We report the observation and the theoretical explanation of the parametric down-conversion nonlinear susceptibility at the $K$-absorption edge of diamond and at the $L_{23}$-absorption edge of a silicon crystal. Using arguments similar to those invoked to successfully predict resonant inelastic x-ray spectra, we derive an expression for the renormalization term of the nonlinear susceptibility at the x-ray edges, which can be evaluated by using first-principles calculations of the atomic scattering factor $f_1$. Our model is shown to reproduce the observed enhancement of the parametric down-conversion at the diamond $K$ and the Si $L_{23}$ edges rather than the suppression previously claimed.

Motivated by its role as a central pillar of current theories of the dynamics of spin ice in and out of equilibrium, we study the single-ion dynamics of the magnetic rare-earth ions in their local environments, subject to the effective fields set up by the magnetic moments with which they interact. This effective field has a transverse component with respect to the local easy axis of the crystal electric field, which can induce quantum tunneling. We go beyond the projective spin-1/2 picture and use instead the full crystal-field Hamiltonian. We find that the Kramers versus non-Kramers nature, as well as the symmetries of the crystal-field Hamiltonian, result in different perturbative behavior at small fields ($\lesssim 1\mathrm{T}$), with transverse field effects being more pronounced in ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ than in ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Remarkably, the energy splitting range we find is consistent with time scales extracted from experiments. We also present a study of the static magnetic response, which highlights the anisotropy of the system in the form of an off-diagonal $g$ tensor, and we investigate the effects of thermal fluctuations in the temperature regime of relevance to experiments. We show that there is a narrow but accessible window of experimental parameters where the anisotropic response can be observed.

We discuss the mechanisms behind the electrically driven insulator-metal transition in single-crystalline ${\mathrm{VO}}_2$ nanobeams. Our dc and ac transport measurements and the versatile harmonic analysis method employed show that nonuniform Joule heating causes electronic inhomogeneities to develop within the nanobeam and is responsible for driving the transition in ${\mathrm{VO}}_2$. A Poole-Frenkel–like purely electric-field-induced transition is found to be absent, and the role of percolation near and away from the electrically driven transition in ${\mathrm{VO}}_2$ is also identified. The results and the harmonic analysis can be generalized to many strongly correlated materials that exhibit electrically driven transitions.

In polycrystalline compound semiconductor thin films, structural defects such as grain boundaries as well as lateral stress can form during film growth, which may deteriorate their electronic performance and mechanical stability. In Cu-based chalcogenide semiconductors such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ or ${\mathrm{Cu}}_2\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_4$, temporary Cu excess during film growth leads to improved microstructure such as a reduced grain boundary density, a strategy that has been used for decades for high-efficiency chalcopyrite thin film solar cells. However, the mechanisms responsible for the beneficial effect of Cu excess are yet not fully clarified. Here, we investigate the evolution of lateral stress, grain growth, and Cu-Se segregation during Cu-Se deposition onto Cu-poor ${\mathrm{CuInSe}}_2$. Real-time x-ray diffraction and fluorescence analysis with a double-detector setup reveals that sudden stress relaxation occurs shortly prior to Cu-Se segregation at the surface and precisely coincides with domain growth and change of texture. Numerical reaction-diffusion modeling provides an explanation for the observed delay of Cu-Se segregation. Our results show that partial recrystallization of the film can be already reached without the necessity of an overall Cu-rich film composition and thus suggest a new synthesis route for the fabrication of high-quality chalcopyrite absorber films.

