We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy on detwinned single crystals. Across the structural transition, the electronic structures around the $\mathrm{\Gamma }$ and $M$ points are modified from fourfold to twofold symmetry due to the lifting of degeneracy in $d_{xz}/d_{yz}$ orbitals. The $d_{xz}$ band shifts upward at the $\mathrm{\Gamma }$ point, while it moves downward at the $M$ point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. Due to this sign-changing orbital order, the elongated directions of the elliptical Fermi surfaces at the $\mathrm{\Gamma }$ and $M$ points are rotated by 90° with respect to each other, which makes the nesting condition between these FSs imperfect. The present result, supported by calculations, indicates the possible suppression of the spin-fluctuation mediated superconductivity in the orthorhombic FeSe, as compared to the orbital-ordered state without sign change.

We report the three-dimensional (3D) mapping of strain and tilts of crystal planes in an extended InP nanostructured layer bonded onto silicon, measured without sample preparation. Our approach takes advantage of 3D x-ray Bragg ptychography combined with an optimized inversion process. The excellent agreement with the sample nominal structure validates the reconstruction while the evidence of spatial fluctuations hardly observable by other means underlines the specificities of Bragg ptychography.

We investigate the spontaneous emission rate of a two-level quantum emitter near a graphene-coated substrate under the influence of an external magnetic field or strain induced pseudomagnetic field. We demonstrate that the application of the magnetic field can substantially increase or decrease the decay rate. We show that a suppression as large as $99\%$ in the Purcell factor is achieved even for moderate magnetic fields. The emitter's lifetime is a discontinuous function of $\left|\mathbf{B}\right|$, which is a direct consequence of the occurrence of discrete Landau levels in graphene. We demonstrate that, in the near-field regime, the magnetic field enables an unprecedented control of the decay pathways into which the photon/polariton can be emitted. Our findings strongly suggest that a magnetic field could act as an efficient agent for on-demand, active control of light-matter interactions in graphene at the quantum level.

With the recent achievement of extremely high-quality epitaxial interfaces between InAs nanowires and superconducting Al shells with strong superconductor-semiconductor tunnel coupling, a new regime of proximity-induced superconductivity in semiconductors can be explored where the induced gap may be similar in value to the bulk Al gap (large gap) with negligible subgap conductance (hard gap). We propose several experimentally relevant consequences of this large-gap strong-coupling regime for tunneling experiments, and we comment on the prospects of this regime for topological superconductivity. In particular, we show that the advantages of having a strong spin-orbit coupling and a large spin $g$ factor in the semiconductor nanowire may both be compromised in this strongly coupled limit, and somewhat weaker interface tunneling may be necessary for achieving optimal proximity superconductivity in the semiconductor nanowire. We derive a minimal, generic theory for the strong-coupling hard-gap regime obtaining good qualitative agreement with the experiment and pointing out future directions for further progress toward Majorana nanowires in hybrid semiconductor-superconductor structures.

We investigate effects of ordinary nonmagnetic disorder in the bulk of a superconductor on magnetic adatom-induced Shiba states and on the proximity-induced superconductivity in a nanowire that is tunnel coupled to the bulk superconductor. Within the formalism of self-consistent Born approximation, we show that, contrary to widespread belief, the proximity-induced topological superconductivity can be adversely affected by the bulk superconducting disorder even in the absence of any disorder in the nanowire (or the superconductor-nanowire interface) when the proximity tunnel coupling is strong. In particular, bulk disorder can effectively randomize the Shiba-state energies. In the case of a proximate semiconductor nanowire, we numerically compute the dependence of the effective disorder and pairing gap induced on the wire as a function of the semiconductor-superconductor tunnel coupling. We find that the scaling exponent of the induced disorder with respect to coupling is always larger than that of the induced gap, implying that at weak coupling, the proximity-induced pairing gap dominates, whereas at strong coupling, the induced disorder dominates. These findings bring out the importance of improving the quality of the bulk superconductor itself (in addition to the quality of the nanowire and the interface) in the experimental search for solid-state Majorana fermions in proximity-coupled hybrid structures and, in particular, points out the pitfall of pursuing strong coupling between the semiconductor and the superconductor in a goal toward having a large proximity gap. In particular, our work establishes that the bulk superconductor in strongly coupled hybrid systems for Majorana studies must be in the ultraclean limit, since otherwise the bulk disorder is likely to completely suppress all induced topological superconductivity effects.

A recent publication [Q. Meng, L. Wu, D. O. Welch, and Y. Zhu, Phys. Rev. B 91, 224305 (2015)] examines the quantum normal modes of the Frenkel-Kontorova chain. The authors compare their results to those of an older work [A. D. Novaco, Phys. Rev. B 22, 1645 (1980)], attributing the differences to limitations in the numerical analysis of that 1980 paper. We show here that it is not numerical limitations that cause the differences between the two papers, and we argue that the cause of these differences resides with the approaches used in the modeling.

