标题： $\textit{a}$-MoGe 薄膜中的超导性：厚度减小时相位涨落的影响以及在低频交流激励下涡旋动力学的研究 显示英文标题

标题： Superconductivity in $\textit{a}$-MoGe thin films: effect of phase fluctuations with decreasing thickness and study of vortex dynamics in presence of low-frequency ac excitation

摘要： 在本论文中，我们研究了非晶态钼锗（$\textit{a}$-MoGe）薄膜中超导性的演化。 这项工作可分为两部分。 在第一部分中，我们研究了厚度减小对$\textit{a}$-MoGe 薄膜中超导性抑制的影响。 厚的$\textit{a}$-MoGe 薄膜是一种典型的II型超导体，遵循传统的巴丁-库珀-施里弗（BCS）方程。 传统上认为，由于逐渐失去屏蔽作用，厚度减小会降低有效的吸引配对相互作用，这在$\textit{a}$-MoGe 的大厚度情况下是成立的。 但对于较薄的厚度，一种新的机制出现，其中超流密度被抑制，使超导体容易受到相位涨落的影响。 这被称为玻色子机制，在这种机制下，即使在转变温度以上吸引力配对幅度仍然有限，超导性也会由于超导态相位相干性的丧失而被破坏。 在第二部分中，我们通过低频双线圈互感技术研究了$\textit{a}$-MoGe 薄膜中涡旋的电磁响应。 以前，通过双线圈技术测量的穿透深度被用来确定超流密度。 然而在本工作中，通过借助Coffey和Clem提出的平均场模型分析场内穿透深度数据，我们展示了一种提取涡旋参数（如钉扎恢复力常数或Labusch参数、涡旋晶格拖拽系数以及涡旋热激活运动的钉扎势垒）的程序。 涡旋参数的温度变化表明热涨落起主导作用。 显示英文摘要

摘要： In this thesis, we have studied the evolution of superconductivity in amorphous Molybdenum Germanium ($\textit{a}$-MoGe) thin films. The work can be broken down into two parts. In the first part, we investigate the effect of decreasing thickness on the suppression of superconductivity in $\textit{a}$-MoGe thin films. Thick $\textit{a}$-MoGe thin film is a typical type-II superconductor and follows the conventional Bardeen-Cooper-Schrieffer (BCS) equation. Conventionally, it is believed that decreasing thickness will decrease the effective attractive pairing interaction because of the gradual loss of screening which holds true for large thicknesses in $\textit{a}$-MoGe. But for lower thicknesses, a new mechanism comes into the picture where superfluid density is suppressed making the superconductor vulnerable to phase fluctuations. This is known as the Bosonic mechanism, where superconductivity is destroyed due to the loss of phase coherence of the superconducting state even though the attractive pairing amplitude remains finite above the transition temperature. In the second part, we explore the electromagnetic response of vortices in $\textit{a}$-MoGe thin film by low-frequency two-coil mutual inductance technique. Penetration depth measured from the two-coil technique was earlier used to determine superfluid density. However in the present work, by analyzing the in-field penetration depth data with the help of a mean-field model proposed by Coffey and Clem, we have demonstrated a procedure of extraction of vortex parameters such as pinning restoring force constant or Labusch parameter, vortex lattice drag coefficient and pinning potential barrier for the thermally activated motion of vortices. The temperature variation of vortex parameters suggests the dominant effect of thermal fluctuations.