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

研究了二维(2D)系统中由无序和场驱动的超导体-绝缘体(SI)相变。对于薄膜，我们发现由霍尔电阻R_&lt；xy&gt；确定的临界场B_&lt；xy C&gt；总是大于由纵向电阻R_&lt；xx&gt；确定的B_&lt；xxc&gt；。表示为B_&lt；xxc&gt；&lt；B&lt；B_&lt；B_&lt；xyC&gt；的场区对应于在零温度下R_&lt；xx&>；发散且R_&lt；xy&>；很小或为零的不寻常绝缘区。由于随着薄膜变得更脏和/或更薄，这个区域变得更加明显，我们提出了一个可能性，即这个绝缘区对应于玻色玻璃绝缘体，在那里二维增强的量子涨落起着至关重要的作用。我们还观察到非晶态MoxSilt；1-x&gt；超薄膜在SI相变附近的反常绝缘区。对于绝缘膜，磁阻始终为正，而对于超导膜，在T&lt；0.1K出现一个奇特的峰值，然后随着B的增加而减小。这表明在场驱动的SI相变的绝缘侧存在局域库珀对。我们还研究了厚(3D)铟薄膜中涡旋-玻璃(VG)相变的存在。在直流线性电阻率B=0.1T的起始点附近，我们可以从交流电阻率相位的温度和频率关系中清楚地确定VG转变温度。在有限的相变临界区内，电流和频率相关的电阻率等温线崩塌到普适标度函数上，这与高T_C超导体中的结果很相似。这些结果为铟薄膜中的VG相变和无序II型超导体中VG相变的普遍性提供了有力的证据。

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.