Motivated by recent progress in understanding the interplay between lattice and electronic topological phases, we consider quantum-melting transitions of weak quantum liquid crystals, a crystal and a nematic phase, in which electrons form a quantum Hall state. In certain classes of Chern band insulators and quantum Hall phases, it has been previously demonstrated that there are topological Chern-Simons terms such as a Hall viscosity term and a gravitational Chern-Simons term for local lattice deformations. The Chern-Simons terms can induce anyonic statistics for the topological lattice defects and, furthermore, dress the defects with certain symmetry quantum numbers. On the other hand, the melting transitions of such liquid-crystalline orders are driven by the condensation of lattice defects. Based on these observations, we show how the topological terms can change the nature of the proximate disordered phases of the quantum liquid-crystalline phases. We derive and study the effective dual field theories for the liquid-crystalline phases with the geometric Chern-Simons terms, and carefully examine the symmetry quantum numbers and statistics of defects. We show that a crystal may go through a continuous phase transition into another crystal with the different discrete translational symmetries because the dislocation, the topological defect in the crystal, carries nonzero crystal momentum due to the Hall viscosity term. For the nematic phase, the disclination will condense at the phase transition to the isotropic phase, and we show that the isotropic phase may support a deconfined fractionally charged excitation due to the Wen-Zee term, and thus the isotropic phase and the nematic phase have different electromagnetic Hall responses.

A solution of the $\mathbf{k}·\mathbf{p}$ model is presented for bulk and quantum spin hall (QSH) edge states in semiconductor topological insulator (TI) quantum wells (QWs), bounded at the edge by an infinite wall potential. The edge states are exponentially localized, with a nonzero amplitude at the QW edge, and obey standard boundary conditions for the wave function and its derivative. Single helical edge states with spin locked to the direction of motion are found in the TI band gap $\left(E_{\mathrm{TI}}\right)$ of QWs with both strong (HgTe/CdTe) and weak (InAs/GaSb/AlSb) $s\text{−}p$ hybridization, but in the second case only below a small critical band gap, $E_{\mathrm{crit}}\sim 1.6\mathrm{meV}$. For $E_{\mathrm{TI}}>E_{\mathrm{crit}}$, there appear to be two degenerate states for each spin direction. It is suggested that $Z_2$-like topological properties can still be maintained if one of these states is spurious or suppressed by disorder. The effect of interface band mixing, and band mixing due to structural inversion asymmetry and bulk inversion asymmetry is also considered. Simple model Hamiltonians are developed for the bulk and edge states which are calibrated against a bulk eight-band $\mathbf{k}·\mathbf{p}$ calculation close to the TI transition. At the transition, the zero gap bulk states exhibit a spin splitting, essentially changing the Dirac point to a circle. In the TI phase, there is a small change in the dispersion of the QSH edge states. These results confirm the robustness of the QSH edge states to spatial symmetry breaking interactions.

We present evidence of a drastic renormalization of the optical conductivity of graphene on $\mathrm{SrTiO}{}_3$ resulting in almost full transparency in the ultraviolet region. These findings are attributed to resonant excitonic effects further supported by ab initio Bethe-Salpeter equation and density functional theory calculations. The $(\pi ,\pi *)$ orbitals of graphene and Ti-$3d$ $t_{2g}$ orbitals of $\mathrm{SrTiO}{}_3$ are strongly hybridized and the interactions of electron-hole states residing in those orbitals play dominant role in the graphene optical conductivity. These interactions are present much below the optical band gap of bulk $\mathrm{SrTiO}{}_3$. These results open a possibility of manipulating interaction strengths in graphene via $d$ orbitals, which could be crucial for optical applications.

We analytically study vacancy effects on electronic and transport properties of graphene nanoribbons and nanodots using Green's function approach. For semiconducting systems, the presence of a vacancy induces a zero-energy midgap state. The spatial pattern of the wave functions critically depends on the atomistic edge structures and can be used as an unambiguous probe of the edge structure. For metallic systems, the midgap vacancy state does not exist. In these systems, the vacancy mainly works as a source of electronic scattering and modifies electronic transmission. We derive that the electronic transmission coefficient can be written as $\mathcal{T}={cos}^2\left(\alpha \right)$, where $\alpha$ denotes the phase angle of the on-site Green's function at the vacancy site of the ideal systems. At small energies, $\mathcal{T}$ exhibits distinctly different functional form depending on edge structures.

