The light reflection oscillations resulting from the coherent scattering of polariton states and propagating coherent acoustic phonons (CAPs) in a ZnO single crystal are studied by using a two-color femtosecond pump-probe technique. The oscillation frequency manifested in the photoinduced reflectance shows a nonlinear dependence on the probe-beam wave vector owing to the variation of the refractive index near the band gap. The oscillation amplitude is affected by the refractive index modulation derived from the CAP-detuning polariton states. The larger oscillation for shorter probe-beam wavelengths is consistent with the stronger scattering between the CAP and excitonlike polariton states closer to the band gap.

We consider the interplay of disorder and interactions upon the gapless surface states of 3D topological superconductors. The combination of topology and superconducting order inverts the action of time-reversal symmetry, so that extrinsic time-reversal invariant surface perturbations appear only as “pseudomagnetic” fields (Abelian and non-Abelian vector potentials, which couple to spin and valley currents). The main effect of disorder is to induce multifractal scaling in surface state wave functions. These critically delocalized, yet strongly inhomogeneous states renormalize interaction matrix elements relative to the clean system. We compute the enhancement or suppression of interaction scaling dimensions due to the disorder exactly, using conformal field theory. We determine the conditions under which interactions remain irrelevant in the presence of disorder for symmetry classes AIII and DIII. In the limit of large topological winding numbers (many surface valleys), we show that the effective field theory takes the form of a Finkel'stein nonlinear sigma model, augmented by the Wess-Zumino-Novikov-Witten term. The sigma model incorporates interaction effects to all orders and provides a framework for a controlled perturbative expansion; the inverse spin or thermal conductance is the small parameter. For class DIII, we show that interactions are always irrelevant, while in class AIII, there is a finite window of stability, controlled by the disorder. Outside of this window, we identify new interaction-stabilized fixed points.

We investigate quantum transport through a quantum dot connected to source and drain leads and side coupled to a topological superconducting nanowire (Kitaev chain) sustaining Majorana end modes. Using a recursive Green's-function approach, we determine the local density of states of the system and find that the end Majorana mode of the wire leaks into the dot, thus, emerging as a unique dot level pinned to the Fermi energy ${\varepsilon }_F$ of the leads. Surprisingly, this resonance pinning, resembling, in this sense, a “Kondo resonance,” occurs even when the gate-controlled dot level ${\varepsilon }_{\mathrm{dot}}\left(V_g\right)$ is far above or far below ${\varepsilon }_F$. The calculated conductance $G$ of the dot exhibits an unambiguous signature for the Majorana end mode of the wire: In essence, an off-resonance dot [${\varepsilon }_{\mathrm{dot}}\left(V_g\right)\ne {\varepsilon }_F$], which should have $G=0$, shows, instead, a conductance $e^2/2h$ over a wide range of $V_g$ due to this pinned dot mode. Interestingly, this pinning effect only occurs when the dot level is coupled to a Majorana mode; ordinary fermionic modes (e.g., disorder) in the wire simply split and broaden (if a continuum) the dot level. We discuss experimental scenarios to probe Majorana modes in wires via these leaked/pinned dot modes.

We present switching field distributions of spin-transfer-assisted magnetization reversal in perpendicularly magnetized Co/Ni multilayer spin-valve nanopillars at room temperature. Switching field measurements of the Co/Ni free layer of spin-valve nanopillars with a 50 nm$\times$300 nm ellipse cross section were conducted as a function of current. The validity of a model that assumes a spin-current-dependent effective barrier for thermally activated reversal is tested by measuring switching field distributions under applied direct currents. We show that the switching field distributions deviate significantly from the double exponential shape predicted by the effective barrier model, beginning at applied currents as low as half of the zero field critical current. Barrier heights extracted from switching field distributions for currents below this threshold are a monotonic function of the current. However, the thermally induced switching model breaks down for currents exceeding the critical threshold.