We use a nonequilibrium implementation of extended dynamical mean-field theory in combination with a noncrossing approximation impurity solver to study the effect of dynamical screening in photoexcited Mott insulators. The insertion of doublons and holes adds low-energy screening modes and leads to a reduction of the Mott gap. The coupling to low-energy bosonic modes furthermore opens new relaxation channels and significantly speeds up the thermalization process. We also investigate the effect of the energy distribution of the photo doped carriers on the screening.

We utilize the variational method to study the Kondo screening of a $\text{spin}-1/2$ magnetic impurity in a three-dimensional (3D) Weyl semimetal with two Weyl nodes along the $k_z$ axis. The model reduces to a 3D Dirac semimetal when the separation of the two Weyl nodes vanishes. When the chemical potential lies at the nodal point, $\mu =0$, the impurity spin is screened only if the coupling between the impurity and the conduction electron exceeds a critical value. For finite but small $\mu$, the impurity spin is weakly bound due to the low density of states, which is proportional to ${\mu }^2$, contrary to that in a 2D Dirac metal such as graphene and 2D helical metal, where the density of states is proportional to $\left|\mu \right|$. The spin-spin correlation function $J_{uv}\left(\mathbf{r}\right)$ between the spin $v$ component of the magnetic impurity at the origin and the spin $u$ component of a conduction electron at spatial point $\mathbf{r}$ is found to be strongly anisotropic due to the spin-orbit coupling, and it decays in the power law. The main difference of the Kondo screening in 3D Weyl semimetals and in Dirac semimetals is in the spin $x\left(y\right)$ component of the correlation function in the spatial direction of the $z$ axis.

The edge states which are observed on a linear edge of graphene may also persist on a curved edge. We calculate the elastic transport scattering cross section on a graphene nanohole supporting the edge states. Resonant peaks in the gate voltage dependence of conductivity of graphene with such nanoholes are obtained. Position and height of the resonances are determined by the localization depth of the quasibound edge states, and width by their lifetime. The scattering amplitude near the resonant energies has a strong valley asymmetry. We evaluate the effect of moderate edge rippling, inhomogeneity of boundary parameter along the edge, and Coulomb effects (charged nanohole) on the edge states and show that they do not affect the presence of the resonances but can substantially influence their position, height, and width. The local density of states near the nanohole also demonstrates a resonant dependence on gate voltage.

Helical Luttinger liquids, appearing at the edge of two-dimensional topological insulators, represent a new paradigm of one-dimensional systems, where peculiar quantum phenomena can be investigated. Motivated by recent experiments on charge fractionalization, we propose a setup based on helical Luttinger liquids that allows one to time-resolve, in addition to charge fractionalization, also spin-charge separation and pure spin fractionalization. This is due to the combined presence of spin-momentum locking and interactions. We show that electric time-resolved measurements can reveal both charge and spin properties, avoiding the need of magnetic materials. Although challenging, the proposed setup could be achieved with present-day technologies, promoting helical liquids as interesting playgrounds to explore the effects of interactions in one dimension.

Angle-resolved photoemission spectroscopy is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictide systems. We find the scattering rate for any given band to depend linearly on energy but to be independent of the control parameter. We demonstrate that the linear energy dependence gives rise to a weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. The resulting small effective Fermi energy favors a BCS [J. Bardeen et al., Phys. Rev. 108, 1175 (1957)] –Bose-Einstein [S. N. Bose, Z. Phys. 26, 178 (1924)] crossover state in the superconducting phase.

Based on recent developments in the first-principles theory of flexoelectricity, I generalize the concept of absolute deformation potential to arbitrary nonpiezoelectric insulators and deformation fields. To demonstrate this formalism, I calculate the response of the band edges of ${\mathrm{SrTiO}}_3$ to both dynamic (sound waves) and static (bending) mechanical loads, respectively, at the bulk level and in a slab geometry. The results have important implications for the understanding of strain-gradient-related phenomena in crystalline insulators, formally unifying the description of band-structure and electrostatic effects.

Transfer-matrix methods are used for a tight-binding description of electron transport in graphenelike geometries in the presence of spin-orbit couplings. Application of finite-size scaling and phenomenological renormalization techniques shows that, for strong enough spin-orbit interactions and increasing on-site disorder, this system undergoes a metal-insulator transition characterized by the exponents $\nu =2.71\left(8\right), \eta =0.174\left(2\right)$. We show how one can extract information regarding spin polarization decay with distance from an injection edge, from the evolution of wave-function amplitudes in the transfer-matrix approach. For (relatively weak) spin-orbit coupling intensity $\mu$, we obtain that the characteristic length ${\mathrm{\Lambda }}_s$ for spin-polarization decay behaves as ${\mathrm{\Lambda }}_s\propto {\mu }^{-2}$.

