Using first-principles density functional theory calculations, we investigated the electronic structure and magnetic properties of (LaMnO${}_3$)${}_m$/(LaNiO${}_3$)${}_n$ superlattices stacked along the (001) direction. The electrons are transferred from Mn to Ni, and the magnetic moments are induced at Ni sites that are paramagnetic in bulk and other types of superlattices. The size of the induced moment is linearly proportional to the amount of transferred electrons, but it is larger than the net charge transfer. The charge transfer and magnetic properties of the $(m,n)$ superlattice can be controlled by changing the $m/n$ ratio. Considering the ferromagnetic couplings between Mn and Ni spins and the charge-transfer characteristic, we propose the (2,1) superlattice as the largest moment superlattice. carrying $\sim$8${\mu }_B$ per formula unit.

We have constructed the Green's tensor for two-dimensional gyrotropic photonic clusters and have calculated their optical local density of states (LDOS). For clusters that support the chiral Hall edge states we calculate the LDOS as a function of wavelength, position, size, and shape of the cluster. It is shown that the LDOS of Hall edge states is a strong function of the cluster shape and position. The LDOS can be orders of magnitude higher at the edges of the cluster compared to the free space value while it vanishes towards the cluster center. It is shown that the LDOS in such photonic clusters can withstand a very strong disorder due to their topological protection. The spatial profiles of chiral Hall edge modes, their quality factors, and their excitations have been calculated. Both gyroelectric and gyromagnetic (ferrite) clusters have been treated.

In Bi${}_2$Te${}_2$Se, the period of quantum oscillations arising from surface Dirac fermions can be increased sixfold using ionic liquid gating. At high gate voltages, the Fermi energy reaches the $N=1$ Landau level in a 14-T field. This enables the $\frac{1}{2}$ shift predicted for the Dirac spectrum to be measured accurately. A surprising result is that liquid gating strongly enhances the surface mobility. By analyzing the Hall conductivity, we show that the enhancement occurs on only one surface. We present evidence that the gating process is fully reversible (hence consistent with band bending by the $E$ field from the anion layer accumulated). In addition to the surface carriers, the experiment yields the mobility and density of the bulk carriers in the impurity band. By analyzing the charge accumulation vs gate voltage, we also obtain estimates of the depletion width and the areal depletion capacitance $C_d/A$. The value of $C_d/A$ implies an enhanced electronic polarizability in the depletion region.

We propose a hierarchical dynamics approach for evaluation of nonequilibrium dynamic response properties of quantum impurity systems. It is based on a hierarchical equations of motion formalism, in conjunction with a linear response theory established in the hierarchical Liouville space. This provides an accurate and universal tool for characterization of a variety of response and correlation functions of local impurities, as well as transport related response properties. The practicality of our proposed approach is demonstrated via the evaluation of various dynamical properties of a single-impurity Anderson model. These include the impurity spectral density, local charge fluctuation, local magnetic susceptibility, and current-voltage admittance, in both equilibrium and nonequilibrium situations. The numerical results are considered to be quantitatively accurate, as long as they converge quantitatively with respect to the truncation of the hierarchy.

We investigate theoretically the effect of an external magnetic field on polarization patterns appearing in quantum microcavities due to the optical spin Hall effect. We show that increase of the magnetic field perpendicular to the plane of the cavity resulting in the increase of the Zeeman splitting leads to the transition from azimuthal separation of polarizations to their radial separation. This effect can be straightforwardly detected experimentally.

We fabricate a vertical thin-film barristor device consisting of highly doped silicon (gate), 300 nm SiO${}_2$ (gate dielectric), monolayer graphene, pentacene, and a gold top electrode. We show that the current across the device is modulated by the Fermi energy level of graphene, tuned with an external gate voltage. We interpret the device current within the thermionic emission theory, showing a modulation of the energy barrier between graphene and pentacene as large as 300 meV.

Electrostatic interactions between the conducting tip of a scanning probe microscope and a flat conductor coated with a thin or thick dielectric layer are treated analytically and numerically. Exact and compact approximate expressions for the capacitance, force, force gradient, electric field profiles, and their effective widths are derived for a spherical model tip by generalizing known solutions for the conducting sphere and sample problem. These expressions allow convenient modeling of various measurements involving voltage-biased probes, estimation of lateral resolution, and prediction of trends as a function of relevant parameters.

To understand the recently established unique magnetic and superconducting phase diagram of LaFeAsO${}_{1-x}$H${}_x$, we analyze the realistic multiorbital tight-binding model for $x=0–0.4$ beyond the rigid-band approximation. Both the spin and orbital susceptibilities are calculated in the presence of the Coulomb and charge quadrupole interactions. It is found that both orbital and spin fluctuations strongly develop at both $x\sim 0$ and $0.4$, due to the strong violation of the rigid-band picture in LaFeAsO${}_{1-x}$H${}_x$. Based on this result, we discuss the experimental phase diagram, especially the double-dome superconducting phase. Moreover, we show that the quadrupole interaction is effectively produced by the vertex correction due to Coulomb interaction, resulting in the mutual development of spin and orbital fluctuations.

