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

为了更好地理解二维超导体-绝缘体相变，从涡旋态的角度进行研究是必不可少的。理论上，2D SIT对应于2D涡旋玻璃化转变（VGT）。然而，2D 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）膜以及薄（2D）膜的系统研究是必要的。我们首次测量了非晶Mo_xSi_2厚膜（100，300 nm）<1-x>在低温（&gt;0.04 K）下不同磁场B下的输运性质。在交流复涡度中观察到了与二级相变有关的临界涡动力学。这是在具有微观钉扎中心的均匀无序低T_c超导体中存在VGT的第一个证据。我们发现，VGT从T ≤ 0.1 T_<c0>到B ≤ 0.9 B_<c2>（0），T_<c0>和B_<c2>（0）分别为平均场转变温度和上临界场。 关于我们 d在T=0时。当T →0时，VGT线B_g（T）几乎与T无关，并外推到B_（0）以下的场<c2>，表明在B_g（0）&lt;B&lt;B_（0）区域存在T=0的量子涡旋液体（QVL）相<c2>。对于更无序的薄膜，QVL相的宽度更宽，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（4 nm）薄膜，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），表明局域库珀对的存在。对于较厚的膜或平行于膜表面施加的场，没有观察到负MR。这与2D VG玻色玻璃化转变的观点一致，其中在B&gt;B_c中预期存在移动的涡旋以及局部库珀对。然而，我们注意到，在我们获得该区域涡旋的直接证据之前，对这些数据的其他解释是可能的。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