We present a general theory about electron orbital motions in topological insulators. An in-plane electric field drives spin-up and spin-down electrons bending to opposite directions, and skipping orbital motions, a counterpart of the integer quantum Hall effect, are formed near the boundary of the sample. The accompanying Zitterbewegung can be found and controlled by tuning external electric fields. Ultrafast flipping electron spin leads to a quantum side jump in the topological insulator, and a snake-orbit motion in two-dimensional electron gas with spin-orbit interactions. This feature provides a way to control electron orbital motion by manipulating electron spin.

The high cost and low abundance of indium has driven research to find In-free $n$-type transparent conducting oxides (TCOs). La-doped cubic perovskite BaSnO${}_3$ has been reported to possess electron mobilities as high as $320{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ for carrier concentrations of $8\times {10}^{19}$ ${\mathrm{cm}}^{-3}$, comparable to the very best TCOs. To date, however, the origins of conductivity in this material have remained unclear. Here we study the defect chemistry of BaSnO${}_3$ using a hybrid density functional theory approach, in order to understand how to control the $n$-type conductivity. We show that in undoped samples, native defects cannot cause high levels of $n$-type conductivity; however, adventitious H present in samples can act as shallow donors. By studying the effect of a range of donor dopants, we pinpoint ideal growth conditions and donor dopants for high-performance $n$-type BaSnO${}_3$ samples.

We theoretically consider $g$ factor and spin lifetimes of holes in a longitudinal Ge/Si core/shell nanowire quantum dot that is exposed to external magnetic and electric fields. For the ground states, we find a large anisotropy of the $g$ factor which is highly tunable by applying electric fields. This tunability depends strongly on the direction of the electric field with respect to the magnetic field. We calculate the single-phonon hole spin relaxation times $T_1$ for zero and small electric fields and propose an optimal setup in which very large $T_1$ of the order of tens of milliseconds can be reached. Increasing the relative shell thickness or the longitudinal confinement length further prolongs $T_1$. In the absence of electric fields, the dephasing vanishes and the decoherence time $T_2$ is determined by $T_2=2T_1$.

We derive an effective medium model to characterize the macroscopic electromagnetic response of metallic nanowire arrays embedded in a host dielectric with a Kerr-type nonlinear permittivity function. It is shown that the macroscopic electromagnetic fields are coupled to the conduction current in the nanowires and to an additional quasistatic potential through a system of nonlinear equations. We prove that a weak nonlinearity leads to an electromagnetic response closer to that of an indefinite medium, and to isofrequency contours with increased hyperbolicity. For high field intensities the negative refraction of electromagnetic waves at an air-nanowire material interface is enhanced when the nanowires are embedded in a self-focusing Kerr medium.

We demonstrate the effectiveness of a generalized renormalized perturbational approach to calculate the induced magnetization for the single impurity Anderson model with a strong on-site interaction, using flow equations for renormalized parameters to scale from a weak correlation to a strong correlation regime. We show that, using simple approximation schemes in different parameter regimes, remarkably accurate results can be obtained for all magnetic field values by comparing the results with those from direct numerical renormalization group and Bethe ansatz calculations.

We consider the effect of electron-electron interaction on the density of states of a disordered paramagnetic conductor in the presence of spin accumulation and magnetic field. We show that interaction correction to electron density of states of the paramagnet may exhibit singularities at energies corresponding to the difference between chemical potentials of electrons with opposite spins. We also discuss correlation effects on conductivity in metallic as well as in hopping regimes.

Although GeTe-${\mathrm{Sb}}_2{\mathrm{Te}}_3$ (GST) alloys are widely used in data storage, many fundamental issues are still under debate. Here, we demonstrate that the presence of vacancies in the crystalline phase has far-reaching consequences, namely, a triad of twofold coordinated Te atoms with lone-pair electrons generated around the vacancy enables the formation of soft three-center four-electron bonds, whose properties provide an explanation for the unusual characteristics of GST, in particular, the increase in local disorder upon crystallization, the co-existence of a very fast switching rate with a large property contrast, the possibility of a solid-solid amorphization process that excludes conventional melting, and the drastic difference in crystallization behavior between GST and the ideal binary GeTe. Anisotropy of the three-center bonds may serve as an additional degree of freedom for information recording and provide a unified explanation for a variety of unique effects observed in lone-pair semiconductors.

Armchair and zigzag edge terminations in planar hexagonal and trigonal graphene nanorings are shown to underlie one-dimensional topological states associated with distinctive energy gaps and patterns (e.g., linear dispersion of the energy of an hexagonal ring with an armchair termination versus parabolic dispersion for a zigzag terminated one) in the bands of the tight-binding spectra as a function of the magnetic field. A relativistic Dirac-Kronig-Penney model analysis of the tight-binding Aharonov-Bohm behavior reveals that the graphene quasiparticle in an armchair hexagonal ring is a condensed-matter realization of an ultrarelativistic fermion with a position-dependent mass term, akin to the zero-energy fermionic solitons with fractional charge familiar from quantum-field theory and from the theory of polyacetylene. The topological origins of the above behavior are highlighted by contrasting it with the case of a trigonal armchair ring, where we find that the quasiparticle excitations behave as familiar Dirac fermions with a constant mass. Furthermore, the spectra of a zigzag hexagonal ring correspond to the low-kinetic-energy nonrelativistic regime of a lepton-like massive fermion. A one-dimensional relativistic Lagrangian formalism coupling a fermionic and a scalar bosonic field via a Yukawa interaction, in conjunction with the breaking of the $Z_2$ reflectional symmetry of the scalar field, is shown to unify the above dissimilar behaviors.