We report that the finite thickness of three-dimensional topological insulator (TI) thin films produces an observable magnetoresistance (MR) in phase coherent transport in parallel magnetic fields. The MR data of Bi${}_2$Se${}_3$ and (Bi${}_{1-x}$Sb${}_x$)${}_2$Te${}_3$ thin films are compared with existing theoretical models of parallel field magnetotransport. We conclude that the TI thin films bring parallel field transport into a unique regime in which the coupling of surface states to bulk and to opposite surfaces is indispensable for understanding the observed MR. The $\beta$ parameter extracted from parallel field MR can in principle provide a figure of merit for searching TI compounds with more insulating bulk than existing materials.

We present a comprehensive study of plasmon dispersions in simple metals and Heusler compounds based on an accurate ab initio evaluation of the momentum- and frequency-dependent dielectric function $\varepsilon (\vec{q},\omega )$ in the random-phase approximation. Using a momentum-dependent tetrahedron method for the computation of the dielectric function, we extract and analyze “full” and “intraband” plasmon dispersions: The “full” plasma dispersion is obtained by including all bands, the intraband plasma dispersion by including only intraband transitions. For the simple metals silver and aluminum, we show that the intraband plasmon dispersion has an unexpected downward slope and is therefore markedly different from the results of an effective-mass electron-gas model and the full plasmon dispersion. For the two Heusler compounds ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ and ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$, we present spectra for the dielectric function, their loss functions, and plasmon dispersions. The latter exhibit the same negative intraband plasmon dispersion as found in the simple metals. We also discuss the influence of spin mixing on the plasmon dispersion.

Compressed hydrogen passes through a series of layered structures in which the layers can be viewed as distorted graphene sheets. The electronic structures of these layered structures can be understood by studying simple model systems—an ideal single hydrogen graphene sheet and three-dimensional model lattices consisting of such sheets. The energetically stable structures result from structural distortions of model graphene-based systems due to electronic instabilities towards Peierls or other distortions associated with the opening of a band gap. Two factors play crucial roles in the metallization of compressed hydrogen: (i) crossing of conduction and valence bands in hexagonal or graphenelike layers due to topology and (ii) formation of bonding states with $2p_z$ $\pi$ character.

Spin orientation of photoexcited carriers and their energy relaxation are investigated in bulk Ge by studying spin-polarized recombination across the direct band gap. The control over parameters such as doping and lattice temperature is shown to yield a high polarization degree, namely larger than 40$\%$, as well as a fine tuning of the angular momentum of the emitted light with a complete reversal between right- and left-handed circular polarization. By combining the measurement of the optical polarization state of band-edge luminescence and Monte Carlo simulations of carrier dynamics, we show that these very rich and complex phenomena are the result of the electron thermalization and cooling in the multivalley conduction band of Ge. The circular polarization of the direct-gap radiative recombination is indeed affected by energy relaxation of hot electrons via the $X$ valleys and the Coulomb interaction with extrinsic carriers. Finally, thermal activation of unpolarized $L$ valley electrons accounts for the luminescence depolarization in the high temperature regime.

We present a unified hydroelectrodynamical approach to study electron dynamics in a laser field in cold metal nanoparticles with a general nonspherical surface. This is the generalization to complex shapes of the model used earlier to study both linear and nonlinear plasmon excitations in thin metal films and spherical nanoparticles. Along with a general formulation allowing a direct nonlinear extension of the model for nanoparticles with nonspherical shapes, the linear plasmon excitations in metal cubic and rectangular parallelepiped nanoparticles are calculated as an example of using the hydrodynamic model in a cold charge-compensated electron plasma approximation. The results obtained for the cubic and parallelepipedlike nanoparticles are analyzed and compared with the ones of earlier approaches such as the discrete dipole approximation.

During the synthesis of ultrathin materials with hexagonal lattice structure Stone-Wales (SW) type of defects are quite likely to be formed and the existence of such topological defects in the graphenelike structures results in dramatic changes of their electronic and mechanical properties. Here we investigate the formation and reactivity of such SW defects in silicene. We report the energy barrier for the formation of SW defects in freestanding ($\sim$2.4 eV) and Ag(111)-supported ($\sim$2.8 eV) silicene and found it to be significantly lower than in graphene ($\sim$9.2 eV). Moreover, the buckled nature of silicene provides a large energy barrier for the healing of the SW defect and therefore defective silicene is stable even at high temperatures. Silicene with SW defects is semiconducting with a direct band gap of 0.02 eV and this value depends on the concentration of defects. Furthermore, nitrogen substitution in SW-defected silicene shows that the defect lattice sites are the least preferable substitution locations for the N atoms. Our findings show the easy formation of SW defects in silicene and also provide a guideline for band gap engineering in silicene-based materials through such defects.