We investigate the tunable and switchable optical radiators and metamaterials formed by metallic nanodipole antennas with submicroscopic gaps (1.2 nm), of which linear and third-order nonlinear quantum conductivities are observed due to the photon-assisted tunneling effect. The quantum conductivities induced at the nanogap are relevant to power dissipations, which can be enhanced by the strongly localized optical fields associated with the plasmonic resonance. We demonstrate that the scattering property of an individual quantum nanoantenna and the transparency of a metamasurface constituted of it can be tuned by electrostatically controlling the linear conductivity (electronic tuning) or by adjusting the irradiation intensity that varies the nonlinear quantum conductivity (all-optical tuning).

We formulate a minimal model of point contact Andreev reflection spectroscopy of a normal-metal/multiband superconductor interface. The theory generalizes the Blonder-Tinkham-Klapwijk formulation to a multiband superconductor and it is based on the quantum waveguide theory. The proposed approach allows an analytic evaluation of the Andreev and normal reflection coefficients and thus is suitable for a data fitting of point contact experiments. The obtained differential conductance curves present distinctive features similar to those measured in the experiments on multiband superconductors, such as iron-based pnictides and ${\mathrm{MgB}}_2$.

The electronic structure and the transport properties of nitrogen-doped carbon nanotubes are investigated using a tight-binding model and a real-space Kubo-Greenwood approach, respectively. The transport regimes of various axial and helical doping configurations, from perfectly periodic to fully random disordered cases, are examined through the time dependence of the diffusivity. By varying the degree of disorder, a rich set of transient regimes is predicted going from persisting quasiballistic to momentarily localized regimes. A spectacular long-time ballistic regime is also observed for a specific semi-random disorder doping configuration owing to symmetry effects.

The highly conductive layered metallic oxide ${\text{PdCoO}}_2$ is a near-perfect analog to an alkali metal in two dimensions. It is distinguished from other two-dimensional (2D) electron systems where the Fermi surface does not reach the Brillouin zone boundary by a high planar electron density exceeding ${10}^{15}{\text{cm}}^{-2}$. The simple single-band quasi-2D electronic structure results in strongly anisotropic transport properties and limits the effectiveness of electron-phonon scattering. Measurements on single crystals in the temperature range from 10 to 300 K show that the thermal conductivity is much more weakly anisotropic than the electrical resistivity, as a result of significant phonon heat transport. The in-plane thermoelectric power is linear in temperature at 300 K and displays a purity-dependent peak around 50 K. Given the extreme simplicity of the band structure, it is possible to identify this peak with phonon drag driven by normal electron-phonon scattering processes.

The mixed-chalcogenide compound TlBiSSe realizes a three-dimensional (3D) Dirac semimetal state. In clean, low-carrier-density single crystals of this material, we found Shubnikov–de Haas oscillations to signify its 3D Dirac nature. Moreover, we observed very large linear magnetoresistance (MR) approaching 10 000% in 14 T at 1.8 K, which diminishes rapidly above 30 K. Our analysis of the magnetotransport data points to the possibility that the linear MR is fundamentally governed by the Hall field; although such a situation has been predicted for highly inhomogeneous systems, inhomogeneity does not seem to play an important role in TlBiSSe. Hence, the mechanism of large linear MR is an intriguing open question in a clean 3D Dirac system.

The search for new large band gap quantum spin Hall (QSH) and quantum anomalous Hall (QAH) insulators is critical for their realistic applications at room temperature. Here we predict, based on first-principles calculations, that the band gap of QSH and QAH states can be as large as 1.01 and 0.35 eV in an H-decorated Bi(111) film. The origin of this giant band gap lies in both the large spin-orbit interaction of Bi and the H-mediated exceptional electronic and structural properties. Moreover, we find that the QAH state also possesses the properties of a quantum valley Hall state, thus intrinsically realizing the so-called valley-polarized QAH effect. We further investigate the possibility of large gap QSH and QAH states in an H-decorated $\mathrm{Bi}\left(\overline{1}10\right)$ film and $X\text{-decorated}(X=\text{F},\mathrm{Cl},\mathrm{Br},\text{and}\mathrm{I})\mathrm{Bi}\left(111\right)$ films.

