Electronic states near the two-dimensional superconductor-insulator transition and vortex dynamics

从涡旋态的角度对超导体-绝缘体相变(SIT)进行深入的研究，是深入理解二维超导体-绝缘体相变(SIT)的必要条件。理论上，2D SIT对应于2D涡旋-玻璃转变(VGT)。然而，二维VGT的实验验证是困难的，因为它只发生在零温度(T=0)。因此，对厚(3D)膜和薄(2D)膜进行系统的研究是必要的。我们首次测量了非晶态Mo_xSi_1-x&gt；厚膜(100,300 nm)在低温(0.04K)下的输运性质。在交流复电阻率中观察到与二级转变有关的临界涡旋动力学。这是具有微观钉扎中心的均匀无序低T_c超导体中VGT的首次证据。我们发现VGT一直持续到T~0.1T；c0&gt；直到B~0.9B&lt；c2&gt；(0)，其中T_&lt；c0&gt；和B_&lt；c2&gt；(0)是平均场转变温度和上临界场…在T=0时，分别增加了d。ATT→0VGT线B_g(T)几乎与T无关，并且外推到B_&lt；c2&gt；(0)以下的场，这表明在B_g(0)&lt；B&lt；B_&lt；c2&gt；(0)区域中存在T=0的量子涡旋液体(Qvl)相。对于较无序的薄膜，QVL相的宽度较宽，这意味着QVL相是由量子涨落驱动的，量子涨落随着无序度的增加而增强。对于薄膜(4 Nm)，我们在T=0时观察到了无序驱动和场驱动的SIT。在场驱动SIT的临界场B_c以上，观察到了反常的负磁电阻(MR)现象，表明存在局域库珀对。对于较厚的膜或在平行于膜表面施加的磁场中，未观察到负MR。这与二维VG-玻色-玻璃转变的观点是一致的，在该观点中，B&gT；B_c中应该存在移动涡旋和局域Cooper对。然而，我们注意到，在我们获得该区域涡旋的直接证据之前，对该数据的另一种解释是可能的。较少

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 … More 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