We investigate theoretically the edge state transport in a HgTe topological insulator under periodic electrical modulation. We find constructive interference of the backscattering amplitudes, leading to the formation of superlattice minigaps and hence complete suppression of the edge state transmission. Consequently, the edge channel can be switched on/off by appropriately tuning the modulation amplitude via gate voltages, even for wide Hall bar with a small finite size effect. We also find efficient conversion between spin-up and spin-down edge channels by the gate-induced Rashba spin-orbit interaction.

We perform first-principles, density-functional-theory calculations of the electronic structure for the layered bulk materials SnS and GeS which are of interest for photovoltaic applications. Band gap corrections are computed within the GW approximation to the electron self-energy. The resulting quasiparticle gaps in both SnS and GeS are in excellent agreement with the majority of existing experimental measurements. In order to better understand the possible use of GeS layers as a carrier-confining barrier within a SnS-based photovoltaic device, we compute the band offsets for different orientations of a SnS/GeS heterojunction. We find the valence band offsets to be almost independent of interfacial direction while the conduction band offsets show a strong anisotropy as a result of the variation in the band gap caused by epitaxial strain along the different directions.

We investigate the influence of the Förster interaction between semiconductor quantum dots on the quantum light emission in proximity to a metal nanoparticle. A fully quantized theory for the excitons in the quantum dots, the plasmons in metal nanoparticles, and their interaction is used. Using an operator equation approach, we derive the Rayleigh emission spectra and the corresponding quantum statistics of the emission. For both observables, we investigate the influence of the exciton-plasmon coupling and the Förster interaction between the semiconductor quantum dots. Surprisingly, the influence of the Förster interaction is barely seen in the Rayleigh spectra whereas the second-order correlation function is strongly affected. In particular, we show that the Förster interaction is capable to tune the emission statistics: depending on the system parameters, the Förster interaction between the semiconductor quantum dots induces strong bunching and antibunching of the emission, respectively. We analyze the quasiparticles formed in the coupled system to explain the observed features.

We study the influence of a magnetic flux tube on the possibility to electrostatically confine electrons in a graphene quantum dot. Without a magnetic flux tube, the graphene pseudospin is responsible for a quantization of the total angular momentum to half-integer values. On the other hand, with a flux tube containing half a flux quantum, the Aharonov-Bohm phase and Berry phase precisely cancel, and we find a state at zero angular momentum that cannot be confined electrostatically. In this case, true bound states only exist in regular geometries for which states without zero-angular-momentum component exist, while nonintegrable geometries lack confinement. We support these arguments with a calculation of the two-terminal conductance of a gate-defined graphene quantum dot, which shows resonances for a disk-shaped geometry and for a stadium-shaped geometry without flux tube, but no resonances for a stadium-shaped quantum dot with a $\pi$-flux tube.

We study the spin-polarized $\nu =0$ Landau-level state of graphene. Due to the electron-hole attractive interaction, electrons and holes can bound into pairs. These pairs can then condense into a spin-triplet superfluid ground state: a spin superconductor state. In this state, a gap opens up in the edge bands as well as in the bulk bands, thus it is a charge insulator, but it can carry the spin current without dissipation. These results can well explain the insulating behavior of the spin-polarized $\nu =0$ state in the recent experiments.

A fully self-consistent treatment for gap generation and Coulomb screening in excitonic insulators is presented. The method is based on the equations of motion for the relevant dynamical variables combined with a variational approach. Applying the theory for a model system of bilayer graphene, an excitonic ground state with a gap exceeding 10 meV is predicted.

Dynamic dipolar toroidal response is demonstrated by an optical plasmonic metamaterial composed of double disks. This response with a hotspot of localized $E$-field concentration is a well-behaved toroidal cavity mode that exhibits a large Purcell factor due to its deep-subwavelength mode volume. All-optical Hall effect (photovoltaic) is observed numerically attributed to the nonlinear phenomenon of unharmonic plasmon oscillations, which exhibits an interesting analogy with the magnetoelectric effect in multiferroic systems. The result shows a promising avenue to explore various intriguing optical phenomena associated with this dynamic toroidal moment.

We investigate the currently debated issue of the existence of the Dirac cone in silicene on an Ag(111) surface, using first-principles calculations based on density functional theory to obtain the band structure. By unfolding the band structure in the Brillouin zone of a supercell to that of a primitive cell, followed by projecting onto Ag and silicene subsystems, we demonstrate that the Dirac cone in silicene on Ag(111) is destroyed. Our results clearly indicate that the linear dispersions observed in both angular-resolved photoemission spectroscopy [P. Vogt et al., Phys. Rev. Lett. 108, 155501 (2012)] and scanning tunneling spectroscopy [L. Chen et al., Phys. Rev. Lett. 109, 056804 (2012)] come from the Ag substrate and not from silicene.