We develop a theoretical description of electromagnon solitons in a coupled ferroelectric-ferromagnetic heterostructure. The solitons are considered in the weakly nonlinear limit as a modulation of plane waves corresponding to two electriclike and magneticlike branches in the spectrum. Emphasis is put on magneticlike envelope solitons that can be created by an alternating electric field. It is shown also that the magnetic pulses can be amplified by an electric field with a frequency close to the band edge of the magnetic branch.

We study a hybrid system consisting of a narrow-band atomic optical resonance and the long-range periodic order of an opaline photonic nanostructure. To this end, we have infiltrated atomic cesium vapor in a thin silica opal photonic crystal. With increasing temperature, the frequencies of the opal's reflectivity peaks shift down by $>20\%$ due to chemical reduction of the silica. Simultaneously, the photonic bands and gaps shift relative to the fixed near-infrared cesium $D_1$ transitions. As a result the narrow atomic resonances with high finesse $\left(\omega /\Delta \omega =8\times {10}^5\right)$ dramatically change shape from a usual dispersive shape at the blue edge of a stop gap, to an inverted dispersion line shape at the red edge of a stop gap. The line shape, amplitude, and off-resonance reflectivity are well modeled with a transfer-matrix model that includes the dispersion and absorption of Cs hyperfine transitions and the chemically reduced opal. An ensemble of atoms in a photonic crystal is an intriguing hybrid system that features narrow defectlike resonances with a strong dispersion, with potential applications in slow light, sensing, and optical memory.

We present a comprehensive study of single-point kinetic energy density functionals (KEDFs) to be used in orbital-free density functional theory (DFT) calculations. We first propose a form of KEDFs based on a pointwise Kohn-Sham (KS) kinetic energy density (KED) and electron localization function (ELF) analysis. We find that the ELF and modified enhancement factor have a very strong and transferable correlation with the reduced density in various bulk metals. The non-self-consistent kinetic energy errors predicted by our KEDF models are decreased greatly compared to previously reported generalized gradient approximation (GGA) KEDFs. Second, we perform self-consistent calculations with various single-point KEDFs and investigate their numerical convergence behavior. We find striking numerical instabilities for previous GGA KEDFs; most of the GGA KEDFs fail to converge and show unphysical densities during the optimization. In contrast, our KEDFs demonstrate stable convergence, and their self-consistent results of various bulk properties agree reasonably well with KSDFT. A further detailed KED analysis reveals an interesting bifurcation phenomenon in defective metals and alloys, which may shed light on directions for future KEDF development.

The structural and transport properties of polycrystalline ${\mathrm{Ti}}_{1\text{−}x}{\mathrm{Pt}}_x{\mathrm{Se}}_{2\text{−}y}(x\le 0.13,y\le 0.2)$ are studied, revealing highly tunable electrical properties, spanning nearly ten orders of magnitude in scaled resistivity. Using x-ray and neutron diffraction, Pt is found to dope on the Ti site. In the absence of Pt doping (for $x=0$), Se deficiency ($y>0$) increases the metallic character of ${\mathrm{TiSe}}_2$, while a large increase of the low-temperature resistivity is favored by a lack of Se deficiency ($y=0$) and increasing amounts of doped Pt ($x>0$). The chemical tuning of the resistivity in ${\mathrm{Ti}}_{1\text{−}x}{\mathrm{Pt}}_x{\mathrm{Se}}_{2\text{−}y}$ with Se deficiency and Pt doping results in a metal-to-insulator transition. Simultaneous Pt doping and Se deficiency ($x,y>0$) confirms the competition between the two opposing trends in electrical transport, with the main outcome being the suppression of the charge density wave transition below 2 K for $y=2x=0.18$. Band structure calculations on a subset of ${\mathrm{Ti}}_{1\text{−}x}{\mathrm{Pt}}_x{\mathrm{Se}}_{2\text{−}y}$ compositions are in line with the experimental observations.

