We have studied the evolution of the superconducting energy gap of indium oxide through the disorder-driven superconductor-to-insulator transition (SIT) using two distinct experimental methods that allow us to test the influence of metallic screening: tunneling spectroscopy in which a metallic electrode is adjacent to the studied sample, thus screening Coulomb interactions, and contactless terahertz spectroscopy which probes the unperturbed sample. The tunneling measurements reveal a similar superconducting gap on both sides of the SIT and at temperatures above and below $T_c$. Terahertz measurements detect the superconducting gap below but not above the critical temperature nor in the insulating state. This difference between the two spectroscopy methods is attributed to the effect of Coulomb interactions which are screened in the tunneling experiments. Our study reveals the importance of Coulomb interactions on the energy gap of disordered superconductors.

We have performed density functional theory calculations using the linearized augmented plane wave method (LAPW) with the local density approximation (LDA) functional to study the electronic structure of the iron-based superconductor iron-selenium (FeSe). In our study, we have performed a comprehensive set of calculations involving structural, atomic, and spin configurations. All calculations were executed using the tetragonal lead-oxide or $P4$/$nmm$ structure, with various volumes, $c/a$ ratios, and internal parameters. Furthermore, we investigated the spin polarization using the LDA functional to assess ferromagnetism in this material. The paramagnetic LDA calculations find the equilibrium configuration of FeSe in the $P4$/$nmm$ structure to have a volume of 472.5 a.u.${}^3$ with a $c/a$ ratio of 1.50 and internal parameter of 0.255, with the ferromagnetic having comparable results to the paramagnetic case. In addition, we calculated total energies for FeSe using a pseudopotential method, and found comparable results to the LAPW calculations. Superconductivity calculations were done using the Gaspari-Gyorffy and the McMillan formalisms and found substantial electron-phonon coupling. Under pressure, our calculations show that the superconductivity critical temperature continues to rise, but underestimates the measured values.

We use temporally resolved intensity cross-correlation measurements to identify the biexciton-exciton radiative cascades in a negatively charged QD. The polarization sensitive correlation measurements show unambiguously that the excited two-electron triplet states relax nonradiatively to their singlet ground state via a spin nonconserving flip-flop with the ground state heavy hole. We explain this mechanism in terms of resonant coupling between the confined electron states and an LO phonon. This resonant interaction together with the electron-hole exchange interaction provides an efficient mechanism for this otherwise spin-blockaded, electronic relaxation.

We observed the antibound state, as well as the bound state, for cavity polariton pairs in a planar CuCl microcavity by spectrally resolved four-wave mixing. We obtained dispersion curves of the bound and antibound states by changing the incident angle of the pump pulses corresponding to the cavity detuning. The dispersion curve for the bound state suggests that the bound state is mainly composed of a bare biexciton and is weakly coupled to the cavity photons. The dephasing time of the bound state was faster than that of a bare biexciton in a thin sample, supporting the hypothesis that the bound state is coupled to the cavity photons. On the other hand, the antibound state consists of two lower polaritons having the same spin. The clear observation of the antibound state can be qualitatively explained by the phase-space filling, which reduces the Rabi splitting.

We investigate the effect of hydrogen coverage on the optical conductivity of single-side hydrogenated graphene from first-principles calculations. To account for different degrees of uniform hydrogen coverage we calculate the complex optical conductivity for graphene supercells of various sizes, each containing a single additional hydrogen atom. We use the linearized augmented plane wave method, as implemented in the wien2k density functional theory code, to show that the hydrogen coverage strongly influences the complex optical conductivity and thus the optical properties, such as absorption, of hydrogenated graphene. We find that the optical conductivity of graphene in the infrared, visible, and ultraviolet range has different characteristic features depending on the degree of hydrogen coverage. This opens up new possibilities to tailor the optical properties of graphene by reversible hydrogenation, and to determine the hydrogen coverage of hydrogenated graphene samples in the experiment by contact-free optical absorption measurements.