Using first-principles calculations, we show that the (001) surface of the ferromagnet La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ under an epitaxial compressive strain favors antiferromagnetic (AF) order in the surface layers, coexisting with ferromagnetic (FM) bulk order. Surface antiferromagnetism is accompanied by a very marked surface-related spectral pseudogap, signaling an incomplete metal-insulator transition at the surface. The different relaxation and rumpling of the MnO${}_2$ and LaO surface planes in the two competing magnetic phases cause distinct work-function changes, which are of potential diagnostic use. The AF phase is recognized as an extreme surface-assisted case of the combination of in-plane AF super-exchange and vertical FM double-exchange couplings that rules magnetism in manganites under in-plane compression.

We use exact symmetry properties of the two-dimensional quantum compass model to derive nonequivalent invariant subspaces in the energy spectra of $L\times L$ clusters up to $L=6$. The symmetry allows one to reduce the original $L\times L$ compass cluster to the $(L-1)\times (L-1)$ one with modified interactions. This step is crucial and enables (i) exact diagonalization of the $6\times 6$ quantum compass cluster and (ii) finding the specific heat for clusters up to $L=6$, with two characteristic energy scales. We investigate the properties of the ground state and the first excited states and present extrapolation of the excitation energy with increasing system size. Our analysis provides physical insights into the nature of nematic order realized in the quantum compass model at finite temperature. We suggest that the quantum phase transition at the isotropic interaction point is second order with some admixture of the discontinuous transition, as indicated by the entropy, the overlap between two types of nematic order (on horizontal and vertical bonds), and the existence of the critical exponent. Extrapolation of the specific heat to the $L\to \infty$ limit suggests the classical nature of the quantum compass model and high degeneracy of the ground state with nematic order.

The magnetic vortex dynamics in thickness-modulated Ni${}_{80}$Fe${}_{20}$ disks in the form of a Ni${}_{80}$Fe${}_{20}$ lens on top of a Ni${}_{80}$Fe${}_{20}$ disk are investigated. The vortex core location can be systematically controlled via the geometry, and the propagation and annihilation can be detected using ferromagnetic resonance measurements. The thickness modulation provides an additional shape anisotropy, which defines the vortex chirality in the disk depending on the magnetization reversal history and the magnetic interactions between the disk and the lens. The vortex propagation and annihilation in each layer were identified by their resonance modes.

We use angle-resolved photoemission spectroscopy and full-potential linearized augmented-plane-wave (FP-LAPW) calculations to study the electronic structure of BaCo${}_2$As${}_2$. The Fermi surface (FS) maps and the corresponding band dispersion data (at 90 and 200 K) reveal a small electron pocket at the center and a large electron pocket at the corner of the Brillouin zone. Therefore the nesting between electron and hole FS pockets is absent in this compound, in contrast to the parent compounds of FeAs-based high-$T$${}_{\mathrm{c}}$ superconductors. The electron pockets at the center of the zone are surrounded by two sets of four smaller electron pockets. The electronic structure at about 500 meV binding energy is very similar to features at the chemical potential in BaFe${}_2$As${}_2$. This indicates that complete substitution of Co for Fe causes a nearly rigid shift in the chemical potential by adding two electrons per formula unit at higher binding energies. However at lower binding energies $\sim$270 meV, the electron pocket at the center of the zone is absent, unlike in the Co-substituted Fe-based materials. This demonstrates that the rigid band picture is valid only at higher binding energies and breaks down closer to the chemical potential in BaCo${}_2$As${}_2$. We also observed the presence of a flat band near the Fermi energy that may have consequences for transport and thermodynamical properties. The experimental FS topology as well as band dispersion data are in reasonable agreement with the FP-LAPW calculations.

Majorana fermions, strange particles that are their own antiparticles, were predicted in 1937 and have been sought after ever since. In condensed matter they are predicted to exist as vortex core or edge excitations in certain exotic superconductors. These are topological superconductors whose order parameter phase winds nontrivially in momentum space. In recent years, a new and promising route for realizing topological superconductors has opened due to advances in the field of topological insulators. Current proposals are based on semiconductor heterostructures, where spin-orbit-coupled bands are split by a band gap or Zeeman field and superconductivity is induced by proximity to a conventional superconductor. Topological superconductivity is obtained in the interface layer. The proposed heterostructures typically include two or three layers of different materials. In the current work we propose a device based on materials with inherent spin-orbit coupling and an intrinsic tendency for superconductivity, eliminating the need for a separate superconducting layer. We study a lattice model that includes spin-orbit coupling as well as on-site and nearest-neighbor interaction. Within this model we show that topological superconductivity is possible in certain regions of parameter space. These regions of nontrivial topology can be understood as a nodeless superconductor with $d$-wave symmetry which, due to the spin-orbit coupling, acquires an extra phase twist of $2\pi$.

It is shown that the thermodynamic Rutgers relation for the second-order phase transitions can be used for the analysis of the superfluid density data irrespective of complexities of the Fermi surface, structure of the superconducting gap, pairing strength, or scattering. The only limitation is that critical fluctuations should be weak so that the mean-field theory of the second-order phase transitions is applicable. By using the Rutgers relation, the zero-temperature value of the London penetration depth $\lambda \left(0\right)$ is related to the specific heat jump $\Delta C$ and the slope of upper critical field $d{H}_{c2}/dT$ at the transition temperature $T_c$, provided the data on $\Delta \lambda =\lambda \left(T\right)-\lambda \left(0\right)$ are available in a broad temperature domain. We then provide a way to determine $\lambda \left(0\right)$, the quantity difficult to determine within many techniques.

