Within the continuum model, i.e., the Dirac equation approach, we study the boundary states around the line defect in graphene in the quantum Hall regime. We find that the boundary states localize at the opposite sides of the line defect if they propagate toward the opposite directions. Meanwhile, some boundary states show simultaneously the valley and sublattice polarizations. In addition, a pseudomagnetic field induced by an inhomogeneous strain field, in place of the real magnetic field, can drive the line defect embedded graphene to the quantum valley Hall regime, due to the occurrence of the valley polarized boundary states.

Manifestations of fluctuating Cooper pairs formed by electrons and holes populating opposite surfaces of a topological insulator film in the Coulomb drag effect are considered. A fluctuational Aslamazov-Larkin contribution to the transresistance between surfaces of the film is calculated. The contribution is the most singular one in the vicinity of critical temperature $T_{\mathrm{d}}$ and diverges in the critical manner as ${(T-T_{\mathrm{d}})}^{-1}$. In the realistic conditions $\gamma \sim T_{\mathrm{d}}$, where $\gamma$ is the average scattering rate of electrons and holes, the Aslamazov-Larkin contribution plays an important role and can dominate the fluctuation transport. The macroscopic theory based on the time-dependent Ginzburg-Landau equation is developed for a description of the transport in the system. The results can be easily generalized for other realizations of an electron-hole bilayer.

We calculated Coulomb scattering rates from quantum dots (QDs) coupled to a two-dimensional (2D) carrier reservoir and QDs coupled to a three-dimensional (3D) reservoir. For this purpose we used a microscopic theory in the limit of Born-Markov approximation, in which the numerical evaluation of high dimensional integrals is done via a quasi-Monte Carlo method. Via a comparison of the so determined scattering rates, we investigated the question whether scattering from 2D is generally more efficient than scattering from 3D. In agreement with experimental findings, we did not observe a significant reduction of the scattering efficiency of a QD directly coupled to a 3D reservoir. In turn, we found that 3D scattering benefits from its additional degree of freedom in the momentum space.

By using the Kubo linear response theory with the Keldysh Green function approach, we investigate the mechanism leading to the negative differential transmission in a system with the equilibrium electron density much smaller than the photon-excited one. It is shown that the negative differential transmission can appear at low probe-photon energy (in the order of the scattering rate) or at high energy (much larger than the scattering rate). For the low probe-photon energy case, the negative differential transmission is found to come from the increase of the intraband conductivity due to the large variation of electron distribution after the pumping. As for the high probe-photon energy case, the negative differential transmission is shown to tend to appear with the hot-electron temperature being closer to the equilibrium one and the chemical potential higher than the equilibrium one but considerably smaller than half of the probe-photon energy. We also show that this negative differential transmission can come from both the inter- and the intraband components of the conductivity. Especially, for the interband component, its contribution to the negative differential transmission is shown to come from both the Hartree-Fock self-energy and the scattering. Furthermore, the influence of the Coulomb-hole self-energy is also addressed.

Combining group-theoretical analysis and first-principles density functional theory calculations, we confirm theoretically the antiferrodistortive phase transition in ultrathin SrTiO${}_3$ (001) TiO${}_2$-terminated films and compare it with a similar transition in the bulk. We demonstrate phonon softening at the $M$ point of the surface Brillouin zone and analyze the change in the calculated electronic and phonon properties upon phase transition.

We investigate the interplay between topological effects and Mott physics in two dimensions on a graphenelike lattice, via a tight-binding model containing an anisotropic spin-orbit coupling on the next-nearest-neighbor links and the Hubbard interaction. We thoroughly analyze the resulting phases, namely, a topological band insulator phase or anisotropic quantum spin Hall phase until moderate-strength interactions and a Néel and a spiral phase at large interactions in the Mott regime, as well as the formation of a spin-orbital texture in the bulk at the Mott transition. The emergent magnetic orders at large interactions are analyzed through a spin-wave analysis and mathematical arguments. At weak interactions, by analogy with the Kane-Mele model, the system is described through a ${\mathbb{Z}}_2$ topological invariant. In addition, we describe how the anisotropic spin-orbit coupling already produces an exotic spin texture at the edges. The physics at the Mott transition is described in terms of a U(1) slave-rotor theory. Taking into account gauge fluctuations around the mean-field saddle-point solution, we show how the spin texture now proliferates into the bulk above the Mott critical point. The latter emerges from the response of the spinons under the insertion of monopoles and this becomes more pronounced as the spin-orbit coupling becomes prevalent. We discuss implications of our predictions for thin films of the iridate compound Na${}_2$IrO${}_3$ and also graphenelike systems.

