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，使我们能够制造清洁和清洁（或颗粒状）薄膜的铟，这不能通过常规的沉积在室温下获得，并测量他们在原位低于0.5 K下的磁场高达11 T。使用这些超导薄膜，我们研究了量子涨落以及热涨落对相变的影响。我们测量了无序铟薄膜的霍耳电阻和纵向电阻，发现在T*0处出现了R_<xx>发散而R_<xy>很小或为零的反常绝缘相。从结果中，我们提出了一种可能性，绝缘相对应的玻色玻璃绝缘体的相位波动量子力学和库珀对局域化。为了对玻色玻璃绝缘体做出明确的声明，需要在更低的温度（-10mK）下进行进一步的实验。本文研究了铟薄膜在不同磁场B下的非线性电阻率（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曲线发生了变化，并根据最近的三维涡环理论进行了解释。因此，从2D到3D的涡流的维数交叉被清楚地观察到。(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。我们得到了与场无关的临界指数和标度函数，它们与高T_C超导体中的临界指数和标度函数相似，表明VG转变在II型超导体中是普遍存在的。对于较薄的薄膜（2D脏超导体）以及具有较大晶粒尺寸的厚膜（3D约瑟夫森网络），可以看到晶粒尺寸效应，这也与VG图片一致。

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.