In this work, we propose an accurate first-principles approach to calculate the transient photoabsorption spectrum measured in pump-and-probe experiments. We formulate a condition of adiabaticity and thoroughly analyze the simplifications brought about by the fulfillment of this condition in the nonequilibrium Green's function (NEGF) framework. Starting from the Kadanoff-Baym equations, we derive a nonequilibrium Bethe-Salpeter equation (BSE) for the response function that can be implemented in most of the already existing ab initio codes. In addition, the adiabatic approximation is benchmarked against full NEGF simulations in simple model Hamiltonians, even under extreme, nonadiabatic conditions in which it is expected to fail. We find that the nonequilibrium BSE is very robust and captures important spectral features in a wide range of experimental configurations.

We calculate the energy deposition by very short laser pulses in ${\mathrm{SiO}}_2$ ($\alpha$-quartz) with a view to establishing systematics for predicting damage and nanoparticle production. The theoretical framework is time-dependent density functional theory, implemented by the real-time method in a multiscale representation. For the most realistic simulations we employ a meta-GGA Kohn-Sham potential similar to that of Becke and Johnson. We find that the deposited energy in the medium can be accurately modeled as a function of the local electromagnetic pulse fluence. The energy-deposition function can in turn be quite well fitted to the strong-field Keldysh formula for a range of intensities from below the melting threshold to well beyond the ablation threshold. We find reasonable agreement between the damage threshold and the energy required to melt the substrate. Also, the depth of the ablated crater at higher energies is fairly well reproduced assuming that the material ablated with the energy exceeds that required to convert it to an atomic fluid. However, the calculated ablation threshold is higher than experiment, suggesting a nonthermal mechanism for the surface ablation.

We investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities in both single layer and Bernal stacked bilayer graphene, finding a number of striking anomalies in the temperature dependence of this interaction. In undoped single layer graphene the strength of the RKKY interaction for substitutional impurities anomalously increases upon increasing temperature, an effect that persists up to and beyond room temperature. For impurities intercalated in the Bernal stacked bilayer and a doping that places the chemical potential near the antibonding band edge, a qualitative change of the RKKY interaction with temperature occurs: a low-temperature oscillatory interaction develops into a high-temperature antiferromagnetic coupling, accompanied by an overall increase of the interaction strength. The origin of the temperature anomalies can be traced back to specific features of the density of states: the vanishing density of states at the apex of the Dirac cone in single layer graphene, and the “kink” in the density of states at the antibonding band edge in the case of the Bernal bilayer.

By means of ab initio molecular dynamics calculations, the thermal evolution of vibrational properties in $\alpha$-U is studied at low temperature. The phase transition undergone by this material around 50 K was previously studied extensively using ab initio calculations in the framework of the linear response at 0 K. Although these previous efforts capture successfully the complexity of the experimental phonon spectrum at room temperature, in particular the soft-phonon mode and its pressure dependence, they fail to reproduce the transition to the charge-density-wave state at ambient pressure as a function of temperature. In the present work, by going beyond the quasiharmonic approximation and taking into account the temperature effects explicitly, we are able to reproduce the behavior of both phonon spectrum and elastic constants of U-$\alpha$ as a function of temperature.

We demonstrate that superconductors that break time-reversal symmetry can exhibit thermoelectric properties, which are entirely different from the Ginzburg mechanism. As an example, we show that in the $s+is$ superconducting state there is a reversible contribution to thermally induced supercurrent, whose direction is not invariant under time-reversal operation. Moreover in contrast to Ginzburg mechanism it has a singular behavior near the time-reversal symmetry breaking phase transition. The effect can be used to confirm or rule out the $s+is$ state, which is widely expected to be realized in pnictide compounds ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ and stoichiometric LiFeAs.

Motivated by recent synthesis of the hyperhoneycomb material $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$, we study the dynamical structure factor (DSF) of the corresponding 3D Kitaev quantum spin-liquid (QSL), whose fractionalized degrees of freedom are Majorana fermions and emergent flux loops. The properties of this 3D model are known to differ in important ways from those of its 2D counterpart—it has a finite-temperature phase transition, as well as distinct features in the Raman response. We show, however, that the qualitative behavior of the DSF is broadly dimension-independent. Characteristics of the 3D DSF include a response gap even in the gapless QSL phase and an energy dependence deriving from the Majorana fermion density of states. Since the majority of the response is from states containing a single Majorana excitation, our results suggest inelastic neutron scattering as the spectroscopy of choice to illuminate the physics of Majorana fermions in Kitaev QSLs.