Transport in Ionic Liquid Gated and Superconducting Nanostructures

Abstract

Electrons, when subjected to low temperatures and to very short length scales, exhibit an array of unique quantum mechanical properties. In order to access this regime, one needs cryogenic measurement and nanofabrication techniques. Both experiments in this thesis, which were performed on nanoscale devices, used a 3He cryostat to access low temperatures, and electron beam lithography to fabricate the devices. The first experiment focused on electron tunneling and transport in ionic liquid gated narrow channels (nanowires) of strontium titanate while the second experiment focused on transport in superconducting aluminum nanowires and magnetic field tuned reentrant superconductivity. Measurements of the current-voltage (I-V) characteristics of ionic liquid gated nanometer scale channels of strontium titanate have been carried out. At low gate voltages, the I-V characteristics exhibited a large voltage threshold for conduction and a nonlinear power law behavior at all temperatures measured. The source-drain current of these nanowires scaled as a power law of the difference between the source-drain voltage and the threshold voltage. The scaling behavior of the I-V characteristic is reminiscent of collective electronic transport through an array of quantum dots. At large gate voltages, the narrow channel acts as a quasi-1D wire whose conductance follows Landauer's formula for multichannel transport. Reenterant superconductivity in quasi-one dimensional superconductors, through the application of a magnetic field, is a counter-intuitive phenomenon. It was not until recently that a microscopic mechanism describing the phenomenon was developed in which superconductivity and phase slip driven dissipation coexist in a non-equilibrium state. Here we present new results on magnetic field induced reentrance to superconductivity in quasi-1D aluminum nanowires for in-plane magnetic fields both transverse to and longitudinal along the wire axis. Measurements in the transverse field configuration result in an abrupt transition into the superconducting state as well as quantized behavior related to the flux quantum in an area determined by the product of the distance between voltage probes and the film thickness. These results are different from those found for the case of a perpendicular field, suggesting that different mechanism may be involved.