We analyze the structure of the pairing interaction and superconducting gap in LiFeAs by decomposing the pairing interaction for various $k_z$ cuts into $s$- and $d$-wave components and by studying the leading superconducting instabilities. We use the ten-orbital tight-binding model, derived from ab initio LDA calculations with hopping parameters extracted from the fit to ARPES experiments. We find that the pairing interaction almost decouples between two subsets; one consists of the outer hole pocket and two electron pockets, which are quasi-2D and are made largely out of the $d_{xy}$ orbital, and the other consists of the two inner hole pockets, which are quasi-3D and are made mostly out of $d_{xz}$ and $d_{yz}$ orbitals. Furthermore, the bare interpocket and intrapocket interactions within each subset are nearly equal. In this situation, small changes in the intrapocket and interpocket interactions due to renormalizations by high-energy fermions give rise to a variety of different gap structures. We focus on $s$-wave pairing which, as experiments show, is the most likely pairing symmetry in LiFeAs. We find four different configurations of the $s$-wave gap immediately below $T_c$: one in which the superconducting gap changes sign between two inner hole pockets and between the outer hole pocket and two electron pockets, one in which the gap changes sign between two electron pockets and three hole pockets, one in which the gap on the outer hole pocket differs in sign from the gaps on the other four pockets, and one in which the gaps on two inner hole pockets have one sign and the gaps on the outer hole pockets and on electron pockets have different sign. Different $s$-wave gap configurations emerge depending on whether the renormalized interactions increase attraction within each subset or increase the coupling between particular components of the two subsets. We discuss the phase diagram and experimental probes to determine the structure of the superconducting gap in LiFeAs. We argue that the state with opposite sign of the gaps on the two inner hole pockets has the best overlap with ARPES data. We also argue that at low $T$, the system may enter into a “mixed” $s+is$ state, in which the phases of the gaps on different pockets differ by less than $\pi$ and time-reversal symmetry is spontaneously broken.

We consider the optical conductivity of a clean two-dimensional metal near a quantum spin-density-wave transition. Critical magnetic fluctuations are known to destroy fermionic coherence at “hot spots” of the Fermi surface but coherent quasiparticles survive in the rest of the Fermi surface. A large part of the Fermi surface is not really “cold” but rather “lukewarm” in a sense that coherent quasiparticles in that part survive but are strongly renormalized compared to the noninteracting case. We discuss the self-energy of lukewarm fermions and their contribution to the optical conductivity $\sigma \left(\Omega \right)$, focusing specifically on scattering off composite bosons made of two critical magnetic fluctuations. Recent study [S. A. Hartnoll et al., Phys. Rev. B 84, 125115 (2011)] found that composite scattering gives the strongest contribution to the self-energy of lukewarm fermions and suggested that this may give rise to a non-Fermi-liquid behavior of the optical conductivity at the lowest frequencies. We show that the most singular term in the conductivity coming from self-energy insertions into the conductivity bubble ${\sigma }^{\prime }\left(\Omega \right)\propto {ln}^3\Omega /{\Omega }^{1/3}$ is canceled out by the vertex-correction and Aslamazov-Larkin diagrams. However, the cancellation does not hold beyond logarithmic accuracy, and the remaining conductivity still diverges as $1/{\Omega }^{1/3}$. We further argue that the $1/{\Omega }^{1/3}$ behavior holds only at asymptotically low frequencies, well inside the frequency range affected by superconductivity. At larger $\Omega$, up to frequencies above the Fermi energy, ${\sigma }^{\prime }\left(\Omega \right)$ scales as $1/\Omega$, which is reminiscent of the behavior observed in the superconducting cuprates.

Solar cells based on a light absorbing layer of the organometal halide perovskite ${\mathrm{CH}}_3{\mathrm{NH}}_3$PbI${}_3$ have recently surpassed $15\%$ conversion efficiency, though how these materials work remains largely unknown. We analyze the electronic structure and optical properties within the quasiparticle self-consistent GW approximation. While this compound bears some similarity to conventional $sp$ semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: Band gaps are much larger than what is predicted by the local-density approximation (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addition, spin-orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The average hole mass is small, which partially accounts for the long diffusion lengths observed. The surface ionization potential (work function) is calculated to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO${}_2$ and Au electrical contacts.

A two-dimensional spin-directed ${\mathbb{Z}}_2^{}$ network model is constructed that describes the combined effects of dimerization and disorder for the surface states of a weak three-dimensional ${\mathbb{Z}}_2^{}$ topological insulator. The network model consists of helical edge states of two-dimensional layers of ${\mathbb{Z}}_2^{}$ topological insulators which are coupled by time-reversal-symmetric interlayer tunneling. It is argued that, without dimerization of interlayer couplings, the network model has no insulating phase for any disorder strength. However, a sufficiently strong dimerization induces a transition from a metallic phase to an insulating phase. The critical exponent $\nu$ for the diverging localization length at metal-insulator transition points is obtained by finite-size scaling analysis of numerical data from simulations of this network model. It is shown that the phase transition belongs to the two-dimensional symplectic universality class of Anderson transition.

We define a class of insulators with gapless surface states protected from localization due to the statistical properties of a disordered ensemble, namely, due to the ensemble's invariance under a certain symmetry. We show that these insulators are topological, and are protected by a ${\mathbb{Z}}_2$ invariant. Finally, we prove that every topological insulator gives rise to an infinite number of classes of statistical topological insulators in higher dimensions. Our conclusions are confirmed by numerical simulations.

