By applying the coherent potential approximation (CPA) to simple models, we have studied the temperature $\left(T\right)$ dependence of the normalized magnetization $M\left(T\right)$, and electrical resistivity $\rho \left(T\right)$ of highly rare-earth-doped EuO. The present result reveals that in degenerate EuO, the magnetization is described by an electron-doped EuO model; the strong double-dome feature of $M\left(T\right)$ of Gd-doped EuO is a consequence of the half-metallicity and low dopant activation. In degenerate EuO, the temperature dependence of the resistivity is well described by Matthiessen's rule as $\rho \left(T\right)={\rho }_C+{\rho }_m\left(M\right)$, where ${\rho }_C$ is the nonmagnetic scattering contribution (independent of $T$) and ${\rho }_m\left(M\right)$ is the magnetic scattering contribution due to the exchange interaction with localized $f$ spins. ${\rho }_C$ is proportional to $x(1-x)/n^{\frac{2}{3}}$, while the amplitude of the change in ${\rho }_m\left(M\right)$ is proportional to $n^{-\frac{2}{3}}$, where $x$ is the doped rare-earth density and $n$ is the electron density. The difference in $M\left(T\right)$ and $\rho \left(T\right)$ between Gd- and La-doped EuO is also discussed.

We reveal the electronic level structure of the Mn acceptor, which consists of a valence-band hole bound to an ${\mathrm{Mn}}^{2+}$ ion, in presence of applied uniaxial stress and an external magnetic field in bulk GaAs. Resonant spin-flip Raman scattering is used to measure the $g$ factor of the ${\mathrm{A}}_{\text{Mn}}^0$ center in the ground and excited states with the total angular momenta $F=1$ and $F=2$ and characterize the optical selection rules of the spin-flip transitions between these Mn-acceptor states. We determine the random stress fields near the Mn acceptor, the constant of the antiferromagnetic exchange interaction between the valence-band holes and the electrons of the inner ${\mathrm{Mn}}^{2+}$ shell as well as the deformation potential for the exchange energy. The $p\text{−}d$ exchange energy, in particular, decreases significantly with increasing compressive stress. By combining the experimental Raman study with the developed theoretical model on the scattering efficiency, in which also the random local and external uniaxial stresses and magnetic field are considered, the fine structure of the Mn acceptor is determined in full detail.

We carry out an improved ab initio calculation of surface second-harmonic generation (SSHG) from the Si(111)($1\times 1$):H surface. This calculation includes three new features in one formulation: (i) the scissors correction, (ii) the contribution of the nonlocal part of the pseudopotentials, and (iii) the inclusion of a cut function to extract the surface response, all within the independent particle approximation. We apply these improvements on the Si(111)($1\times 1$):H surface and compare with various experimental spectra from several different sources. We also revisit the three-layer model for the SSHG yield and demonstrate that it provides more accurate results over several, more common, two-layer models. We demonstrate the importance of using properly relaxed coordinates for the theoretical calculations. We conclude that this approach to the calculation of the second-harmonic spectra is versatile and accurate within this level of approximation. This well-characterized surface offers an excellent platform for comparison with theory and allows us to offer this study as an efficient benchmark for this type of calculation.

Using the Kubo formalism we have calculated the local dynamic conductivity of a bulk, i.e., three-dimensional (3D), Dirac semimetal (BDS). We obtain that at frequencies lower than Fermi energy the metallic response in a BDS film manifests in the existence of surface-plasmon polaritons, but at higher frequencies the dielectric response is dominated and it occurs that a BDS film behaves as a dielectric waveguide. At this dielectric regime we predict the existence inside a BDS film of novel electromagnetic modes, a 3D analog of the transverse electric waves in graphene. We also find that the dielectric response manifests as the wide-angle passband in the mid-infrared (IR) transmission spectrum of light incident on a BDS film, which can be used for the interferenceless omnidirectional mid-IR filtering. The tuning of the Fermi level of the system allows us to switch between the metallic and the dielectric regimes and to change the frequency range of the predicted modes. This makes BDSs promising materials for photonics and plasmonics.

Graphene, the thinnest material in the world, can form moiré structures on different substrates, including graphite, $h$-BN, or metal surfaces. In such systems, the structure of graphene, i.e., its corrugation, as well as its electronic and elastic properties, are defined by the combination of the system geometry and local interaction strength at the interface. The corrugation in such structures on metals is heavily extracted from diffraction or local probe microscopy experiments, and it can be obtained only via comparison with theoretical data, which usually simulate the experimental findings. Here we show that graphene corrugation on metals can be measured directly employing atomic force spectroscopy, and the obtained value coincides with state-of-the-art theoretical results. The presented results demonstrate an unexpected space selectivity for the $\mathrm{\Delta }f\left(z\right)$ signal in the atomic force spectroscopy in the moiré graphene lattice on Ru(0001), which is explained by the different response of the graphene layer on the indentation process. We also address the elastic reaction of the formed graphene nanodoms on the indentation process by the scanning tip that is important for the modeling and fabrication of graphene-based nanoresonators on the nanoscale.

