R. Bojko

Anisotropic localization in periodic superconducting networks

We report measurements of the normal-superconducting phase boundary Tc(H) for a series of square wire networks with a range of ratios for the wire widths in the two perpendicular directions. We find indirect evidence for anisotropic localization of the superconducting order parameter through the vanishing of structure in Tc(H) at rational fields, and direct evidence in the field-dependent lowering of the resistive transition for the narrow wires relative to the wide wires. This represents experimental evidence for anisotropic localization in a periodic system in a homogeneous field as originally predicted by Aubry and Andre.

Fabrication of submicron wire networks using electron beam lithography

Abstract We have fabricated and studied superconducting wire networks with various geometrical properties including fractal, periodic, quasi-periodic, and random. We will discuss design and fabrication of these aluminum networks, which consist of nearly one million elements and span a square millimeter. We will present results of several studies of these networks.

Commensurate noise in superconducting networks

Abstract Voltage noise in the presence of DC currents in periodic superconducting Al wire networks exhibits a striking dependence on the applied field which determines commensurability of vortices with the underlying lattice. AC currents suppress the noise. We find analogies with charge-density-wave depinning and use a relative phase-slip model to account for these effects.

Nonlocal paraconductance of small superconducting loops

Abstract The nonlocal current response to an electric field characteristic of the superconducting state has been predicted to be observable in the paraconductive regime above the transition temperature T c over a length scale set by the correlation length for suerconducting fluctuations, ξ(T). A manifestation of this effect has been found in a multi-terminal measurement on small superconducting Al loops.

Nonlocal conductance of small superconducting loops in the paraconductive regime

Abstract Both quasiparticles and superconducting fluctuations contribute to the conductance of superconductors in the paraconductive regime slightly above the transition temperature. The fluctuation contribution consists of the Maki-Thompson correction, which results from the scattering of electrons by a superconducting fluctuation, and the Aslamazov-Larkin contribution which describes the conductivity of a fluctuating electron pair. The latter is nonlocal over a length scale set by the fluctuation correlation length, ξ( T ). The observation of this nonlocality involves a multi-terminal measurement of voltages in the paraconductive regime of a square superconducting loop of characteristic size, L ∼ ξ( T ). The ratio of the voltages measured by two different sets of probes attached to the loop at different positions is found to oscillate as a function of the flux threading the loop. This effect is distinct from phenomena associated with the coherent transport of electrons in mesoscopic structures of a size smaller than the dephasing length, L φ .

Localized order parameter on superconducting wire networks

The rich structure in the magnetic field dependence of the mean field transition temperature of square superconducting wire networks from which bonds have been removed randomly washes out with decreasing bond occupation probability p. These results appear to be associated with localization of the mean field superconducting order parameter on the network.