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

我们已经建造了一个低温蒸发器，使我们能够制造干净和超薄（或颗粒）的铟薄膜，这是在室温下无法通过常规沉积获得的，并在0.5 K以下的磁场下原位测量它们高达11 t .使用这些超导薄膜，我们研究了量子涨落和热涨落对相变的影响。我们测量了无序铟薄膜的霍尔电阻和纵向电阻，发现了R_<xx>发散而R_<xy>在T*0时很小或为零的不寻常的绝缘相。根据结果，我们提出了一种可能性，即绝缘相位对应于玻色-玻璃绝缘体，其中相位量子涨落并且库珀对是局域的。为了对玻色玻璃绝缘体做出明确的结论，需要在更低的温度下（-10mK）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)迈斯纳相（B=0）：二维KT理论很好地描述了薄膜（<14nm）的I-V特性。随着薄膜变厚，I-V曲线发生改变，并根据考虑三维涡圈的最新理论进行解释。因此，可以清楚地观察到涡旋从二维到三维的维度交叉。(2)混合态（B*0）：根据三维涡-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，它们与在高T_C超导体中得到的相似，表明VG跃迁在II型超导体中是普遍存在的。对于较薄的薄膜（2D脏超导体）和具有较大晶粒尺寸的厚薄膜（3D Josephson网络），可以看到有限尺寸效应，这也与VG图一致。

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.