By ab initio based tight-binding calculations, we show that nontrivial electronic states exist at an interface of a ${\mathcal{Z}}_2$ topological insulator and a topological crystalline insulator. At the exemplary (111) interface between Bi${}_2$Te${}_3$ and SnTe, the two Dirac surface states at the Brillouin zone center $\overline{\Gamma }$ annihilate upon approaching the semi-infinite subsystems but one topologically protected Dirac surface state remains at each time-reversal invariant momentum $\overline{M}$. This leads to a highly conducting spin-momentum-locked channel at the interface but insulating bulk regions. For the Sb${}_2$Te${}_3$/Bi${}_2$Te${}_3$ interface, we find complete annihilation of Dirac states because both subsystems belong to the same topology class. Our proof of principle may have impact on planar electric transport in future spintronics devices with topologically protected conducting channels.

We report on the observation of the reststrahl band-assisted photocurrents in epitaxial graphene on SiC excited by infrared radiation. The peculiar spectral dependence for frequencies lying within the reststrahl band of the SiC substrate provides a direct and noninvasive way to probe the electric field magnitude at atomic distances from the material's surface. Furthermore our results reveal that nonlinear optical and optoelectronic phenomena in two-dimensional crystals and other atomic scale structures can be giantly enhanced by their deposition on a substrate with negative dielectric constant.

Low-temperature electronic conductance in nanocontacts, scanning tunneling microscopy (STM), and metal break junctions involving magnetic atoms or molecules is a growing area with important unsolved theoretical problems. While the detailed relationship between contact geometry and electronic structure requires a quantitative ab initio approach such as density functional theory (DFT), the Kondo many-body effects ensuing from the coupling of the impurity spin with metal electrons are most properly addressed by formulating a generalized Anderson impurity model to be solved with, for example, the numerical renormalization group (NRG) method. Since there is at present no seamless scheme that can accurately carry out that program, we have in recent years designed a systematic method for semiquantitatively joining DFT and NRG. We apply this DFT-NRG scheme to the ideal conductance of single wall (4,4) and (8,8) nanotubes with magnetic adatoms (Co and Fe), both inside and outside the nanotube, and with a single carbon atom vacancy. A rich scenario emerges, with Kondo temperatures generally in the Kelvin range, and conductance anomalies ranging from a single channel maximum to destructive Fano interference with cancellation of two channels out of the total four. The configuration yielding the highest Kondo temperature (tens of Kelvins) and a measurable zero-bias anomaly is that of a Co or Fe impurity inside the narrowest nanotube. The single atom vacancy has a spin, but a very low Kondo temperature is predicted. The geometric, electronic, and symmetry factors influencing this variability are all accessible, which makes this approach methodologically instructive and highlights many delicate and difficult points in the first-principles modeling of the Kondo effect in nanocontacts.

We report tight-binding (TB) calculations of magnetocrystalline anisotropy energy (MAE) of iron slabs and nanoclusters with a particular focus on local analysis. After clarifying various concepts and formulations for the determination of MAE, we apply our realistic TB model to the analysis of the magnetic anisotropy of $\mathrm{Fe}\left(001\right)$, $\mathrm{Fe}\left(110\right)$ slabs and of two large Fe clusters with $\left(001\right)$ and $\left(110\right)$ facets only: a truncated pyramid and a truncated bipyramid containing 620 and 1096 atoms, respectively. It is shown that the MAE of slabs originates mainly from outer layers; a small contribution from the subsurface gives rise, however, to an oscillatory behavior for large thicknesses. Interestingly, the MAE of the nanoclusters considered is almost solely due to $\left(001\right)$ facets and the base perimeter of the pyramid. We believe that this fact could be used to efficiently control the anisotropy of iron nanoparticles and could also have consequences on their spin dynamics.

