Tunneling and Transport Studies of Strongly Disordered Ultra-Thin Films of Metals and Superconductors

本实验研究项目涉及超薄超冷凝聚态金属薄膜中超导体-绝缘体量子相变的机理。这种转变可能发生在无序超导薄膜上，这种薄膜薄到每平方的正常电阻为h/4e2数量级，并且类似绝缘体的状态也被磁场促进。该研究项目涉及超薄层在低温基底上的原位蒸发，以及磁输运、STM形貌和光谱的原位测量。目标是建立表现SIT跃迁的宏观输运与STM测量所揭示的微观性质之间的相关性。要解决的一个关键问题是相变附近超导序参量的相位和振幅波动的相对重要性。这个研究项目本质上是跨学科的，通常涉及几个本科生和研究生的活动。这些参与有利于学生为进一步的学习和在工业、政府实验室或学术界的职业生涯做准备。这个实验研究项目涉及极薄金属层在极低温度下的导电性。如果金属是超导体，如铅，通常的行为是在金属状态和超导状态之间的过渡，也就是说，金属薄膜的电阻将在过渡温度以下变为零。然而，当薄膜只有几个原子层厚时，可以观察到完全不同的行为，在绝缘状态和超导状态之间发生转变。这种不寻常的行为是这个项目的重点。实验装置允许在液态氦温度的衬底上真空沉积金属层，即所谓的淬火冷凝，这允许超薄和均匀厚度的薄膜显示感兴趣的效果。实验装置的设置是为了允许在同一装置中对薄膜进行电阻测量，并且在不干扰沉积金属薄膜的情况下进行扫描隧道显微镜地形图和电气测量。温度和磁场也可以变化。这项基础研究的目的是了解这种不寻常的绝缘体-超导体转变的机制。这样的理解将增加对材料、微电子和其他器件中导电的整体理解。这个研究项目本质上是跨学科的，通常涉及几个本科生和研究生的活动。这些参与有利于学生为进一步的学习和在工业、政府实验室或学术界的职业生涯做准备。＊＊＊

w:\awards\awards96\*.doc 9801983 Valles This experimental research project is concerned with the mechanism of the superconductor-insulator quantum phase transition exhibited in ultra-thin quench condensed metal films. Such a transition may occur for a disordered superconducting film so thin that its normal resistance per square is on the order of h/4e2, and the insulator like state is also promoted by a magnetic field. The research project involves in-situ evaporation of ultra thin layers onto cryogenic substrates, and in-situ measurements of magnetotransport, STM topography and spectroscopy. The goals are to establish correlations between the macroscopic transport exhibiting the SIT transition and the microscopic properties as revealed by the STM measurements. A key issue to resolve is the relative importance of phase and amplitude fluctuations of the superconducting order parameter in the vicinity of the transition. This research program is interdisciplinary in nature and has typically involved several undergraduate and graduate students in its activities. These involvements are beneficial in the preparation of students for further study and for careers in industry, government laboratories or academia. %%% This experimental research project is concerned with electrical conduction in extremely thin layers of metal, at very low temperature. If the metal is a superconductor, like lead, the usual behavior is a transition between a metallic state and a superconducting state, that is the resistance of the metal film will go to zero below a transition temperature. However, when the films are only a few atomic layers thick, an entirely different behavior can be observed, in which a transition occurs between an insulating state and a superconducting state. This unusual behavior is the focus of this project. The experimental apparatus allows vacuum deposit of a metal layer onto a liquid-helium- temperature substrate, so called quench condensing, which permits the ultra thin and even-thickness films that show the effect of interest. The experimental apparatus is set up to allow electrical resistance measurements of the film to be made in the same apparatus, and also scanning tunneling microscope topographs and electrical measurements to be made without disturbing the as deposited metal film. The temperature and magnetic field can also be varied. The goal of this basic research is to understand the mechanism of the unusual insulator-superconductor transition. Such understanding will add to the overall understanding of electrical conduction in materials and also in microelectronic and other devices. This research program is interdisciplinary in nature and has typically involved several undergraduate and graduate students in its activities. These involvements are beneficial in the preparation of students for further study and for careers in industry, government laboratories or academia. ***