The anharmonic behavior of phonons and intrinsic thermal conductivity associated with the umklapp scattering in monolayer MoS${}_2$ sheet are investigated via first-principles calculations within the framework of density functional perturbation theory. In contrast to the negative Grüneissen parameter ($\gamma$) occurring in low-frequency modes in graphene, positive $\gamma$ in the whole Brillouin zone is demonstrated in monolayer MoS${}_2$ with much larger $\gamma$ for acoustic modes than that for the optical modes, suggesting that monolayer MoS${}_2$ sheet possesses a positive coefficient of thermal expansion. The calculated phonon lifetimes of the infrared active modes are 5.50 and 5.72 ps for $E^{\prime }$ and $A_2^{\prime \prime }$, respectively, in good agreement with experimental results obtained by fitting the dielectric oscillators with the infrared reflectivity spectrum. The lifetime of the Raman $A_1^{\prime }$ mode (38.36 ps) is about seven times longer than those of the infrared modes. The dominated phonon mean free path of monolayer MoS${}_2$ is less than 20 nm, about 30-fold smaller than that of graphene. Combined with the nonequilibrium Green's function calculations, the room temperature thermal conductivity of monolayer MoS${}_2$ is found to be around 23.2 W m${}^{-1}$ K${}^{-1}$, two orders of magnitude lower than that of graphene.

We present ab initio electronic structure calculations based on density functional theory for the thermoelectric properties of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ films. Conductivity and thermopower are computed in the diffusive limit of transport based on the Boltzmann equation. Bulk and surface contributions to the transport coefficients are separated by a special projection technique. As a result we show clear signatures of the topological surface state in the thermoelectric properties.

We study properties of a class of spin-singlet Mott states for arbitrary spin $S$ bosons on a lattice, with particle number per cite $n=S/l+1$, where $l$ is a positive integer. We show that such a singlet Mott state can be mapped to a bosonic Laughlin wave function on a sphere with a finite number of particles at filling $\nu =1/2l$. Spin, particle, and hole excitations in the Mott state are discussed, among which the hole excitation can be mapped to the quasihole of the bosonic Laughlin wave function. We show that this singlet Mott state can be realized in a cold-atom system on an optical lattice and can be identified using Bragg spectroscopy and Stern-Gerlach techniques. This class of singlet Mott states may be generalized to map to bosonic Laughlin states with filling $\nu =q/2l$.

The interest in SnTe has recently increased due to its topological crystalline insulator nature, despite the fact that SnTe is always heavily $p$ type. Here, using first-principles calculations, we identify the microscopic origin of the $p$-type conductivity of SnTe. It is found that the negatively charged Sn vacancy ($V_{\mathrm{Sn}}^{2-}$) dominates the electronic properties of SnTe: regardless of the growth conditions, $V_{\mathrm{Sn}}^{2-}$ always has a negative formation energy within the band gap, which forces the Fermi level below the valence band maximum (VBM), leading to degenerate $p$-type doping. In contrast, the deeper VBM of PbTe increases the formation energy of the Pb vacancy ($V_{\mathrm{Pb}}^{2-}$), allowing for either $n$-type or $p$-type PbTe. So even though standard n doping of SnTe is very difficult, alloying with Pb can achieve $n$ doping by lowering the VBM, thereby inhibiting the formation of cation vacancy and, thus, probably producing a topological crystalline insulator with the Fermi level located near the Dirac point.

We consider an interacting nanowire/superconductor heterostructure attached to metallic leads. The device is described by an unusual low-energy model involving spin-1 conduction electrons coupled to a nonlocal spin-$\frac{1}{2}$ Kondo impurity built from Majorana fermions. The topological origin of the resulting Kondo effect is manifest in distinctive non-Fermi-liquid (NFL) behavior, and the existence of Majorana fermions in the device is demonstrated unambiguously by distinctive conductance line shapes. We study the physics of the model in detail, using the numerical renormalization group, perturbative scaling, and Abelian bosonization. In particular, we calculate the full scaling curves for the differential conductance in ac and dc fields, onto which experimental data should collapse. Scattering $t$ matrices and thermodynamic quantities are also calculated, recovering asymptotes from conformal field theory. We show that the NFL physics is robust to asymmetric Majorana-lead couplings, and here we uncover a duality between strong and weak coupling. The NFL behavior is understood physically in terms of competing Kondo effects. The resulting frustration is relieved by inter-Majorana coupling which generates a second crossover to a regular Fermi liquid.

