Indications of pair electron effect above field induced superconductor-insulator transition in amorphous ultra thin W films with Si/W/Si structure

We present the measurements of the superconductor-insulator transition (SIT) driven by both thickness and magnetic field for the Si/a-W/Si system. The thickness of the W-film was varied over a range from 1 to 5 nm. We found the universal quantum sheet resistance at SIT driven by thickness. Field induced SIT for two samples with 1.6 and 3 nm W thick was investigated in detail. In the insulator side, the evidence of Bose glass state for our homogeneous disordered two dimensional system is inferred by comparing SIT field B_c with the upper critical field at 0 K H_c2(0) estimated from measurements of dH_c2/dT\_Tc and the field dependent magneto-resistance at various temperatures.