| Budget Amount *help |
¥9,700,000 (Direct Cost: ¥9,700,000)
Fiscal Year 2001: ¥4,700,000 (Direct Cost: ¥4,700,000)
Fiscal Year 2000: ¥5,000,000 (Direct Cost: ¥5,000,000)
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| Research Abstract |
To understand the superconductor-insulator transition (SIT) in two dimensions (2D) thoroughly, studies from the viewpoint of vortex states are indispensable. Theoretically, the 2D SIT corresponds to a 2D vortex-glass transition (VGT). However, experimental verification for the 2D VGT is difficult, because it occurs only at zero temperature (T=0). Therefore, systematic studies for thick (3D) films as well as thin (2D) films are necessary. We have first measured the transport properties for thick (100,300 nm) films of amorphous Mo_xSi_<1-x> at low T (>0.04 K) in various fields B. Critical vortex dynamics associated with the second-order transition has been observed in ac complex resistivities. This is the first evidence for the VGT in the homogeneously disordered low-T_c superconductors with microscopic pinning centers. We have found that the VGT persists down to T〜0.1T_<c0> up to B〜0.9B_<c2>(0), where T_<c0> and B_<c2>(0) are the mean-field transition temperature and upper critical fiel
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d at T=0, respectively. AtT→0 the VGT line B_g(T) is nearly independent of T and extrapolates to a field below B_<c2>(0), indicative of the presence of a T=0 quantum-vortex-liquid (QVL) phase in the region B_g(0)<B<B_<c2>(0). The width of the QVL phase is wider for a more disordered film, implying that the QVL phase is driven by quantum fluctuations, which are enhanced with increasing disorder.
For thin (4 nm) films we have observed both the disorder-driven and field-driven SIT's at T=0. Above the critical field B_c for the field-driven SIT, an anomalous negative magnetoresistance (MR) suggesting the presence of the localized Cooper pairs has been observed. The negative MR is not observed for thicker films or in fields applied parallel to the film surface. This is consistent with a view of the 2D VG-Bose-glass transition where the existence of mobile vortices, as well as localized Cooper pairs, is expected in B>B_c. We note, however, that alternative explanations for the data may be possible until we obtain direct proof for the vortices in the region. Less
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