We investigate the hole spin relaxation due to the Rashba spin-orbit coupling induced by an external perpendicular electric field in bilayer ${\mathrm{WSe}}_2$. The Rashba spin-orbit coupling coefficients in bilayer ${\mathrm{WSe}}_2$ are constructed from the corresponding monolayer ones. In contrast to monolayer ${\mathrm{WSe}}_2$, the out-of-plane component of the bilayer Rashba spin-orbit coupling acts as a Zeeman-like field with opposite directions but identical values in the two valleys. For in-plane spins, this Zeeman-like field, together with the intervalley hole-phonon scattering, opens an intervalley spin relaxation channel, which is found to dominate the in-plane spin relaxation in bilayer ${\mathrm{WSe}}_2$ even at low temperature. For out-of-plane spins, this Zeeman-like field is superimposed by the identical Hartree-Fock effective magnetic fields in the two valleys, and hence different total effective magnetic fields between two valleys are obtained. Owing to the large difference of the total fields at large spin polarization, different out-of-plane spin relaxation times in the two valleys are obtained when the intervalley hole-phonon scattering is weak at low temperature and low hole density. This difference in the spin relaxation times can be suppressed by enhancing the intervalley hole-phonon scattering through increasing temperature or hole density. Moreover, at large spin polarization and low temperature, due to the weak intravalley hole-phonon scattering but relatively strong hole-hole Coulomb scattering, the fast spin precessions are found to result in a quasi-hot-hole Fermi distribution characterized by an effective hot-hole temperature larger than the temperature, which also enhances the intervalley scattering. During this process, it is interesting to discover that the initially equal hole densities in the two valleys are broken in the temporal evolution, and a valley polarization is built up. It is further revealed that this comes from the different spin relaxation processes at large spin polarization and different spin-conserving intervalley scattering rates between spin-up and -down holes due to the different effective hot-hole temperatures.

We study numerically, analytically, and experimentally the resonant transmission of light in a waveguide formed by a periodic array of high-index dielectric nanoparticles with a side-coupled resonator. We demonstrate that a resonator with high enough $Q$-factor provides the conditions for the Fano-type interference allowing one to control the resonant transmission of light. We suggest a practical realization of this resonant effect based on the quadrupole resonance of a dielectric particle and demonstrate it experimentally for ceramic disks at microwave frequencies.

First-principles (generalized gradient approximation) calculations are presented for symmetric ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3$ (001) orientation slab models with varying thickness (3, 4, 5 unit cells) of the ${\mathrm{LaAlO}}_3$ (LAO) layers. The buckling of the layers and their effect on the slope of the layer-averaged electrostatic potential and layer-projected densities of states are studied. We find the buckling of the LAO layers to increase from the interface toward the surface, while the buckling of the ${\mathrm{AlO}}_2$ layers decreases toward the surface. The critical layer thickness for obtaining electrons in the Ti-$d$ band of the ${\mathrm{SrTiO}}_3$ (STO) is determined to be 4 layers within this model. Beyond this point, the sloped potential is confined to the 4 layers of LAO nearest to the interface. The electrons in the Ti-$d$ states extend throughout the 5.5-layer-thick STO region of our calculation. The sheet charge density of electrons in the STO conduction band is determined and found to be of order $(1–3)\times {10}^{13}e/{\mathrm{cm}}^2$, in fair agreement with experimental values and an order of magnitude smaller than required by the polar discontinuity model. We also find still a significant change in the sheet density between the 4-LAO layer and 5-LAO layer model. It results in only $d_{xy}$-like states being occupied for the 4-LAO layer case but other $t_{2g}$ bands becoming occupied for the 5-LAO layer case. The effects of H adsorption on surface O and OH adsorption on the surface Al are investigated for a model with $1/8$ coverage of H and $1/4$ coverage of OH. The former leads to electron doping of the STO layer while the latter leads to a p-type surface. When both together are present, they cancel each other. For high H coverage, we find that only a certain fraction of the electrons donated by H can be accommodated at the interface while the remaining go to the surface and lead to a reversal of the slope of the potential in the LAO region. The addition of 25% Ti on Al sites into the first layer of LAO already leads to a cancellation of the field in the LAO layer. It does not lead to ${\mathrm{Ti}}^{3+}$ embedded in the LAO site but rather the Ti donates its additional electron to the interface two-dimensional electron gas (2DEG) confined to the STO ${\mathrm{TiO}}_2$ layers. A swap of Al with Ti in the layers closest to the interface does not produce a 2DEG because the Al in the ${\mathrm{TiO}}_2$ interface layer provides holes compensating the electron doping from the Ti. Interdiffusion of Sr and La between the layers nearest to the interface does not lead to a 2DEG. These species are just electron donors in their own respective materials. In a swap compensating dipoles result from the different nuclear charges but the electronic states near the gap are not affected. Thus no 2DEG formation occurs. On the other hand a ${\mathrm{Sr}}_{\mathrm{La}}$ placed in the middle of the LAO layer is found to facilitate electron transfer from the surface to the interface and could lead to a 2DEG. However, the latter had only a small sheet density.