The current dependence of the spin torque switching rate in a thermally activated region of an in-plane magnetized system was studied. The logarithm of the switching rate depended nonlinearly on current in the high-current region, $I_{\mathrm{c}}\lesssim I<I_{\mathrm{c}}^{*}$, where $I_{\mathrm{c}}$ and $I_{\mathrm{c}}^{*}$ are critical currents distinguishing the stability of the magnetization. We also found that the attempt frequency had a minimum around $I_{\mathrm{c}}$, and that the attempt frequency at $I_{\mathrm{c}}$ was three orders of magnitude smaller than that at zero current, contrary to the assumption in previous analyses of experiments that it remains constant.

We present a study of the magnetic and crystallographic structure of TbMnO${}_3$ in the presence of crossed electric and magnetic fields using circularly polarized x-ray nonresonant scattering. A comprehensive account is presented of the scattering theory and data analysis methods used in our earlier studies, and in addition we present interesting high magnetic field data and its analysis. We discuss in detail how polarization analysis was used to reveal structural information, including the arrangement of Tb moments which we proposed for $H=0$ T, and how the diffraction data for $H<H_C$ can be used to determine specific magnetostrictively induced atomic displacements with fm accuracy. The connection between the electric polarization and magnetostrictive mechanisms is discussed. Similar magnetostrictive displacements have been observed for $H>H_C$ as for $H<H_C$. Finally some observations regarding the kinetics and the conservation of domain population at the transition are described.

By pump-probe optical reflectivity measurements, we studied the photoinduced dynamics of MnV${}_2$O${}_4$, which is a $d^2$ Mott insulator and exhibits a simultaneous spin and orbital ordering accompanied by a structural phase transition at 57 K. We compared the present experimental results with the results of the steady-state optical measurement of the same compound and the steady-state and pump-probe optical measurements of Al-doped MnV${}_2$O${}_4$, in which the orbital ordering and structural phase transition are absent but the magnetic ordering exists. We found that the photoinduced dynamics of MnV${}_2$O${}_4$ can be separated into three components: (1) the suppression of the Mott-excitation peak, (2) the melting of the orbital ordering, and (3) the melting of the magnetic ordering. Furthermore, we found that the lattice distortion associated with the photoinduced structural phase transition from tetragonal to cubic induces a strain wave whose propagation velocity is discernibly smaller than that of a conventional shockwave.

We report the discovery of superconductivity at $\sim 9$ K in Fe${}_{1.05}$Te single crystals that have been exposed to air for more than six months. The superconductivity is induced due to the oxygen incorporation and only exists in the surface layer of the samples. Our high-resolution transmission electron microscopy experiments and density functional theory calculations show the oxygen prefers to locate at the interstitial site in the Fe-Te layer. X-ray photoelectron spectra characterize an enhancement of the itinerant character of Fe $3d$ electrons, which may reduce the local moment and hence suppress the long-range antiferromagnetism associated with superexchange interactions. This work suggests the bicollinear antiferromagnetism in Fe${}_{1+y}$Te can be suppressed by oxygen incorporation to induce superconductivity and thus could be considered as the parent phase of the superconductivity.

We compute self-consistently the Josephson current in a superconductor-antiferromagnet-superconductor junction using a lattice model, focusing on $0-\pi$ transitions occurring when the width of the antiferromagnetic region changes from an even to an odd number of lattice sites. Previous studies predicted $0-\pi$ transitions when alternating between an even and an odd number of sites for sufficiently strong antiferromagnetic order. We study numerically the magnitude of the threshold value for this to occur, and also explain the physics behind its existence in terms of the phase shifts picked up by the quasiparticles constituting the supercurrent in the antiferromagnet. Moreover, we show that this threshold value allows for pressure-induced $0-\pi$ transitions by destroying the antiferromagnetic nesting properties of the Fermi surface, a phenomenon which has no counterpart in ferromagnetic Josephson junctions, offering a way to tune the quantum ground state of a Josephson junction without the need for multiple samples.

