Preparation of Ultrathin and Granular Superconducting Films, and Studies on Electronic Properties at Very Low Temperatures

我们建造了一个低温蒸发器，使我们能够制备在室温下常规沉积无法获得的干净的超薄(或颗粒状)铟薄膜，并在高达11T的磁场下对其在0.5K以下进行原位测量。利用这些超导薄膜，我们研究了量子涨落和热涨落对相变的影响。我们测量了无序薄膜的霍尔电阻和纵向电阻，发现了一个不寻常的绝缘相，在T*0时，R_&lt；xx&gt；发散，但R_&lt；xy&gt；很小或为零。根据结果，我们提出了一种可能性，即绝缘相对应于玻色-玻璃绝缘体，其中位相以量子力学的方式起伏，库珀对处于局域化状态。为了对玻色玻璃绝缘体做出明确的声明，有必要在更低的温度(-10mK)下进行进一步的实验。研究了不同磁场下铟薄膜的非线性电阻率(I-V特性)。(1)Meissner相(B=0)：薄膜(~lt；14 nm)的I-V特性可用二维KT理论很好地描述。随着薄膜厚度的增加，I-V曲线发生变化，并根据最近考虑3D涡旋循环的理论进行了解释。因此，可以清楚地观察到涡旋从2D到3D的维度交叉。(2)混合态(B*0)：根据三维涡旋玻璃理论分析了厚膜的I-V曲线。我们发现了与场无关的临界指数和标度函数，它们与高T_C超导体的结果相似，表明VG相变在II型超导体中是普遍存在的。对于较薄的薄膜(2D脏超导体)和具有较大颗粒尺寸的厚膜(3D约瑟夫森网络)，有限尺寸效应是可见的，这也与VG图像一致。

We have constructed a cryogenic evaporator which allows us to fabricate clean and ultrathin (or granular) films of indium, which could not be obtained by conventional deposition at room temperatures, and to measure them in situ below 0.5 K under magnetic fields up to 11 T.Using these superconducting films, we have studied effects of quantum fluctuations as well as thermal fluctuations on the phase transition.1. We have measured the Hall resistance as well as the longitudinal resistance for disordered thin indium films and found the unusual insulating phase where R_<xx> diverges but R_<xy> is small or zero at T*0. From the result, we have proposed a possibility that the insulating phase corresponds to the Bose-glass insulator where the phase fluctuates quantum-mechanically and Cooper pairs are localized. In order to make a definitive statement on Bose-glass insulator, further experiments at even lower temperatures (-10mK) are necessary.2. Nonlinear resistivities (I-V characteristics) for indium films have been studied in various magnetic fields B.(1) The Meissner phase (B=0) : The I-V characteristics for thin films (<14nm) are well described by the two-dimensional (2D) KT theory. As the films become thicker, I-V curves are altered and are explained in terms of the recent theory considering 3D vortex loops. Thus, the dimensionality crossover for vortices from 2D to 3D is clearly observed. (2) The mixed state (B*0) : The I-V curves for thick films are analyzed according to the 3D vortex-glass theory. We have found the field-independent critical exponents and the scaling functions, which are similar to those obtained in high T_C superconductors, indicating that the VG transition is universal in type II superconductors. Finite-size effects are visible for thinner films (2D dirty superconductor) as well as for thick films with larger grain sizes (3D Josephson network), which are also consistent with the VG picture.