Quantum Fluctuations and Anomalous Insulating Phase in Disordered Two-Dimensional Superconductors

本文研究了二维系统中由无序和场驱动的超导体-绝缘体（SI）转变。对于薄铟膜，我们发现<xyC>由霍尔电阻R_确定的临界场B_<xy>总是大于<xxC>由纵向电阻R_确定的临界场B_<xx>。表示为B_<xxC>&lt;B&lt;B_<xyC>的场区对应于一个不寻常的绝缘区，其中R_<xx>发散并且<xy>在零温度下R_很小或为零。由于该区域变得更加明显，因为膜变得更脏和/或更薄，我们提出了一种可能性，即该绝缘区域对应于玻色玻璃绝缘体，其中在2D中增强的量子波动起着至关重要的作用。我们还观察到非晶Mo_xSi<1-x>对于绝缘膜，磁电阻始终为正，而对于超导膜，在T&lt;0.1K时，磁电阻随B的增加出现一个特殊的峰，随后又随B的增加而减小，这表明在场驱动的SI跃迁的绝缘侧存在定域的库珀对.我们还研究了厚（3D）铟膜中的涡旋玻璃（VG）跃迁.在B= 0.1T的直流线性电阻率的起始点附近，我们可以从交流电阻率相位的温度和频率依赖性清楚地确定VG转变温度。在有限的相变临界区内，电阻率随电流和频率变化的等温线塌缩到与高温超导体相似的普适标度函数上。这些结果为铟膜中的VG跃迁和无序II型超导体中VG跃迁的普遍性提供了强有力的证据。

We have studied the superconductor-insulator (SI) transition driven by disorder and field for two-dimensional (2D) systems. For thin indium films we have found that the critical field B_<xyC> determined from the Hall resistance R_<xy> is always larger than that B_<xxC> determined from the longitudinal resistance R_<xx>. The field region expressed as B_<xxC><B<B_<xyC> corresponds to an unusual insulating region where R_<xx> diverges and R_<xy> is small or zero at zero temperature. Since this region turns out to be more pronounced as the film becomes dirtier and/or thinner, we propose a possibility that this insulating region corresponds to the Bose-glass insulator where quantum fluctuations enhanced in 2D play an essential role. We have also observed the anomalous insulating region near the SI transition for ultrathin films of amorphous Mo_xSi_<1-x>. For insulating films the magnetoresistance is always positive, while for superconducting films it shows an peculiar peak and a subsequent decrease with increasing B at T<0.1K.This suggests the presence of the localized Cooper pairs on the insulating side of the field-driven SI transition.We have also investigated the presence of the vortex-glass (VG) transition in thick (3D) indium films. Near the onset of the dc linear resistivity in B=0.1T,we can clearly determine the VG transition temperature from the temperature and frequency dependence of the phase of the ac resistivity. Within a limited critical regime about the phase transition, the isotherms of the current-and frequency-dependent resistivity collapse onto the universal scaling functions, which well resemble those found in high-T_C superconductors. These results offer strong evidence for the VG transition in the indium film and the universality of the VG transition in disordered type-II superconductors.