We discuss the sign change of superconducting order parameters in both real and reciprocal spaces when the odd parity spin singlet pairing proposed recently by Hu [Phys. Rev. X 3, 031004 (2013)] is allowed. We show that in this case an nodeless antiphase $s^{\pm }$ can be generated. In a 2-Fe Brilliouin zone (BZ), sign change exists between two hole pockets and between two electron pockets. In a 1-Fe BZ, which includes two 2-Fe BZs, the sign change is between two 2-Fe BZs, which leads to a $d$-wave type sign distribution on the electron pockets, namely, an antiphase $s^{\pm }$ state with no symmetry protected gapless node on the electron pockets. This sign change character consistently explains experimental results related to sign change properties measured on both iron-pnictides and iron-chalcogenides.

Conventional understanding implies that the ground state of a nonmagnetic quantum mechanical system should be nodeless. While this notion also provides a valuable guidance in understanding the ordering of energy levels in semiconductor nanostructures, there are reports that nodal ground states for holes are possible. However, the existence of such nodal states has been debated and even viewed merely as an artifact of a $\mathit{k}·\mathbf{p}$ model. Using complementary approaches of both $\mathit{k}·\mathbf{p}$ and tight-binding models, further supported by an effective Hamiltonian for a continuum model, we reveal that nodal ground states in quantum dots are not limited to a specific approach. Remarkably, the emergence of nodal hole states at the top of the valence band can be attributed to the formation of orbital vortex textures through competition between the hole kinetic energy and the coupling to the conduction band states. We suggest an experimental test for our predictions of the reversed energy ordering and the existence of nodal ground states. We discuss how our findings and the studies of orbital textures could be also relevant for other materials systems.

We demonstrate a method for control of the polarization of emission of quantum dots (QDs) embedded in an active layer of a planar microcavity. This method involves a modification of the electromagnetic mode structure in a planar microcavity which is achieved by fabrication of a chiral gammadion layer structure with partial etching of the upper Bragg mirror. A polarization degree as high as 81% has been demonstrated experimentally without the use of a static magnetic field or birefringent wave plates; this is in full agreement with the theoretical simulations for the fabricated structure. Theoretical optimization has shown that a polarization degree of up to 99% can be achieved in optimized structures with randomly positioned quantum dots, and to an even higher degree when the QDs have controlled positions.

Spin-noise measurements can serve as a direct probe for the microscopic decoherence mechanism of an electronic spin in semiconductor quantum dots (QDs). We have calculated the spin-noise spectrum in the anisotropic central spin model using a Chebyshev expansion technique which exactly accounts for the dynamics up to an arbitrary long but fixed time in a finite-size system. In the isotropic case, describing QD charge with a single electron, the short-time dynamics is in good agreement with quasistatic approximations for the thermodynamic limit. The spin-noise spectrum, however, shows strong deviations at low frequencies with a power-law behavior of ${\omega }^{-3/4}$ corresponding to a $t^{-1/4}$ decay at intermediate and long times. In the Ising limit, applicable to QDs with heavy-hole spins, the spin-noise spectrum exhibits a threshold behavior of ${(\omega -{\omega }_L)}^{-1/2}$ above the Larmor frequency ${\omega }_L=g{\mu }_{B}B$. In the generic anisotropic central spin model we have found a crossover from a Gaussian type of spin-noise spectrum to a more Ising-type spectrum with increasing anisotropy in a finite magnetic field. In order to make contact with experiments, we present ensemble averaged spin-noise spectra for QD ensembles charged with single electrons or holes. The Gaussian-type noise spectrum evolves to a more Lorentzian shape spectrum with increasing spread of characteristic time scales and $g$ factors of the individual QDs.

The Casimir force between purely dielectric materials is generally attractive and can lead to increased friction and stiction effects in nanoscale devices. While prospective quantum levitating systems have been proposed for high dielectric constant host materials, reversal of the Casimir force with air/vacuum as the intermediate medium remains challenging. Here, the problem of quantum levitation is studied for a system consisting of two parallel magnetodielectric plates. A simple analytical treatment of the problem is provided through the introduction of an upper bound of the force. An explicit sufficient condition for the manifestation of Casimir force repulsion is derived in terms of the plate's material parameters and temperature. The sufficient condition can serve as a useful tool in designing quantum levitating systems with air as the intermediate medium, which is the natural environment for practical microscopic devices.

