Abstract
We study vortex, charge and spin transport in small superconducting tunnel junctions in two aspects, the collective behavior of small Josephson junctions, and the spin accumulation in superconducting single electron transistors. For the first part, we experimentally study the magnetic field-induced superconductor-insulator quantum phase transition in one-dimensional arrays of small Josephson junctions. It is found that the critical magnetic field that separates the two phases corresponds to the onset of Coulomb blockade of Cooper pairs tunneling in the current-voltage characteristics. The resistance data are analyzed in the context of the superfluid-insulator transition in one dimension, and a finite temperature scaling analysis isperformed to extract the critical exponents. The dynamical exponents $z$ are determined to be close to 1, and the correlation length exponents $\nu$ are found to be approximately 0.3 and 0.45 in the two groups of measured samples. We also construct an experimental phase diagram using Josephson coupling-to-charging energy ratio($E_J/E_{CP}$) and dissipation strength. For the second part, we both experimentally and theoretically study the spin accumulation in ferromagnet/superconductor/ferromagnet single electron transistors. The measured superconducting gap as a function of magnetic field reveals a dramatic decrease when the magnetizations of the two leads are in nearly anti-parallel orientations. The effect of suppression increases with increasing source-drain voltage. This phenomena can be account for the spin accumulation, which takes place when the ferromagnetic leads are in anti-parallel alignment, suppressing superconductivity in the central island. A comparison with theoretical calculations, in which the charging effect and gap suppression are self-consistently considered is presented. We also theoretically investigated the spin accumulation under the influence of gate voltage. It is found that the gate can be used to tune polarization of current passing through the ferromagnetic single electron transistor when spin accumulates. Processes that weaken this phenomena such as spin-flipping and energy relaxation are modeled and discussed.