We demonstrate that the bandwidth of pulsed electrically detected magnetic resonance can be increased to at least 80 MHz using a radio-frequency-reflectometry detection scheme. Using this technique, we measure coherent spin oscillations in real time during a resonant microwave pulse. We find that the observed signal is in quantitative agreement with simulations based on rate equations modeling the recombination dynamics of the spin system under study. The increased bandwidth opens the way to electrically study faster spin-dependent recombination processes, e.g., in direct semiconductors which so far have almost exclusively been studied by optically detected magnetic resonance.

The role of disorder in the field of three-dimensional time-reversal-invariant topological insulators has become an active field of research recently. However, the computation of ${\mathbb{Z}}_2$ invariants for large, disordered systems still poses a considerable challenge. In this paper, we apply and extend a recently proposed method based on the scattering matrix approach, which allows the study of large systems at reasonable computational effort with few-channel leads. By computing the ${\mathbb{Z}}_2$ invariant directly for the disordered topological Anderson insulator, we unambiguously identify the topological nature of this phase without resorting to its connection with the clean case. We are able to efficiently compute the ${\mathbb{Z}}_2$ phase diagram in the mass-disorder plane. The topological phase boundaries are found to be well described by the self-consistent Born approximation, both for vanishing and finite chemical potentials.

Spin-polarized electron scattering experiments on different metal-organic and metal-carbon interfaces are performed. A completely unexpected behavior of the spin-motion angles as well as of related quantities as a function of the organic layer or carbon coverage is observed. In fact, by deposition of organic molecules or carbon onto ferromagnetic as well as nonmagnetic metal surfaces in the submonolayer thickness range, the electron reflection amplitude, i.e., both the reflectivity and the reflection phase, become spin independent. Our findings show that this behavior is a very general phenomenon which is independent of the electron energy and the choice of the metal as well as of the organic molecules and thus does not depend on the choice of the specific interface.

The Cr${}^{5+}$ ions in Sr${}_3$Cr${}_2$O${}_8$ constitute a strongly correlated spin-1/2 dimer system. Experiments show that the collective singlet-triplet excitations in this system are well defined in the zero-temperature limit, but, when heated, the inelastic neutron scattering peaks decrease rapidly in intensity and acquire a nonzero line width. When including the fluctuations to leading order in $1/z$, where $z$ is the coordination number, the diagrammatic high-density expansion is found to offer an accurate description of the singlet-triplet excitations. The theory explains not only the temperature dependencies of the intensities and line widths, but also that strong correlation causes the lineshapes to become asymmetric at temperatures comparable to the excitation energies.

Scanning Hall probe microscopy has been used to search for spontaneous fields at the well-defined edges of large mesoscopic disks, etched into the $ab$ surface of very high quality single crystal ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$. Such fields are predicted to exist at locations of broken translational symmetry as a consequence of the proposed two-component spin-triplet chiral order parameter $\widehat{d}={\Delta }_0(k_x\pm i{k}_y)\widehat{z}$. We find no evidence for such fields and impose an upper limit of $\pm$2.5 mG on their magnitude. We do, however, observe an abrupt apparent loss of strong bulk pinning and a change in the screening behavior above $H\sim 25$ Oe. At high fields ($H>25$ Oe) pronounced magnetic screening by the disks is very well described by a model containing only strong edge currents, and bulk critical currents do not appear to play a significant role. Our results are discussed in terms of relevant theoretical predictions.

We calculate the efficiency of infrared optical spin injection in single-layer graphene with Rashba spin-orbit coupling and for the in-plane magnetic field. The injection rate in the photon frequency range corresponding to the Rashba splitting is shown to be proportional to the ratio of the Zeeman and Rashba splittings. As a result, a large spin polarization can be controllably achieved for experimentally available values of the spin-orbit coupling and in magnetic fields below 10 T, without using ferromagnetic contacts.

The generalized Langevin equation (GLE) has been recently suggested to simulate the time evolution of classical solid and molecular systems when considering general nonequilibrium processes. In this approach, a part of the whole system (an open system), which interacts and exchanges energy with its dissipative environment, is studied. Because the GLE is derived by projecting out exactly the harmonic environment, the coupling to it is realistic, while the equations of motion are non-Markovian. Although the GLE formalism has already found promising applications, e.g., in nanotribology and as a powerful thermostat for equilibration in classical molecular dynamics simulations, efficient algorithms to solve the GLE for realistic memory kernels are highly nontrivial, especially if the memory kernels decay nonexponentially. This is due to the fact that one has to generate a colored noise and take account of the memory effects in a consistent manner. In this paper, we present a simple, yet efficient, algorithm for solving the GLE for practical memory kernels and we demonstrate its capability for the exactly solvable case of a harmonic oscillator coupled to a Debye bath.

