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Title:The current-voltage characteristics of linear chains of submicron Josephson tunnel junctions
Author(s):Tien, Joseph Jui
Doctoral Committee Chair(s):Van Harlingen, Dale J.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Electronics and Electrical
Physics, Condensed Matter
Abstract:The recent experimental results on granular superconducting films have suggested an universal sheet resistance threshold of 6.4 k$\Omega$/square, below which all films become superconducting with decreasing temperature, and above which all films become insulating with decreasing temperature. At the critical sheet resistance of 6.4 k$\Omega$/square, the conductance of the granular films exhibited unusual quasi-reentrant behavior with decreasing temperature.
Theories have been proposed to explain the temperature dependence of the conductance of the granular films. Most of the theories modeled the granular film as an ordered two dimensional arrays consisting of resistively shunted Josephson junctions (RSJ), and the inter-grain charging energy E$\sb{\rm c}$ = $Q\sp2$/2C is responsible for the insulating behavior in high resistance films at low temperatures, and the quasi-reentrant behavior in the temperature dependence of the conductance is the result of competition between charging and Josephson effects.
The existence of disorder in granular films changes the nature of conduction at the percolation threshold from a two dimensional one, as in two dimensional arrays, to a one dimensional one, as in linear chains.
If disorder is present at the percolation threshold, the current is carried by exactly one path across the film; a linear chain of small superconducting gains stretching across the film. Therefore, the behavior of the granular film at the percolation threshold is strictly an one dimensional problem, and it can be modeled as a linear chain of Josephson junctions. The purpose of this thesis was to model the granular films as linear chains of Josephson junctions and study the temperature dependence of the dynamic resistance of the chains as functions of different normal state resistances in an attempt to understand the origin of the quasi-reentrant behavior.
We have found that, although the competition between charging and Josephson effects may be one of the factors causing the observed quasi-reentrant behavior at very low temperature, it does not appear to be very important near the superconducting transition, where quasiparticle population may be significant. In addition, computer simulations on model based on competition between quasiparticle and Josephson conduction were able to reproduce most of the observed quasi-reentrant results without charging effects.
The presence of charge traps within the junctions within the chains may cause quasi-periodic switching in the dynamic resistance of the chains, causing the appearance of periodic oscillations in the dynamic resistance as temperature is lowered.
In summary, we believed the quasi-reentrant behavior in the granular films near the superconducting transition was caused by competition between quasiparticle and Josephson conduction, and not related to charging effects. In addition, the presence of charge traps can cause the measured dynamic resistance to fluctuate. If the superconducting transition temperatures of the films were inferred from transport measurements, as it was the case in the data of Goldman et. al., such fluctuations in dynamic resistance would give the appearance of superconducting temperature fluctuations.
Issue Date:1994
Rights Information:Copyright 1994 Tien, Joseph Jui
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9512571
OCLC Identifier:(UMI)AAI9512571

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