Nonequilibrium Properties of Superconducting-Normal Metal Boundaries

Author: Palmer, David William

Year: 1975

Degree: Dissertation (Ph.D.)

Advisor: Mercereau, James E.

Committee Member: Unknown, Unknown

Option: Physics

DOI: 10.7907/9zt7-fh22

Abstract

Three different nonequilibrium states of superconductivity were experimentally investigated. These experiments included studies of: the electron transport properties across the interface of a single superconductor and a normal metal, electron transport between two weakly coupled superconductors, and electron transport through periodic arrays of hundreds of weakly coupled superconductors. In all of these investigations most emphasis was given to quantum interference effects. All three nonequilibrium states produced time dependent voltages which were investigated both by inducing step structure on the I-V characteristics with external microwave radiation, and by directly measuring the internal oscillating potentials. Josephson's relationship between frequency and voltage, hν = 2eV , was found to be significantly modified in certain well defined circumstances leading to a better understanding of quantum rules for nonequilibrium weak superconductivity,

To produce the structures necessary for these studies, thin films of soft, hard, and compound superconductors were used. These thin films were defined into strongly superconducting, weakly superconducting, and normal regions using highly refined photolithographic and etching techniques to provide boundary definition of less than 250Å, depth determination to within 5Å, and lateral detail resolution of 2000Å.

All results can be described using concepts of macroscopic quantum mechanics and simple equivalent circuit representations for the nonequilibrium electron state. Macroscopic wave function boundary conditions and coupling effects were examined with particular emphasis on characteristic coupling energy, decay lengths, healing time, and thermal constants. The geometric dimensions and equilibrium electronic properties were used to predict the critical current-temperature dependence and the rf responsivity. In some circumstances a single superconductor-normal metal interface was found to show quantum interference effects much as though the superconductor were interfering with itself. Thermal effects, which can dominate at high voltages, were calculated, measured, and designed around so that the time response of the macroscopic quantum state could be investigated down to 10-12sec.

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