A detailed low-energy electronic structure of a Kondo insulator ${\mathrm{YbB}}_{12}$ was revealed by a synergetic combination of ultra-high-resolution laser photoemission spectroscopy (PES) and time-resolved PES. The former confirmed a 25-meV pseudogap corresponding to the Kondo temperature of this material, and more importantly, it revealed that a 15-meV gap and a Kondo-peak feature developed below a crossover temperature of $T^{*}\sim 110\mathrm{K}$. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below $\sim T^{*}$. This is a clear manifestation of photoexcited carriers due to the bottleneck in the recovery dynamics, which is interpreted as a developing hybridization gap of a hard gap.

The optical properties of layered tungsten ditelluride have been measured over a wide temperature and frequency range for light polarized in the a-b planes. A striking low-frequency plasma edge develops in the reflectance at low temperature where this material is a perfectly compensated semimetal. The optical conductivity is described using a two-Drude model which treats the electron and hole pockets as separate electronic subsystems. At low temperature, one scattering rate collapses by over two orders of magnitude, while the other also undergoes a significant, but less dramatic, decrease; both scattering rates appear to display the quadratic temperature dependence expected for a Fermi liquid. First principles electronic structure calculations reveal that the low-lying optical excitations are due to direct transitions between the bands associated with the electron and hole pockets.

Recent angle-resolved photoemission spectroscopy measurements have identified an inversion-symmetry-breaking Weyl semimetal phase in TaAs and NbAs. In an inversion-symmetry-breaking Weyl semimetal the left and the right handed Weyl points can occur at different energies and the energy mismatch between the Weyl points of opposite chirality is known as the chiral chemical potential. In the presence of the chiral chemical potential, the nontrivial Berry curvature of the Weyl fermions gives rise to the dynamic chiral magnetic effect. This describes how a time-dependent magnetic field leads to an electrical current along the applied field direction, which is also proportional to the field strength. We derive a general formula for the dynamic chiral magnetic conductivity of the inversion-symmetry-breaking Weyl semimetal. We show that the measurement of the natural optical activity or rotary power provides a direct confirmation of the existence of the dynamic chiral magnetic effect in inversion-symmetry-breaking Weyl semimetals.

The silicon vacancy in silicon carbide is a strong emergent candidate for applications in quantum information processing and sensing. We perform room temperature optically detected magnetic resonance and spin echo measurements on an ensemble of vacancies and find the spin echo properties depend strongly on magnetic field. The spin echo decay time varies from less than $10\mu \mathrm{s}$ at low fields to $80\mu \mathrm{s}$ at 68 mT, and a strong field-dependent spin echo modulation is also observed. The modulation is attributed to the interaction with nuclear spins and is well described by a theoretical model.

Quantum dots in cavities have been shown to be very bright sources of indistinguishable single photons. Yet the quantum interference between two such bright quantum dot sources, a critical step for photon-based quantum computation, still needs to be investigated. Here, we report on such a measurement, taking advantage of a deterministic fabrication of the devices. We show that cavity quantum electrodynamics can efficiently improve the quantum interference between remote quantum dot sources: Poorly indistinguishable photons can still interfere with good contrast with high quality photons emitted by a source in the strong Purcell regime. Our measurements and calculations show that cavity quantum electrodynamics is a powerful tool for interconnecting several quantum dot devices.

Results are presented for the optical Hall conductivity of a Floquet topological insulator (FTI) for an ideal closed quantum system, as well as an open system in a nonequilibrium steady state with a reservoir. The steady state, even for the open system, is strongly dependent on the topological phase of the FTI, with certain phases showing a remarkable near cancellation from pockets of Berry curvature of opposite signs, leading to a suppressed low-frequency Hall conductivity that also shows an anomalous temperature dependence by increasing as the temperature of the reservoir is increased. Such a behavior is in complete contrast to heating and arises because of a strong modification of the effective system-reservoir coupling by the laser. The Berry curvature of the Floquet modes is time dependent, and its frequency components are found to control the main features of the high-frequency Hall conductivity.

