Observation of mesoscopic conductance fluctuations in YBaCuO grain boundary Josephson Junctions

Magneto-fluctuations of the normal resistance R_N have been reproducibly observed in high critical temp erature superconductor (HTS) grain boundary junctions, at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line. The Thouless energy appears to be the relevant energy scale. Our findings have significant implications on quasiparticle relaxation and coherent transport in HTS grain boundaries.Comment: Revised version, minor changes. 4 pages, 4 figure

Interaction corrections: temperature and parallel field dependencies of the Lorentz number in two-dimensional disordered metals

The electron-electron interaction corrections to the transport coefficients are calculated for a two-dimensional disordered metal in a parallel magnetic field via the quantum kinetic equation approach. For the thermal transport, three regimes (diffusive, quasiballistic and truly ballistic) can be identified as the temperature increases. For the diffusive and quasiballistic regimes, the Lorentz number dependence on the temperature and on the magnetic field is studied. The electron-electron interactions induce deviations from the Wiedemann-Franz law, whose sign depend on the temperature: at low temperatures the long-range part of the Coulomb interaction gives a positive correction, while at higher temperature the inelastic collisions dominate the negative correction. By applying a parallel field, the Lorentz number becomes a non-monotonic function of field and temperature for all values of the Fermi-liquid interaction parameter in the diffusive regime, while in the quasiballistic case this is true only sufficiently far from the Stoner instability.Comment: 11 pages, 5 figures. Appendix A revised, notes adde

Quantum coherence in a ferromagnetic metal: time-dependent conductance fluctuations

Quantum coherence of electrons in ferromagnetic metals is difficult to assess experimentally. We report the first measurements of time-dependent universal conductance fluctuations in ferromagnetic metal (Ni$_{0.8}$Fe$_{0.2}$) nanostructures as a function of temperature and magnetic field strength and orientation. We find that the cooperon contribution to this quantum correction is suppressed, and that domain wall motion can be a source of coherence-enhanced conductance fluctuations. The fluctuations are more strongly temperature dependent than those in normal metals, hinting that an unusual dephasing mechanism may be at work.Comment: 5 pages, 4 figure

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

Electron-electron interaction corrections to the thermal conductivity in disordered conductors

We evaluate the electron-electron interaction corrections to the electronic thermal conductivity in a disordered conductor in the diffusive regime. We use a diagrammatic many-body method analogous to that of Altshuler and Aronov for the electrical conductivity. We derive results in one, two and three dimensions for both the singlet and triplet channels, and in all cases find that the Wiedemann-Franz law is violated.Comment: 8 pages, 2 figures Typos corrected in formulas (15) and (A.4) and Table 1; discussion of previous work in introduction extended; reference clarifying different definitions of parameter F adde

Manipulating nonequilibrium magnetism through superconductors

Electrostatic control of the magnetization of a normal mesoscopic conductor is analyzed in a hybrid superconductor-normal-superconductor system. This effect stems from the interplay between the non-equilibrium condition in the normal region and the Zeeman splitting of the quasiparticle density of states of the superconductor subjected to a static in-plane magnetic field. Unexpected spin-dependent effects such as magnetization suppression, diamagnetic-like response of the susceptibility as well as spin-polarized current generation are the most remarkable features presented. The impact of scattering events is evaluated and let us show that this effect is compatible with realistic material properties and fabrication techniques.Comment: 5 pages, 4 figure

Anomalous Aharonov-Bohm conductance oscillations from topological insulator surface states

We study transport properties of a topological insulator nanowire when a magnetic field is applied along its length. We predict that with strong surface disorder, a characteristic signature of the band topology is revealed in Aharonov Bohm (AB) oscillations of the conductance. These oscillations have a component with anomalous period $\Phi_0=hc/e$, and with conductance maxima at odd multiples of $\frac12\Phi_0$, i.e. when the AB phase for surface electrons is $\pi$. This is intimately connected to the band topology and a surface curvature induced Berry phase, special to topological insulator surfaces. We discuss similarities and differences from recent experiments on Bi$_2$Se$_3$ nanoribbons, and optimal conditions for observing this effect.Comment: 7 pages, 2 figure