The out-of-plane fluctuations of carbon atoms in a graphene sheet have been studied by means of classical molecular dynamic simulations with an empirical force field as a function of temperature. The Fourier analysis of the out-of-plane fluctuations often applied to characterize the acoustic bending mode of graphene is extended to the optical branch, whose polarization vector is perpendicular to the graphene layer. This observable is inaccessible in a continuous elastic model of graphene but it is readily obtained by the atomistic treatment. Our results suggest that the long-wavelength limit of the acoustic out-of-plane fluctuations of a free layer without stress is qualitatively similar to that predicted by a harmonic model under a tensile stress. This conclusion is a consequence of the anharmonicity of both in-plane and out-of-plane vibrational modes of the lattice. The most striking anharmonic effect is the presence of a linear term, ${\omega }_A=v_{A}k$, in the dispersion relation of the acoustic bending band of graphene at long wavelengths $\left(k\to 0\right)$. This term implies a strong reduction of the amplitude of out-of-plane oscillations in comparison to a flexural mode with a $k^2$ dependence in the long-wavelength limit. Our simulations show an increase of the sound velocity associated to the bending mode, as well as an increase of its bending constant, $\kappa ,$ as the temperature increases. Moreover, the frequency of the optical bending mode, ${\omega }_O(\mathrm{\Gamma })$, also increases with the temperature. Our results are in agreement with recent analytical studies of the bending modes of graphene using either perturbation theory or an adiabatic approximation in the framework of continuous layer models.

Phase-matched second-harmonic generation (SHG) under the Bragg diffraction in the Laue geometry in one-dimensional photonic crystal (PhC) is studied theoretically and experimentally. We demonstrate that the phase-matched SHG can be realized in a PhC by compensation of the material dispersion of the PhC constituent layers of adjustable thickness. The second-order nonlinear susceptibility is introduced in the porous quartz-based PhC by its infiltration by sodium nitrite. We observed that two second-harmonic (SH) beams appear after passing through the PhC under the phase-matched process, which correspond to the transmission and diffraction angular directions. The appearance of the phase-matched SHG is confirmed by a pronounced SH spectral dependence and a narrow SH angular distribution, with the FWHM of the SH peak of approximately 3.5 times smaller as compared to the case of non-phase-matched SHG.

We report on the large-scale density-functional calculations that show the emergence of the spin-polarized ground states in nanofacets self-organized on SiC (0001) surfaces. We first reveal that the nanofacet formed by bunching of single bilayer steps induces peculiar electron states localized at but extended along step edges, showing the flat-band characteristics. The electron states are of C-dangling-bond characters mixed with the back-bond character of neighboring edge Si atoms. We find that the resulting flat bands lead to the spin polarization near the step edges by H passivation of the terrace Si atoms. Interestingly, either ferromagnetic or antiferromagnetic chains appear along the step edges on the SiC nanofacet and the location of such magnetic chains can be controlled by manipulating the H passivation of atoms near the step edges. These findings open a possibility of the appearance of new magnetic functions on the covalent semiconductor surfaces without magnetic elements.

We have performed angle-resolved photoemission spectroscopy on ${\mathrm{Tl}}_{0.5}{\mathrm{Bi}}_2{\mathrm{Te}}_3$, a possible topological superconductor derived from ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. We found that the bulk Fermi surface consists of multiple three-dimensional hole pockets surrounding the Z point, produced by the direct hole doping into the valence band. The Dirac-cone surface state is well isolated from the bulk bands, and the surface chemical potential is variable in the entire band-gap range. ${\mathrm{Tl}}_{0.5}{\mathrm{Bi}}_2{\mathrm{Te}}_3$ thus provides an excellent platform to realize two-dimensional topological superconductivity through a proximity effect from the superconducting bulk. Also, the observed Fermi-surface topology provides a concrete basis for constructing theoretical models for bulk topological superconductivity in hole-doped topological insulators.

The electronic structure, nonlinear electronic polarization induced by a static external electric field, and frequency dependent second-harmonic susceptibility tensor of the borophosphate ${\mathrm{BPO}}_4$ are studied by a first-principles calculation based on density-functional theory. Our calculated results show that the borophosphate ${\mathrm{BPO}}_4$ has a large band gap $\sim 10.4$ eV, which is larger than the band gap of the widely used nonlinear optical crystal ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$. However, ${\mathrm{BPO}}_4$ also has a nonlinear coefficient $d_{36}=0.92$ pm/V at static limit, which also is larger than the nonlinear coefficient $d_{11}=0.47$ pm/V of ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$. The unexpected larger nonlinear coefficient of ${\mathrm{BPO}}_4$ can be interpreted by the relatively strong $s\text{−}p$ hybridization in ${\mathrm{BPO}}_4$, which can enhance the inter-band Berry connections, while the O $2p$ orbitals dominating valence bands in ${\mathrm{KBe}}_2{\mathrm{BO}}_3{\mathrm{F}}_2$ are very flat, resulting from weak $s\text{−}p$ hybridization.