We investigate the nonpolar, cleavage $\left(001\right)\left(2\times 1\right)$ surfaces of antiferromagnetic late transition-metal oxides by means of the density functional theory for non-collinear spins and including the spin-orbit interaction. The effects of strong Coulomb interaction among the localized transition-metal $3d$ electrons are described by an on-site Hubbard $U$ added to the exchange-correlation functional in the local-density approximation. This approach guarantees finite energy gaps also for FeO and CoO, although they are still underestimated. Besides a change in the geometry due to rumpling and relaxation, we find that the size and orientation of the surface magnetic moments are also influenced by the surface. Electronic surface states appear in the fundamental gap close to the edges of the projected bulk bands. Only in the NiO case does an empty, well-separated band of $e_g$-derived surface states occur in the fundamental gap. As a consequence the scanning tunneling microscopy (STM) images exhibit almost the bulk $1\times 1$ translational symmetry. For ferromagnetic tips strong spin contrasts are observed in spin-polarized STM. As a consequence of the bulk antiferromagnetic ordering, mainly chain structures become visible, independent of the tunneling into empty states or out from filled ones. Clear chemical trends with the occupation of the $t_{2g}$-minority-spin channel states along the row $\mathrm{MnO}\to \mathrm{NiO}$ are observed and discussed.

We propose a spin transport experiment to measure the low-energy excitations in insulators with spin degrees of freedom, with a focus on detecting ground states that lack magnetic order. A general formalism to compute the spin current from a metal with a nonequilibrium distribution of spins to an insulator is developed. It is applied to insulating states with and without long range magnetic order, and salient features in the spin conductance are noted.

We report an investigation of transverse Hall resistance and longitudinal resistance on Pt thin films sputtered on epitaxial ${\mathrm{LaCoO}}_3$ (LCO) ferromagnetic insulator films. The ${\mathrm{LaCoO}}_3$ films were deposited on several single crystalline substrates $[{\mathrm{LaAlO}}_3,(\text{La},\text{Sr}\left)\right(\text{Al},\text{Ta}){\mathrm{O}}_3$, and ${\mathrm{SrTiO}}_3]$ with (001) orientation. The physical properties of ${\mathrm{LaCoO}}_3$ films were characterized by the measurements of magnetic and transport properties. The ${\mathrm{LaCoO}}_3$ films undergo a paramagnetic to ferromagnetic (FM) transition at Curie temperatures ranging from 40 to 85 K, below which the Pt/LCO hybrids exhibit significant extraordinary Hall resistance up to 50 $\mathrm{m}\mathrm{\Omega }$ and unconventional magnetoresistance ratio $\mathrm{\Delta }\rho /{\rho }_0$ about $1.2\times {10}^{-4}$, accompanied by the conventional magnetoresistance. The observed spin transport properties share some common features as well as some unique characteristics when compared with well-studied ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$-based Pt thin films. Our findings call for new theories since the extraordinary Hall resistance and magnetoresistance cannot be consistently explained by the existing theories.

We propose a way to break the time-reversal symmetry at the surface of a three-dimensional topological insulator that combines features of both surface magnetic doping and magnetic proximity effect. Based on the possibility of organizing an ordered array of local magnetic moments by inserting them into a two-dimensional matrix of organic ligands, we study the magnetic coupling and electronic structure of such metal-organic coordination networks on a topological insulator surface from first principles. In this way, we find that both Co and Cr centers, linked by the tetracyanoethylenelike organic ligand, are coupled ferromagnetically and, depending on the distance to the topological insulator substrate, can yield a magnetic proximity effect. This latter leads to the Dirac point gap opening indicative of the time-reversal symmetry breaking.

We propose a powerful method for controlling interaction between silicene and a substrate utilizing quantum size effect, which allows growing silicene with tailored electronic properties. As an example we consider silicene on ultrathin Pb(111) layers and demonstrate how the properties of silicene, including the binding energy and the Dirac bands, can easily be tuned by quantum well states of the substrate. We also discover a novel mechanism of protecting the Dirac electrons from the influence of the substrate. This is associated with a special arrangement of a part of the Si atoms in silicene. These findings emphasize the essential role of interfacial coupling and open new routes to create silicenelike two-dimensional structures with controlled electronic properties.

In bilayer graphene an interface between regions with opposite sign of the interlayer asymmetry gap leads to the spatial confinement of low-energy chiral modes [I. Martin, Y. M. Blanter, and A. F. Morpurgo, Phys. Rev. Lett. 100, 036804 (2008)]. Here we study the influence of trigonal warping on the properties of the interface-guided stats and find that in the vicinity of the band edges the group velocity and transverse confinement length exhibit a nonmonotonic dependence on the crystallographic orientation of the crystal with respect to the interface.