Nematic Skyrmions in Odd-Parity Superconductors.

We study topological excitations in two-component nematic superconductors, with a particular focus on Cu_{x}Bi_{2}Se_{3} as a candidate material. We find that the lowest-energy topological excitations are coreless vortices: a bound state of two spatially separated half-quantum vortices. These objects are nematic Skyrmions, since they are characterized by an additional topological charge. The inter-Skyrmion forces are dipolar in this model, i.e., attractive for certain relative orientations of the Skyrmions, hence forming multi-Skyrmion bound states.

Proximity-Induced π Josephson Junctions in Topological Insulators and Kramers Pairs of Majorana Fermions.

We study two microscopic models of topological insulators in contact with an s-wave superconductor. In the first model the superconductor and the topological insulator are tunnel coupled via a layer of randomly distributed scalar and of randomly oriented spin impurities. Here, we demonstrate that spin-flip tunneling dominates over the spin-conserving one. In the second model the tunnel coupling is realized by a spatially nonuniform array of single-level quantum dots with randomly oriented spins. We find that the tunnel region forms a π junction where the effective order parameter changes sign. Because of the random spin orientation, effectively both models exhibit time-reversal symmetry. The proposed π junctions support topological superconductivity without magnetic fields and can be used to generate and manipulate Kramers pairs of Majorana fermions by gates.

Correlations between Majorana fermions through a superconductor.

We consider a model of ballistic quasi-one-dimensional semiconducting wire with intrinsic spin-orbit interaction placed on the surface of a bulk s-wave superconductor (SC), in the presence of an external magnetic field. This setup has been shown to give rise to a topological superconducting state in the wire, characterized by a pair of Majorana-fermion (MF) bound states formed at the two ends of the wire. Here, we demonstrate that besides the well-known direct-overlap-induced energy splitting, the two MF bound states may hybridize via elastic tunneling processes through virtual quasiparticle states in the SC, giving rise to an additional energy splitting between MF states from the same as well as from different wires.

Role of domain wall fluctuations in non-Fermi-liquid behavior of metamagnets.

In this paper we study the resistivity temperature dependence of a three-dimensional metamagnet near the metamagnetic phase transition point. The phase transition is characterized by a phase separation of regions with high and low magnetization. We show that, in the case of weak pinning, the spin relaxation time of the domain wall, which separates the two phases, is much larger than that of the volume spin fluctuations. This opens a temperature range where resistivity temperature dependence is determined by scattering of conducting electrons by the domain wall fluctuations. We show that it leads to quasi-linear low temperature dependence of resistivity.

Organization of modular networks.

We examine the global organization of heterogeneous equilibrium networks consisting of a number of well-distinguished interconnected parts-"communities" or modules. We develop an analytical approach allowing us to obtain the statistics of connected components and the intervertex distance distribution in these modular networks, and to describe their global organization and structure. In particular, we study the evolution of the intervertex distance distribution with an increasing number of interlinks connecting two infinitely large uncorrelated networks. We demonstrate that even a relatively small number of shortcuts unite the networks into one. In more precise terms, if the number of interlinks is any finite fraction of the total number of connections, then the intervertex distance distribution approaches a delta -function peaked form, and so the network is united.

Niobium sputtered Havar foils for the high-power production of reactive [18F]fluoride by proton irradiation of [18O]H2O targets.

Niobium sputtered Havar entrance foils were used for the production of reactive [(18)F]fluoride by proton irradiation of [(18)O]H(2)O targets under pressurized conditions. The synthesis yield in the routine production of 2-[(18)F]fluoro-2-deoxy-glucose (FDG) was used as an indicative parameter of the reactivity of (18)F. The yield of FDG obtained with (18)F produced in a target with Havar foil was used as a baseline. No statistically significant difference was found in the saturated yields of (18)F when using Havar or Havar-Nb sputtered entrance foils. However, the amount of long-lived radionuclidic impurities decreased more than 10-fold using the Havar-Nb entrance foil. The average decay corrected synthesis yield of FDG, evaluated over a period of more than 2 years, was found to be approximately 5% higher when using a Havar-Nb entrance foil and a marked improvement on the FDG yield consistency was noted. In addition, the frequency of target rebuilding was greatly diminished when using the Nb sputtered entrance foil.

Charge transfer between a superconductor and a hopping insulator.

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron-Cooper pair conversion process time-reversal reflection, where electrons tunnel into a superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. A specific feature of this problem is the interplay between the time-reversal reflection at the interface and transport through the percolation cluster. To allow for this interplay, we have generalized the connectivity criterion of the percolation theory to include surface effects. We show that the time-reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.

Mesoscopic oscillations of the conductance of disordered metallic samples as a function of temperature.

We show theoretically and experimentally that the conductance of small disordered samples exhibits random oscillations as a function of temperature. The amplitude of the oscillations decays as a power law of temperature, and their characteristic period is of the order of the temperature itself.

Signature of the electron-electron interaction in the magnetic-field dependence of nonlinear I-V characteristics in mesoscopic systems.

We show that the nonlinear I-V characteristics of mesoscopic samples with metallic conductivity should contain parts which are linear in the magnetic-field and quadratic in the electric field. These contributions to the current are entirely due to the electron-electron interaction and consequently they are proportional to the electron-electron interaction constant. We also note that both the amplitude and the sign of the nonlinear part of the current exhibit random oscillations as a function of temperature.

7Be(p,gamma)8B astrophysical S factor from precision cross section measurements.

We measured the 7Be(p,gamma)8B cross section from E(c.m.) = 186 to 1200 keV, with a statistical-plus-systematic precision per point of better than +/-5%. All important systematic errors were measured including 8B backscattering losses. We obtain S17(0) = 22.3+/-0.7(expt)+/-0.5(theor) eV b from our data at E(c.m.)< or =300 keV and the theory of Descouvemont and Baye.

Mesoscopic sensitivity of speckles in disordered nonlinear media to changes of the scattering potential

We show that the sensitivity of wave speckle patterns in disordered nonlinear media to changes of scattering potential increases with sample size. For large sizes the sensitivity diverges, which implies that for a given coherent wave incident on a sample there are multiple solutions for the spatial distribution of the wave density. The number of solutions increases exponentially with the sample size.