Despite its fundamental importance for nanophysics and chemistry and potential device applications, the relationship between atomic structure and electronic transport in molecular nanostructures is not well understood. Thus the experimentally observed increase of the conductance of some molecular nanojunctions when they are stretched continues to be counterintuitive and controversial. Here we explore this phenomenon in propanedithiolate molecules bridging gold electrodes by means of ab initio computations and semiempirical modeling. We show that in this system it is due to changes in Au-S-C bond angles and strains in the gold electrodes, rather than to the previously proposed mechanisms of Au-S bond stretching and an associated energy shift of the highest occupied molecular orbital and/or Au atomic chain formation. Our findings indicate that conductance enhancement in response to the application of tensile stress should be a generic property of molecular junctions in which the molecule is thiol-bonded in a similar way to gold electrodes.

Nitrogen-vacancy centers in diamond are ideal platforms for quantum simulation, which allows one to handle problems that are intractable theoretically or experimentally. Here we propose a digital quantum simulation scheme to simulate the quantum phase transition occurring in an ultrathin topological insulator film placed in a parallel magnetic field [Zyuzin et al., Phys. Rev. B 83, 245428 (2011)]. The quantum simulator employs high quality spin qubits achievable in nitrogen-vacancy centers. With a series of precise controls, the dynamics of the spin system is made capable of simulating the dynamics of the simulated TI system. The eigenenergies of the TI system are extracted by a quantum algorithm of finding eigenvalues. The scheme can be realized with existing technologies. An operation time of the order of 100 $\mu$s is estimated for each individual run.

The interaction of two resonant impurities in graphene has been predicted to have a long-range character with weaker repulsion when the two adatoms reside on the same sublattice and stronger attraction when they are on different sublattices. We reveal that this attraction results from a single energy level. This opens up a possibility of controlling the sign of the impurity interaction via the adjustment of the chemical potential. For many randomly distributed impurities (adatoms or vacancies) this may offer a way to achieve a controlled transition from aggregation to dispersion.

Changes in temperature or stress state may induce reversible B2$\leftrightarrow$(R)$\leftrightarrow$ B19${}^{\prime }$ martensitic transformations and associated shape memory effects in close-to-stoichiometric nickel-titanium (NiTi) alloys. Recent experimental studies confirmed a considerable impact of the hydrogen-rich aging atmosphere on the subsequent B2 austenite $\leftrightarrow$ B19${}^{\prime }$ martensite transformation path. In this paper, we employ density functional theory to study properties of Ar, He, and H interstitials in B2 austenite and B19${}^{\prime }$ martensite phases. We show that H interstitials exhibit negative formation energies, while Ar and He interstitials yield positive values. Our theoretical analysis of slightly Ni-rich NiTi-based alloys with the austenite B2 structure shows that a slight over-stoichiometry towards Ni-rich compositions in a range $51–52\text{at}.\%$ is energetically favorable. The same conclusion holds for H-doped NiTi with the H content up to $\approx 6\text{at}.\%$. In agreement with experimental data we predict H atoms to have a strong impact on the martensitic phase transformation in NiTi by altering the mutual thermodynamic stability of the high-temperature cubic B2 and the low-temperature monoclinic B19${}^{\prime }$ phase of NiTi. Hydrogen atoms are predicted to form stable interstitial defects. As this is not the case for He and Ar, mixtures of hydrogen and the two inert gases can be used in annealing experiments to control H partial pressure when studying the martensitic transformations in NiTi in various atmospheres.

A model based on the continuous atomic density function (ADF) approach is applied to predict the atomic structure of grain boundaries (GBs) in iron. Symmetrical $\left[100\right]$ and $\left[110\right]$ tilt GBs in bcc iron are modeled with the ADF method and relaxed afterwards in molecular dynamics (MD) simulations. The shape of the GB energy curve obtained in the ADF model reproduces well the peculiarities of the angles of $70.{53}^{\circ }$ [$\Sigma 3\left(112\right)$] and $129.{52}^{\circ }$ [$\Sigma 11\left(332\right)$] for $\left[110\right]$ tilt GBs. The results of MD relaxation with an embedded-atom method potential for iron confirm that the atomic GB configurations obtained in ADF modeling are very close to equilibrium ones. The developed model provides well-localized atomic positions for GBs of various geometries.