We report the observation of a superconducting state below $\sim 8$ K coexistent with a spin-glass state caused by atomic disorder in Ce substituted ${\mathrm{Ca}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$. Measurements of specific heat, resistivity, and magnetism reveal the existence of inhomogeneous superconductivity in samples doped with Ce with superconducting critical temperatures $T_c$ higher than those observed in the parent compound. For ${\mathrm{Ca}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$, the negative value of the change in resistivity $\rho$ with pressure $P$, $d\rho /dP$ correlates well with the calculated decrease in the density of states (DOS) at the Fermi energy with $P$. Based on band-structure calculations performed under pressure, we demonstrate how the change in DOS would affect $T_c$ of ${\mathrm{Ca}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ under negative lattice pressure in samples that are strongly defected by quenching.

Anomalous Hall effect (AHE) and spin Hall effect (SHE) are fundamental phenomena, and their potential for application is great. However, we understand the interaction effects unsatisfactorily, and should have clarified issues about the roles of the Fermi sea term and Fermi surface term of the conductivity of the intrinsic AHE or SHE of an interacting multiorbital metal and about the effects of spin-Coulomb drag on the intrinsic SHE. Here, we resolve the first issue and provide the first step about the second issue by developing a general formalism in the linear response theory with appropriate approximations and using analytic arguments. The most striking result is that even without impurities, the Fermi surface term, a non-Berry-curvature term, plays dominant roles at high or slightly low temperatures. In particular, this Fermi surface term causes the temperature dependence of the dc anomalous Hall or spin Hall conductivity due to the interaction-induced quasiparticle damping and the correction of the dc spin Hall conductivity due to the spin-Coulomb drag. Those results revise our understanding of the intrinsic AHE and SHE. We also find that the differences between the dc anomalous Hall and longitudinal conductivities arise from the difference in the dominant multiband excitations. This not only explains why the Fermi sea term such as the Berry-curvature term becomes important in clean and low-temperature case only for interband transports, but also provides the useful principles on treating the electron-electron interaction in an interacting multiorbital metal for general formalism of transport coefficients. Several correspondences between our results and experiments are finally discussed.

The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field.

The high-temperature superfluidity of two-dimensional dipolar excitons in two parallel transition metal dichalcogenide (TMDC) layers is predicted. We study Bose-Einstein condensation in the two-component system of dipolar $A$ and $B$ excitons. The effective mass, energy spectrum of the collective excitations, the sound velocity, and critical temperature are obtained for different TMDC materials. It is shown that in the Bogoliubov approximation, the sound velocity in the two-component dilute exciton Bose gas is always larger than in any one-component exciton system. The difference between the sound velocities for two-component and one-component dilute gases is caused by the fact that the sound velocity for a two-component system depends on the reduced mass of $A$ and $B$ excitons, which is always smaller than the individual mass of $A$ or $B$ exciton. Due to this fact, the critical temperature $T_c$ for superfluidity for the two-component exciton system in a TMDC bilayer is about one order of magnitude higher than $T_c$ in any one-component exciton system. We propose to observe the superfluidity of two-dimensional dipolar excitons in two parallel TMDC layers, which causes two opposite superconducting currents in each TMDC layer.

By means of first-principles calculations for the SbSI semiconductor, we show that bare density functional theory fails to reproduce the experimentally observed ferroelectric phase, whereas a more advanced approach, based on hybrid functionals, correctly works. When comparing the paraelectric and ferroelectric phases, our results show polar displacements along the $c$ direction of the Sb and S sublattices with respect to the iodine framework, leading to a predicted spontaneous polarization of $P\simeq 20\mu \text{C}/{\text{cm}}^2$, in good agreement with experiments. In the ferroelectric phase, the semiconducting behavior of SbSI is confirmed by relatively large values for the indirect and direct gaps ($\simeq 2.15\text{eV}$ and $2.3\text{eV}$, respectively). An analysis of the electronic structure, in terms of density of states, charge density distribution, and anomalies in the Born effective charges, reveals (i) the clear presence of a Sb(III) lone pair and (ii) a large covalency in the SbSI bonding, based on the hybridization between Sb and S ions, in turn more ionically bonded to iodine anions. Finally, the interplay between ferroelectricity and spin-orbit coupling reveals a coexistence of Dresselhaus and Rashba relativistic effects and a spin texture that can be reversed by switching the polarization, of potential appeal in electrically controlled spintronics.

We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is $\sim 100$ fs. An effective electronic Grüneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.