Spectroscopic modes provide the most sensitive probe of the very weak interactions responsible for the properties of the long-wavelength cycloid in the multiferroic phase of BiFeO${}_3$ below $T_N\approx 640$ K. Three of the four modes measured by terahertz (THz) and Raman spectroscopies were recently identified using a simple microscopic model. While a Dzyaloshinskii-Moriya (DM) interaction $D$ along $[-1,2,-1]$ induces a cycloid with wave vector $(2\pi /a)(0.5+\delta ,0.5,0.5-\delta )$ ($\delta \approx 0.0045$), easy-axis anisotropy $K$ along the $[1,1,1]$ direction of the electric polarization $\mathbf{P}$ induces higher harmonics of the cycloid, which split the ${\Psi }_1$ modes at 2.49 and 2.67 meV and activate the ${\Phi }_2$ mode at 3.38 meV. However, that model could not explain the observed low-frequency mode at about 2.17 meV. We now demonstrate that an additional DM interaction $D^{\prime }$ along $[1,1,1]$ not only produces the observed weak ferromagnetic moment of the high-field phase above 18 T but also activates the spectroscopic matrix elements of the nearly degenerate, low-frequency ${\Psi }_0$ and ${\Phi }_1$ modes, although their scattering intensities remain extremely weak. Even in the absence of easy-axis anisotropy, $D^{\prime }$ produces cycloidal harmonics that split ${\Psi }_1$ and activate ${\Phi }_2$. However, the observed mode frequencies and selection rules require that both $D^{\prime }$ and $K$ are nonzero. This work also resolves an earlier disagreement between spectroscopic and inelastic neutron-scattering measurements.

Measurements of temperature-dependent in-plane resistivity $\rho \left(T\right)$ were used to determine the upper critical field and its anisotropy in high-quality single crystals of the stoichiometric iron arsenide superconductor KFe${}_2$As${}_2$. The crystals were characterized by the residual resistivity ratio $\rho \left(\text{300}\text{K}\right)/\rho \left(0\right)$ up to 3000 and the resistive transition midpoint temperature $T_c=3.8$ K, significantly higher than in previous studies on the same material. We find increased $H_{c2}\left(T\right)$ for both directions of the magnetic field, which scale with the increased $T_c$. This unusual linear $H_{c2}\left(T_c\right)$ scaling is not expected for an orbital-limiting mechanism of the upper critical field in clean materials.

Negative charge transfer $AB$O${}_3$ oxides may undergo electronic metal-insulator transitions (MIT) concomitant with a dilation and contraction of nearly rigid octahedra. On both sides of the MIT are in-phase or out-of-phase (or both) rotations of adjacent octahedra that buckle the $B$-O-$B$ bond angle away from 180${}^{\circ }$. Using density functional theory with the PBEsol $+U$ approach, we describe an octahedral engineering avenue to control the $B$ $3d$ and O $2p$ orbital polarization through enhancement of the $B$O${}_6$ rotation “sense” rather than solely through conventional changes to the $B$-O bond lengths, i.e., crystal field distortions. Using CaFeO${}_3$ as a prototypical material, we show the flavor of the octahedral rotation pattern when combined with strain-rotation coupling and thin film engineering strategies offers a promising avenue to fine tune orbital polarizations near electronic phase boundaries.

We study experimentally and theoretically the electronic and magnetic properties of two insulating double perovskites that show similar atomic and electronic structure but different magnetic properties. In magnetization measurements, La${}_2$ZnIrO${}_6$ displays weak ferromagnetic behavior below 7.5 K, whereas La${}_2$MgIrO${}_6$ shows antiferromagnetic behavior below $T_N$ $=$ 12 K. Electronic structure calculations find that the weak ferromagnetic behavior observed in La${}_2$ZnIrO${}_6$ is in fact due to canted antiferromagnetism. The calculations also predict canted antiferromagnetic behavior in La${}_2$MgIrO${}_6$, but intriguingly, this was not observed. Neutron diffraction measurements confirm the essentially antiferromagnetic behavior of both systems but lack the sensitivity to resolve the small (0.22${\mu }_{\mathrm{B}}$/Ir) ferromagnetic component in La${}_2$ZnIrO${}_6$. Overall, the results presented here indicate the crucial role of spin-orbit coupling (SOC) and the on-site Coulomb repulsion on the magnetic, transport, and thermodynamic properties of both compounds. The electronic structure calculations show that both compounds, like Sr${}_2$IrO${}_4$, are $J_{\mathrm{eff}}$ $=$ 1/2 Mott insulators. Our present findings suggest that La${}_2$ZnIrO${}_6$ and La${}_2$MgIrO${}_6$ provide a new playground to study the interplay between SOC and on-site Coulomb repulsion in a 5d transition-metal oxide.

