Superconductor/Insulator transition in two dimensional amorphous W ultra-thin film

非晶态W薄膜(a-W)将超导电性保持在1.2 nm以下。该材料对研究二维无序系统的超导电性具有一定的参考价值。本研究制备了两种样品--Si/a-W/Si和a-W/Si多层膜，并对其超导特性进行了研究。1.在Si/a-W/Si结构中观察到无序诱导超导体-绝缘体(S/i)转变我们测量了d_W=1-10 nm不同a-W膜厚度的临界温度。在d_W*1 nm处，超导电性消失，方阻R_&lt；sq&gt；与T的关系服从指数(T^&lt；-1/3&gt；)S定律。对于d_W*1.2 nm，超导电性随临界温度T_&lt；Co&gt；与正常R_&lt；Sq&gt；的依赖关系而消失，当R_&lt；Sq&gt；等于量子方阻h/4e^2时，超导电性消失。该系统表现为典型的无序诱导的S/i相变。2.a-W/Si多层膜的Kostelize-Thoughless(KT)相变是第二相变，在此相变中，相的有序参数的热涨落在T_c附近诱发涡旋-反涡对。通过对零场下I-V特性的详细测量，(1)通过改变硅层的厚度研究了KT相变与a-W层之间超导耦合的关系；(2)通过控制双层数，观察到了KT相变处的涡圈激发。3.在Si/a-W/Si结构中观察到了磁致S-I相变。在W厚度为1.2-3 nm的样品中，通过方块电阻的温度关系观察到了场诱导的S-I相变。用Fisher标度定律对实验数据进行了分析，研究了这种转变是否是由超导相涨落引起的涡旋向玻色玻璃转变。

Amorphous W thin film (a-W) keeps a superconductivity down to the thickness of 1.2nm. Then this material is useful on the research of superconductivity of two dimensional disorder systems. In this research, two kinds of samples, Si/a-W/Si and a-W/Si multilayrs, were prepared and their superconducting characteristics were investigated. The following results were produced.1.Observation of disorder induced Superconductor-Insulator (S/I) transition in Si/a-W/Si stuctureWe measured the critical temperatures for various a-W film thickness with d_W=1-10nm. In d_W*1nm, superconductivity disappears and T-dependence of sheet resistance R_<sq> follows to the exp (T^<-1/3>) 's law. In the dependence of critical temperature T_<co> upon normal R_<sq> for d_W*1.2nm, the superconductivity disappears when R_<sq> equals to the quantum sheet resistance h/4e^2. This system indicates a typical disorder induced S/I transition.2.Kostelize-Thoughless (KT) transition in a-W/Si multilayrsKT-transition is the second phase transition where the vortex-antivortex pairs are induced near Tc by the thermal fluctuation of phase in order parameter. Due to the detail measurements of I-V characteristics at zero field, (1) the relation of KT transition and superconducting coupling between a-W layrs was investigated by the change of thickness of Si layr and (2) the vortex loop excitation at KT-transition was observed by controlling the number of bilayrs.3.Observation of magnetic induced S-I transition in Si/a-W/Si structure.In the samples with W-thickness of 1.2-3nm, the field induced S-I transition was observed by the temperature dependence of sheet resistance. Whether such a transition is vortex to Bose glass one caused by the fluctuation of superconducting phase or not was studied by analyzing the data with the Fisher's scaling law.