Superconductor/Insulator transition in two dimensional amorphous W ultra-thin film

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Amorphous W thin film (a-W) keeps a superconductivity down to the thickness of 1.2nm. Then this material is useful on the research of superconductivity of two dimensional disorder systems. In this research, two kinds of samples, Si/a-W/Si and a-W/Si multilayrs, were prepared and their superconducting characteristics were investigated. The following results were produced.1.Observation of disorder induced Superconductor-Insulator (S/I) transition in Si/a-W/Si stuctureWe measured the critical temperatures for various a-W film thickness with d_W=1-10nm. In d_W*1nm, superconductivity disappears and T-dependence of sheet resistance R_<sq> follows to the exp (T^<-1/3>) 's law. In the dependence of critical temperature T_<co> upon normal R_<sq> for d_W*1.2nm, the superconductivity disappears when R_<sq> equals to the quantum sheet resistance h/4e^2. This system indicates a typical disorder induced S/I transition.2.Kostelize-Thoughless (KT) transition in a-W/Si multilayrsKT-transition is the second phase transition where the vortex-antivortex pairs are induced near Tc by the thermal fluctuation of phase in order parameter. Due to the detail measurements of I-V characteristics at zero field, (1) the relation of KT transition and superconducting coupling between a-W layrs was investigated by the change of thickness of Si layr and (2) the vortex loop excitation at KT-transition was observed by controlling the number of bilayrs.3.Observation of magnetic induced S-I transition in Si/a-W/Si structure.In the samples with W-thickness of 1.2-3nm, the field induced S-I transition was observed by the temperature dependence of sheet resistance. Whether such a transition is vortex to Bose glass one caused by the fluctuation of superconducting phase or not was studied by analyzing the data with the Fisher's scaling law.