Superconductor-Insulator Transition in disordered 2D-Superconductor

点击翻译按钮获取中文摘要

In a 2D superconducting system a superconductor-insulator transition occuras at absolute zero as disorderness is increased. We have studied this problem in amorphous Nb ultra-thin films system which are prepared by quecnch-condensation of Nb on a cold Al_2O_3 substarate of liquid helium temperanture. We were able to prepare a-Nb films with thickness from about 3 - 4 * with and the thickness was able to varied in a fine steo of 0.1 *. Furtermore, we have developed a technique to measure electrical resistances of a-Nb thin films in situ in high vacuum and at tempertaures down to 1.6 K.Mean field superconducting transition temperatures (T_<c0>) and Kosterlitz-Thouless transition temperatures (T_<KT>) have been measured with varying film thickness. It has been found that both T_<CO> and T_<KT> decrease with decreasing film thickness, but T_<KT> seems to vanish faster than T_<c0>. This is the first measurement of the dependence of T_<KT> on the thickness of superconducting films. The fast vanishing of T_<KT> means that at absolute zero we have found a new phase where cooper pair exists in the 2D superconducting film, but global phase coherence is absent. This result has been reported in 20 th. Int Conf. on low temperature physics (Eugene, 1993) and reported as a doctor thesis of Atsushi Asamitsu in March of 1994 at Tokyo Institute of Technology. To confirm this result, experiments at temperatures below 1 K are now in progress.