Quasiparticle bound states in normal metal/superconductor structures probed by scanning tunneling microscopy
Tessmer, Stuart Holden
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https://hdl.handle.net/2142/23160
Description
Title
Quasiparticle bound states in normal metal/superconductor structures probed by scanning tunneling microscopy
Author(s)
Tessmer, Stuart Holden
Issue Date
1995
Doctoral Committee Chair(s)
Van Harlingen, Dale J.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Electricity and Magnetism
Physics, Condensed Matter
Language
eng
Abstract
In this work we consider systems in which small normal-metal structures (N) are put into contact with a large superconductor (S), with the goal of spatially characterizing the superconductivity. Confined regions with suppressed superconductivity will support quasiparticle bound states, which can be measured spectroscopically. Using a custom-built cryogenic scanning tunneling microscopy system, we have probed the bound states of an NS system consisting of Au (N) droplets of nanometer dimensions in electrical contact with bulk NbSe$\sb 2$ (S). A quasiparticle bound state was observed even when tunneling directly into the NbSe$\sb 2$, clear evidence for a significant reduction of the superconductivity inside the NbSe$\sb 2$ induced by the proximity of the Au over-layer. By invoking a proximity effect model, we are able to characterize the vertical and lateral variation of the pair potential $\Delta$ inside the superconductor. We find that a severe suppression occurs which is beyond the conventional theory. We believe that this effect arises from the short coherence length of the superconductor, so that the spatial variation of the interaction parameter g becomes important. The profile of $\Delta$ that we extract from our data then combines both the conventional proximity suppression of $\Delta$ and its modulation by the profile of g, representing the first observation of spatial structure of the interaction parameter.
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