We study the magnetic phase diagram and hysteresis behavior of weakly coupled spin dimers on a triangular lattice using the cluster mean-field method with cluster-size scaling. We find that the magnetization curve has plateaus at 1/3 and 2/3 of the total magnetization, in which local singlet and triplet states form a superlattice pattern. Moreover, if increasing (decreasing) the magnetic field from the 1/3 (2/3) plateau, the Bose-Einstein condensation (BEC) of triplons occurs on the superlattice background, leading to the transition into “magnon supersolid” phase. We also find that the first-order transition between these solid states and the standard magnon BEC state exhibits an anomalous hysteresis upon cycling the magnetic field; the transition can occur only from solid to BEC, and the system cannot return to the initial solid state in the reverse process.

The investigation of Li(Ni${}_{1-x}$Fe${}_x$)PO${}_4$ by optical second harmonic generation yields the competition of the antiferromagnetic (AFM) structures and ${180}^{\circ }$ domain patterns found in the end compounds LiNiPO${}_4$ (point group $m{m}^{\prime }m$, spins along $z$) and LiFePO${}_4$ (point group $mm{m}^{\prime }$, spins along $y$). While the AFM order and the distribution of the AFM domains of LiNiPO${}_4$ are unaffected by the ion substitution at $x=0.06$, striking changes are observed at $x=0.20$. Fe${}^{2+}$ is dominant in establishing the magnetic order. For $x=0.20$ the magnetic order of the solid solution interpolates the magnetic order of its end compounds by exhibiting an orientation of the spins in the (100) plane which include an angle of ${40}^{\circ }\pm 3^{\circ }$ with respect to the $y$ axis toward 0 K (point group $m$). The associated AFM domains form rods of a few millimeters length and $\sim$10 $\mu$m width occurring in neither of the end compounds. Mechanisms responsible for the magnetic order and domain pattern (${180}^{\circ }$ domains still being one of the least explored aspects of AFM materials in spite of their omnipresence) are discussed.

We analytically determine the optimal microwave field that allows for the magnetization reversal of a nanomagnet modeled as a macrospin. This is done by minimizing the total injected energy. The results are in good agreement with the fields obtained numerically using the optimal control theory. For typical values of the damping parameter, a weak microwave field is sufficient to induce switching through a resonant process. The optimal field is orthogonal to the magnetization direction at any time and modulated in both amplitude and frequency. The dependence of the pulse shape on the applied field and damping parameter is interpreted. The total injected energy is found to be proportionnal to the energy barrier between the initial state and the saddle point and to the damping parameter. This result may be used as a means for probing the damping parameter in real nanoparticles.

We estimate the anomalous Hall effect of selected Heusler alloys from first principles. An emphasis is put on the effect of the native disorder which is often present in the stoichiometric samples. Such disorder can strongly influence both the magnetic and transport properties of these alloys. We employ a recently developed fully relativistic Kubo-Středa approach adapted to disordered multisublattice systems in which the chemical disorder is described in terms of the coherent potential approximation. As case studies we choose half-metallic Heusler alloys Co${}_2$CrAl and Co${}_2$MnAl, as well as the spin gapless semiconductor alloy Mn${}_2$CoAl for which experimental and theoretical studies appeared recently. We demonstrate that proper inclusion of disorder significantly improves agreement between the experiment and theory.

Magnetization of DyFe${}_4$Ge${}_2$ measured as function of temperature in a 1 kOe magnetic field indicates antiferromagnetic (AFM) ordering at $T_{\mathrm{N}}$ $=$ 62 K followed by two spin reorientation transitions at $T_f$${}_1$ $=$ 52 and $T_f$${}_2$ $=$ 32 K and one unusual anomaly at 15 K ($T_f$${}_3$). Three transitions ($T_f$${}_1$, $T_f$${}_2$, and $T_{\mathrm{N}}$) are further confirmed by heat capacity measurement in a zero magnetic field. The two low-temperature magnetic transitions are broadened and gradually vanish when the applied magnetic field exceeds 30 kOe, and the AFM transition shifts toward low temperatures with an increasing magnetic field. The reentrant magnetic glassy state is observed below the freezing point of $T_f$${}_3$ $=$ 15 K. Two field-induced metamagnetic phase transitions are observed between 2 and 50 K in fields below 140 kOe. A temperature-magnetic-field phase diagram has been constructed. The first-principles electronic structure calculations show that the paramagnetic tetragonal structure of DyFe${}_4$Ge${}_2$ is stable at high temperatures. The calculations with collinear Dy spins confirm ferrimagnetic orthorhombic DyFe${}_4$Ge${}_2$ as the ground-state structure.