We report on the theoretical and experimental study of spin-dependent electronic transition rates which are controlled by a radiation-induced spin-Rabi oscillation of weakly spin-exchange and spin-dipolar coupled paramagnetic states ($S=\frac{1}{2}$). The oscillation components [the Fourier content, $\mathbf{F}\left(s\right)$] of the net transition rates within spin-pair ensembles are derived for randomly distributed spin resonances, with an account of a possible correlation between the two distributions corresponding to individual pair partners. Our study shows that when electrically detected Rabi spectroscopy is conducted under an increasing driving field $B_1$, the Rabi spectrum, $\mathbf{F}\left(s\right)$, evolves from a single peak at $s={\Omega }_R$, where ${\Omega }_R=\gamma B_1$ is the Rabi frequency ($\gamma$ is the gyromagnetic ratio), to three peaks at $s={\Omega }_R$, $s=2{\Omega }_R$, and low $s\ll {\Omega }_R$. The crossover between the two regimes takes place when ${\Omega }_R$ exceeds the expectation value ${\delta }_0$ of the difference in the Zeeman energies within the pairs, which corresponds to the broadening of the magnetic resonance by disorder caused by a hyperfine field or distributions of Landé $g$ factors. We capture this crossover by analytically calculating the shapes of all three peaks at an arbitrary relation between ${\Omega }_R$ and ${\delta }_0$. When the peaks are well developed their widths are $\Delta s\sim {\delta }_0^2/{\Omega }_R$. We find a good quantitative agreement between the theory and experiment.

The stability and growth of three-dimensional (3D) nanostructures in the Ge on Si system is controlled in part by the strain- and overlayer-thickness-dependent surface energies of the crystal facets involved. Here, we use density functional theory (DFT) with local-density approximation calculations to calculate the strain- and thickness-dependent energy of various Ge(113) and Si(113) surface reconstructions. Results of DFT calculations are compared to Tersoff potential calculations to assess the relative importance of stress-strain effects compared to electronic effects not captured by empirical atomistic potentials. We find that the self-interstitial-based $3\times 2$ adatom-dimer-interstitial and $3\times 2$ adatom-interstitial surface reconstructions are stable for Ge overlayer thicknesses from 0 to 4 monolayers and at applied biaxial strains from $\sim$$-4$$\%$ to 0$\%$. We leverage calculated surface energies to determine net effective surface energies of various experimentally observed 3D Ge on Si nanostructures.

Polariton condensates are investigated in periodical potentials created by surface acoustic waves (SAWs) using both coherent and incoherent optical excitation. Under coherent resonant pumping, condensates are formed due to polariton-parametric scattering from the pump. In this case, the excitation spectrum of the condensate shows a strong reduction of the energy gap arising from the acoustic modulation, indicating efficient screening of the SAW potential by spatial modulation of the polariton density. The observed results are in good agreement with a model based on generalized Gross-Pitaveskii equations, with account taken of the spatial dependence of the exciton energy landscape. In the case of incoherent pumping, coexisting nonequilibrium condensates with $s$- and $p$-type wavefunctions are observed, which have different energies, symmetry, and spatial coherence. The energy splitting between these condensate states is also reduced with respect to the gap of the one particle spectrum below threshold, but the screening effect is less pronounced than in the case of coherently pumped system due to weaker modulation of the pump state.

We show that hot carriers scattered by optic phonons in graphene undergo an anomalous parametric resonance when modulated by an ac field, at about half of the frequency ${\omega }_F=2\pi eF{v}_f/\hslash {\omega }_{\mathrm{op}}$, corresponding to ballistic acceleration up to the phonon energy ${\omega }_F=2\pi eF{v}_f/\hslash {\omega }_{\mathrm{op}}$ in the presence of a dc field $F$. The resonance occurs in the terahertz range and is tunable with dc electric fields. Dephasing between the current and the ac field also exhibits a nonzero minimum at resonance for weak elastic scattering, while increasing monotonously with ac frequency for strong elastic scattering. The overall effect would also manifest in a long-range spatially varying periodic potential.

We study the 2$D$ Raman band of in-plane uniaxially strained graphene within a nonorthogonal tight-binding model. At nonzero strain, the obtained 2$D$ band splits into two subbands at strain angles $0^{\circ }$ and ${30}^{\circ }$ or into three subbands at intermediate angles. The evolution of the 2$D$ subbands is calculated systematically in the range of the accessible strains from $-1\%$ to $3\%$ and for the commonly used laser photon energy from 1.5 to $3.0$ eV. The strain rate and dispersion rate of the 2$D$ subbands are derived and tabulated. In particular, these two quantities show large variations up to $50\%$. The results on the 2$D$ subbands can be used for detecting and monitoring strain in graphene for nanoelectronics applications.