We analyze the pairing instabilities for fermions on hexagonal lattices (both honeycomb and triangular ones) in a wide range of fermionic densities ranging from van Hove density at which a single large Fermi surface splits into two disconnected Fermi pockets, to a density at which disconnected pockets shrink to Fermi points (half-filling for a honeycomb lattice and full filling for a triangular lattice). We argue that for a generic doping in this range, superconductivity at weak coupling is of Kohn-Luttinger type, and, due to the presence of electronic interactions beyond on-site repulsion, is a threshold phenomenon, with superconductivity emerging only if the attraction generated by the Kohn-Luttinger mechanism exceeds the bare repulsion in some channel. For disconnected Fermi pockets, we predict that Kohn-Luttinger superconductivity, if it occurs, is likely to be $f$ wave. While the Kohn-Luttinger analysis is adequate over most of the doping range, a more sophisticated analysis is needed near van Hove doping. We treat van Hove doping using a parquet renormalization group, the equations for which we derive and analyze. Near this doping level, superconductivity is a universal phenomenon, arising from any choice of bare repulsive interactions. The strongest pairing instability is into a chiral $d$-wave state ($d+id$). At a truly weak coupling, the strongest competitor is a spin-density-wave instability, however, $d$-wave superconductivity still wins. Moreover, the feedback of the spin density fluctuations into the Cooper channel significantly enhances the critical temperature over the estimates of the Kohn-Luttinger theory. We analyze renormalization group equations at stronger couplings and find that the main competitor to $d$-wave superconductivity away from weak coupling is actually ferromagnetism. We also discuss the effect of the edge fermions and show that they are unimportant in the asymptotic weak coupling limit, but may give rise to, e.g., a charge-density-wave order at moderate coupling strengths.

We report a sudden reversal in the pressure dependence of $T_{\mathrm{c}}$ in the iron-based superconductor CsFe${}_2$As${}_2$, similar to that discovered recently in KFe${}_2$As${}_2$ [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe${}_2$As${}_2$, we observe no change in the Hall coefficient at $T\to 0$, again ruling out a Lifshitz transition across the critical pressure $P_{\mathrm{c}}$. We interpret the $T_{\mathrm{c}}$ reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and we investigate which parameters control this transition. Comparing samples of different residual resistivity ${\rho }_0$, we find that a sixfold increase in impurity scattering does not shift $P_{\mathrm{c}}$. From a study of x-ray diffraction on KFe${}_2$As${}_2$ under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of the various lattice parameters suggests that $P_{\mathrm{c}}$ should be significantly higher in CsFe${}_2$As${}_2$ than in KFe${}_2$As${}_2$, but we find on the contrary that $P_{\mathrm{c}}$ is lower in CsFe${}_2$As${}_2$, indicating that other factors control $T_{\mathrm{c}}$. Resistivity measurements under pressure reveal a change of regime across $P_{\mathrm{c}}$, suggesting a possible link between inelastic scattering and pairing symmetry.

We study the problem of evolution of a density pulse of one-dimensional interacting fermions with a nonlinear single-particle spectrum. We show that, despite the non-Fermi-liquid nature of the problem, nonequilibrium phenomena can be described in terms of a kinetic equation for certain quasiparticles related to the original fermions by a nonlinear transformation which decouples the left- and right-moving excitations. Employing this approach, we investigate the kinetics of the phase-space distribution of the quasiparticles and thus determine the time evolution of the density pulse. This allows us to explore a crossover from the essentially free-fermion evolution for weak or short-range interaction to hydrodynamics emerging in the case of sufficiently strong, long-range interaction.

We calculate the conductance of a two-dimensional bilayer with inverted electron-hole bands to study the sensitivity of the quantum spin Hall insulator (with helical edge conduction) to the combination of electrostatic disorder and a perpendicular magnetic field. The characteristic breakdown field for helical edge conduction splits into two fields with increasing disorder, a field $B_c$ for the transition into a quantum Hall insulator (supporting chiral edge conduction) and a smaller field $B_c^{\prime }$ for the transition to bulk conduction in a quasimetallic regime. The spatial separation of the inverted bands, typical for broken-gap InAs/GaSb quantum wells, is essential for the magnetic-field-induced bulk conduction—there is no such regime in HgTe quantum wells.