We studied the thermoelectric properties of the composite of misfit-layered compounds (BiSe)${}_{1.09}$TaSe${}_2$ and TaSe${}_2$. The x-ray diffraction pattern on the cross-sectional plane of the sintered body shows a preferred orientation of the (00$l$) direction for (BiSe)${}_{1.09}$TaSe${}_2$/TaSe${}_2$ indicating anisotropic alignment during hot pressing. Because of the crystallographic alignment, the temperature-dependent electrical resistivity $\rho \left(T\right)$, Seebeck coefficient $S\left(T\right)$, and the thermal conductivity $\kappa \left(T\right)$ exhibit in-plane and out-of-plane anisotropic transport behavior. The Seebeck coefficient is very low because of the coexistence of electron and hole mixing, as confirmed by the two-carrier model. The lattice thermal conductivity ${\kappa }_L$ of the covalent bonding layer (in-plane) is lower than those of the layer with van der Waals bonding (out-of-plane) implying the existence of a charge density wave along the in-plane. We observed a sign anomaly of the positive Hall coefficient $R_H$ and negative Seebeck coefficient $S$. According to Holstein's small-polaron model, the sign anomaly may come from the odd number of small-polaron hopping sites.

We study the semiclassical dynamics of interacting electrons in a biased crystal lattice. A complex dynamical scenario emerges from the interplay between the Coulomb and the external electric fields. When the electrons are far apart, the Coulomb potential may be small compared to the external potential and the electrons oscillate with effective Bloch frequencies, determined by the local electric field. In the opposite case, nearby electrons either separate or form a bound pair, depending on the initial energy compared to the band width. The pair due to the Coulomb field is stable even in the absence of the external field.

We report on the local magnetic properties of ultrathin bcc Fe films grown on a Ag(1,1,6) stepped surface and covered by a Au layer. We determined the structure of the multilayer stack and the depth-resolved magnetic profile for 5.9 and 13.5 monolayer (ML) thick films. The experiments were carried out on magnetically saturated samples by soft x-ray resonant magnetic reflectivity using both circularly and linearly polarized x rays and two different acquisition configurations. Our results indicate a 20$\%$ enhancement of the magnetic moment in the first 2–3 ML of Fe at both interfaces. A comparison of the magnetization profile for Fe films grown on a flat Ag(001) surface indicates a very similar effect and does not reveal any difference regarding the method's accuracy.

The spectroscopic modes of multiferroic BiFeO${}_3$ provide detailed information about the very small anisotropy and Dzyaloshinskii-Moriya (DM) interactions responsible for the long-wavelength, distorted cycloid below $T_{\mathrm{N}}=640$ K. A microscopic model that includes two DM interactions and easy-axis anisotropy predicts both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic field applied along a cubic axis. While only six modes are optically active in zero field, all modes at the cycloidal wave vector are activated by a magnetic field. The three magnetic domains of the cycloid are degenerate in zero field but one domain has lower energy than the other two in nonzero field. Measurements imply that the higher-energy domains are depopulated above about 6 T and have a maximum critical field of 16 T, below the critical field of 19 T for the lowest-energy domain. Despite the excellent agreement with the measured spectroscopic frequencies, some discrepancies with the measured spectroscopic intensities suggest that other weak interactions may be missing from the model.

Sr${}_3$Ir${}_2$O${}_7$ exhibits a novel $J_{\mathrm{eff}}=\frac{1}{2}$ insulating state that features a splitting between $J_{\mathrm{eff}}=\frac{1}{2}$ and $\frac{3}{2}$ bands due to spin-orbit interaction. We report a metal-insulator transition in Sr${}_3$Ir${}_2$O${}_7$ via either dilute electron doping (La${}^{3+}$ for Sr${}^{2+}$) or application of high pressure up to 35 GPa. Our study of single-crystal Sr${}_3$Ir${}_2$O${}_7$ and (Sr${}_{1-x}$La${}_x$)${}_3$Ir${}_2$O${}_7$ reveals that application of high hydrostatic pressure $P$ leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure $P_C$ $=$ 13.2 GPa, manifesting a closing of the gap; but further increasing $P$ up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles θ. In contrast, slight doping of La${}^{3+}$ ions for Sr${}^{2+}$ ions in Sr${}_3$Ir${}_2$O${}_7$ readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr${}_{0.95}$La${}_{0.05}$)${}_3$Ir${}_2$O${}_7$ where the metallic state occurs. The results are discussed along with comparisons drawn with Sr${}_2$IrO${}_4$, a prototype of the $J_{\mathrm{eff}}=\frac{1}{2}$ insulator.

We extend the composite fermion construction to the torus geometry. We verify the validity of our construction by computing the overlap of the composite fermion state to the exact diagonalization ground state of both Coulomb interaction and Haldane-pseudopotential interaction $V_0$ ($V_1$) for bosonic (fermionic) states.