Spin fluctuations and Magnetic Field Enhanced Superconductivity in two-dimensional localized system

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We obtained the following results for the competitive effect of spin paramagnetic effect on superconductivity and localization in two-dimensional thin Au-Ge films.(1) Tc-enhancement under parallel magnetic fields is confirmed from the observations of the decrease in resistances in resistive transitions for thin films of AuィイD20.5ィエD2GeィイD20.5ィエD2 in thickness region of about 2nm. For those of AuィイD20.3ィエD2GeィイD20.7ィエD2 and AuィイD20.7ィエD2GeィイD20.3ィエD2, no Tc-enhancement is observed.(2) In field-sweep and -modulation experiments on resistive transition, the low field behavior below lkG is found to be different from those expected from the steady state experiments in (1) due to the vortex dynamics in staggered fields.(3) Typical temperature dependence and the nonlinear behavior are not obsereved because the external fields are not completely excluded. At the lowest temperature of T/Tco=0.7, resistance appears linearly with increasing applied perpendicular field H and its temperature dependence is consistent with the vortex flow resistivity due to thermal excitation. This implies that the vortex-glass transition is absent in two dimensional system.(4) Resistive transitions are investigated under exactly parallel fields by using a Hermholtz magnet to compensate the residual perpendicular fields. Positive magnetoresistances due to localization are quadratically proportional to H// and the superconducting resistive transitions are scaled by T/Tc(H//). The Tc-enhancement evaluated from the field dependence of resistance agree with those obtained in steady field experiments in (1).