We provide a detailed study of the Abelian quasiholes of $\nu =1/2$ bosonic fractional quantum Hall states on the torus geometry and in fractional Chern insulators. We find that the density distribution of a quasihole in a fractional Chern insulator can be related to that of the corresponding fractional quantum Hall state by choosing an appropriate length unit on the lattice. This length unit only depends on the lattice model that hosts the fractional Chern insulator. Therefore, the quasihole size in a fractional Chern insulator can be predicted for any lattice model once the quasihole size of the corresponding fractional quantum Hall state is known. We discuss the effect of the lattice embedding on the quasihole size. We also perform the braiding of quasiholes for fractional Chern insulator models to probe the fractional statistics of these excitations. The numerical values of the braiding phases accurately match the theoretical predictions.

We discuss a technique to manipulate and read out strictly resonantly excited exciton polaritons confined in a three-dimensional trap. The polaritons are trapped via their photonic part in a locally elongated microcavity with a high quality factor $(Q\sim 6000)$, giving rise to sharp zero-dimensional resonances. Manipulation of the polaritons is achieved by spectrally tuning the quantum well excitons via the quantum confined Stark effect up to 10 meV, while the signal of the resonantly excited polaritons is simultaneously read out via the photocurrent flowing through the device. The effects of polariton-polariton interactions and interactions with the environment are revealed in the zero detuning regime by excitation power dependent investigations. By increasing the polariton number in our trap via resonant optical injection, we observe a pronounced blueshift of the lower polariton eigenenergy towards the weakly coupled cavity resonance. Furthermore, the photocurrent exhibits pronounced nonlinearities when self-tuned into resonance via its excitation dependent spectral blueshift.

We study the relaxation of the Holstein model after a sudden switch-on of the interaction by means of the nonequilibrium dynamical mean field theory, with the self-consistent Migdal approximation as an impurity solver. We show that there exists a qualitative change in the thermalization dynamics as the interaction is varied in the weak-coupling regime. On the weaker interaction side of this crossover, the phonon oscillations are damped more rapidly than the electron thermalization time scale, as determined from the relaxation of the electron momentum distribution function. On the stronger interaction side, the relaxation of the electrons becomes faster than the phonon damping. In this regime, despite long-lived phonon oscillations, a thermalized momentum distribution is realized temporarily. The origin of the “thermalization crossover” found here is traced back to different behaviors of the electron and phonon self-energies as a function of the electron-phonon coupling. In addition, the importance of the phonon dynamics is demonstrated by comparing the self-consistent Migdal results with those obtained with a simpler Hartree-Fock impurity solver that neglects the phonon self-energy. The latter scheme does not properly describe the evolution and thermalization of isolated electron-phonon systems.

We present a time-domain model for the simulation of light-matter interaction in semiconductors in arbitrary geometries and across a wide range of excitation conditions. The electromagnetic field is treated classically using the finite-difference time-domain method. The polarization and occupation numbers of the semiconductor material are described using the semiconductor Bloch equations including many-body effects in the screened Hartree-Fock approximation. Spontaneous emission noise is introduced using stochastic driving terms. As an application, we present simulations of the dynamics of a nanowire laser including optical pumping, seeding by spontaneous emission, and the selection of lasing modes.

Electronic states of the topological insulator ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$, which is nontrivial in terms of both the $Z_2$ index and the mirror Chern number, were studied by quasiparticle interference (QPI) using low-temperature scanning tunneling spectroscopy. Our Fourier-transform analysis of the QPI patterns reveals the dispersion of the two surface bands above the Fermi energy ($E_F$). Absence of the surface band crossing is unambiguously confirmed, which allowed us to elucidate the mirror Chern number of this system is $-1$. We also found that Co atoms deposited onto a clean surface of ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ create impurity states with a clover-leaf-like shape at around 80 meV above $E_F$. While additional spin-flipping scatterings due to those Co atoms cannot be detected by QPI because of the inherent absence of interference between states with antiparallel spins, we observed clear enhancement of spin-conserving scatterings after Co deposition.