semiconductors, and we have demonstrated that the JGM-G kernel yields an optical response that is close to experiment. These encouraging results indicate that the JGM-G kernel can become a low-cost and practical tool for the description of excitonic effects in semiconductors.

We theoretically investigate several types of plasmonic slot waveguides for enhancing the detected signal in Raman spectroscopy, which is a consequence of electric field and Purcell factor enhancements, as well as an increase in light-matter interaction volume and the Raman signal collection efficiency. An intuitive methodology is presented for calculating the accumulated Raman enhancement factor of an ensemble of molecules in waveguide sensing, which exploits an analytical photon Green function expansion in terms of the waveguide normal modes, and we combine this with a quantum optics formalism of the molecule-waveguide interaction to model Raman scattering. We subsequently show how integrated plasmonic slot waveguides can attain significantly higher Raman enhancement factors: $\sim 5.3\times$ compared to optofluidic fibers and $\sim 3.7\times$ compared to planar integrated dielectric waveguides, with a device size and thus analyte volume of at least three orders of magnitude less. We also provide a comprehensive comparison between the different types of plasmonic slot waveguides based on the important figures of merit, and determine the optimal approaches to maximize Raman enhancement.

We develop a temperature-dependent theory for singlet exciton hopping transport in disordered semiconductors. It draws on the transport level concept within a Förster transfer model and bridges the gap in describing the transition from equilibrium to nonequilibrium time-dependent spectral diffusion. We test the validity range of the developed model using kinetic Monte Carlo simulations and find agreement over a broad range of temperatures. It reproduces the scaling of the diffusion length and spectral shift with the dimensionless disorder parameter and describes in a unified manner the transition from equilibrium to nonequilibrium transport regime. We find that the diffusion length in the nonequilibrium regime does not scale with the the third power of the Förster radius. The developed theory provides a powerful tool for interpreting time-resolved and steady state spectroscopy experiments in a variety of disordered materials, including organic semiconductors and colloidal quantum dots.

The conventional first-principles theory for the thermal and thermodynamic properties of insulators is based on the perturbative treatment of the anharmonicity of crystal bonds. While this theory has been a successful predictive tool for strongly bonded solids such as diamond and silicon, here we show that it fails dramatically for strongly anharmonic (weakly bonded) materials, and that the conventional quasiparticle picture breaks down at relatively low temperatures. To address this failure, we present a unified first-principles theory of the thermodynamic and thermal properties of insulators that captures multiple thermal properties within the same framework across the full range of anharmonicity from strongly bonded to weakly bonded insulators. This theory features a new phonon renormalization approach derived from many-body physics that creates well-defined quasiparticles even at relatively high temperatures, and it accurately captures the effects of strongly anharmonic bonds on phonons and thermal transport. Using a prototypical strongly anharmonic material, sodium chloride (NaCl), as an example, we demonstrate that our new first-principles framework simultaneously captures the apparently contradictory experimental observations of large thermal expansion and low thermal conductivity of NaCl on the one hand, and anomalously weak temperature dependence of phonon modes on the other, while the conventional theory fails in all three cases. We demonstrate that four-phonon scattering due to higher-order anharmonicity significantly lowers the thermal conductivity of NaCl and is required for a proper comparison to experiment. Furthermore, we show that our renormalization framework, along with four-phonon scattering, also successfully predicts the measured phonon frequencies and thermal properties of a weakly anharmonic material, diamond, indicating universal applicability for thermal properties of insulators. Our work gives new insights into the physics of heat flow in solids, and presents a computationally efficient and rigorous framework that captures the thermal and thermodynamic properties of both weakly and strongly bonded insulators simultaneously.

Modal analysis based on the quasinormal modes (QNMs), also called resonant states, has emerged as a promising way for modeling the resonant interaction of light with open optical cavities. However, the fields associated with QNMs in open photonic cavities diverge far away from the scatterer and the possibility of expanding the scattered field with only resonant contributions has not been established. Here, we address these two issues while restricting our study to the case of a dispersionless spherical scatterer. First, we derive the rigorous pole expansion of the $T$-matrix coefficients that link the scattered to the incident fields associated with an optical resonator. This expansion evinces the existence of a nonresonant term. Second, in the time domain, the causality principle allows us to solve the problem of divergence and to derive a modal expansion of the scattered field that does not diverge far from the scatterer.

We consider electrons in tubular nanowires with prismatic geometry and infinite length. Such a model corresponds to a core-shell nanowire with an insulating core and a conductive shell. In a prismatic shell the lowest energy states are localized along the edges (corners) of the prism and are separated by a considerable energy gap from the states localized on the prism facets. The corner localization is robust in the presence of a magnetic field longitudinal to the wire. If the magnetic field is transversal to the wire the lowest states can be shifted to the lateral regions of the shell, relatively to the direction of the field. These localization effects should be observable in transport experiments on semiconductor core-shell nanowires, typically with hexagonal geometry. We show that the conductance of the prismatic structures considerably differs from the one of circular nanowires. The effects are observed for sufficiently thin hexagonal wires and become much more pronounced for square and triangular shells. To the best of our knowledge, the internal geometry of such nanowires is not revealed in experimental studies. We show that with properly designed nanowires these localization effects may become an important resource of interesting phenomenology.

Isothermal simulational data for the three-dimensional Edwards-Anderson (E-A) spin glass are collected at several temperatures below $T_{\mathrm{c}}$ and, in analogy with a recent model of dense colloidal suspensions, interpreted in terms of clusters of contiguous spins overturned by quakes, i.e., nonequilibrium events linked to record-size energy fluctuations. We show numerically that to a good approximation, these quakes are statistically independent and constitute a Poisson process whose average grows logarithmically in time. The overturned clusters are local projections on one of the two ground states of the model, and grow likewise logarithmically in time. Data collected at different temperatures $T$ can be collapsed by scaling them with $T^{-1.75}$, which we relate, on the one hand, to the geometry of configuration space and, on the other, to experimental memory and rejuvenation effects. The rate at which a cluster flips is shown to decrease exponentially with the size of the cluster, as recently assumed in a coarse-grained model of dense colloidal dynamics. The evolving structure of clusters in real space is finally associated to the decay of the thermo-remanent magnetization. Our analysis provides an unconventional coarse-grained description of spin-glass aging as statistically subordinated to a Poisson quaking process and highlights record dynamics as a viable common theoretical framework for aging in different systems.

The magnetic and thermodynamic properties of metallic single-crystalline ${\mathrm{Gd}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$, wherein magnetic Gd-Al layers with a distorted kagome lattice structure and nonmagnetic Ru-Al layers are stacked alternately along the $c$ axis, have been reported. The present investigation demonstrates the gradual formation of ferromagnetic trimers on a distorted kagome lattice with a decrease in temperature by the Ruderman-Kittel-Kasuya-Yosida interaction. This may induce an effective transformation into an antiferromagnetic triangular lattice at low temperatures.