We show that the pair of Majorana modes at each end of a 1D spin triplet superconductor with ${\Delta }_{\uparrow \uparrow }=-{\Delta }_{\downarrow \downarrow }=p{\Delta }_0$ (two time reversed copies of the Kitaev $p$-wave chain) are topologically robust to perturbations such as mixing by the $S_z=0$ component of the order parameter (${\Delta }_{\uparrow \downarrow }={\Delta }_{\downarrow \uparrow }$), transverse hopping, nonagnetic disorder, and also, importantly, to time-reversal (TR) breaking perturbations such as applied Zeeman fields/magnetic impurities and the mixing by the $S_y=0$ component of the order parameter (${\Delta }_{\uparrow \uparrow }={\Delta }_{\downarrow \downarrow }$). We show that the robustness to TR-breaking results from a hidden chiral symmetry, which places the system in the BDI class in the presence of the generic TR-breaking perturbations (the TR-invariant system is both DIII and BDI). Our work has important implications for the quasi-1D organic superconductors (TMTSF)${}_2$$X$ ($X=\text{P}$F${}_6$, CIO${}_4$) (Bechgaard salts) and Li${}_{0.9}$Mo${}_6$O${}_{17}$, which have been proposed as triplet superconductors with equal spin pairing (${\Delta }_{\uparrow \uparrow },{\Delta }_{\downarrow \downarrow }\ne 0,{\Delta }_{\uparrow \downarrow }=0$) in the presence of magnetic fields.

We report low-temperature specific heat, $C\left(T\right)$, measurements on (Yb${}_{1-x}$Lu${}_x$)${}_4$As${}_3$, with $x=0.01$ and $x=0.03$, where nonmagnetic Lu atoms are randomly distributed on antiferromagnetic $S=1/2$ Heisenberg chains with $J/k_{\mathrm{B}}=28$ K. The observed reduction of $C$ below 15 K with increasing $x$ is accurately described by quantum transfer matrix simulations without any adjustable parameter, implying that the system is an excellent experimental realization of segmented quantum spin chains. Finite-size effects consistent with conformal-field theory predictions are leading to the formation of an effective low-energy gap. The size of the gap increases with Lu content and accounts for the impurity-driven reduction of the specific heat. For both concentrations our results verify experimentally the low-temperature scaling behavior established theoretically and also confirm the value of $J$ determined from pure Yb${}_4$As${}_3$.

We report on current-induced domain-wall propagation in a patterned GaMnAs microwire with perpendicular magnetization. A slowing down of the propagation velocity has been found when the moving domain wall extends over only half of the width of the wire. This slowing down is related to the elongation of a longitudinal wall along the axis of the wire. By using an energy balance argument, the expected theoretical dependence of the velocity change has been calculated and compared with the experimental results. According to this, the energy associated with the longitudinal domain wall is found to change when a current passes through the wire. These results provide possible evidence of transverse spin effects when a current flows parallel to a magnetic domain wall.

We studied the possibility of exciton condensation in a strongly correlated bilayer extended Hubbard model using determinant quantum Monte Carlo. To model both the on-site repulsion $U$ and the interlayer interaction $V$ we introduced an update scheme extending the standard Sherman-Morrison update. We observe that the sign problem increases dramatically with the inclusion of the interlayer interaction $V$, which prohibits at this stage an unequivocal conclusion regarding the presence of exciton condensation. However, enhancement of the interlayer tunneling results suggest that the strongest exciton condensation tendency lies around $10–20\%$ $p$/$n$ doping. Magnetic properties and conductivity turn out to be relatively independent of the interlayer interaction.