Impurity effects on superconductors and the electron-phonon interaction
Author
Savran, Kerim
Advisor
Kim, Yong-Jihn
Date
2000Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
In this thesis effects of impurities on superconductors and electron-phonon
interactions in metals are studied.
The first part deals with the effect of magnetic impurities on superconductors.
In particular, we focus on the experimental observation that the effect of magnetic
impurities in a superconductor is drastically different depending on whether the
host superconductor is in the crystalline or the amorphous state. Based on the
recent theory of Kim and Overhauser, it is shown that as the disorder in the
system increases, the initial slope of the Tc depression decreases by a factor
when the mean free path I becomes smaller than the BCS coherence length
which is in agreement with experimental findings. Additionally, the transition
temperature Tc for a superconductor, which is in a pure crystalline state, drops
sharply from about 50% of Tco (transition temperature of a pure system) to zero
near the critical impurity concentration. This pure limit behavior was found in
crystalline Cd by Roden and Zimmermeyer.
In the second part, the effect of weak localization on electron-phonon
interactions in metals is investigated. As weak localization leads to the same correction term to both conductivity and electron-phonon coupling constant
A (and Xtr), the temperature dependence of the thermal electrical resistivity
is decreasing as the conductivity is decreasing due to weak localization.
Consequently, the temperature coefficient of resistivity (TCR) decreases, while
t he residual resistivit}' increases. As the coupling constant A approaches zero, only
the residual resistivity part remains and accordingly TCR becomes negative. In
other words, the Mooij rule turned out to be a manifestation of weak localization
correction to the conductivity and the electron-phonon interaction.
Keywords
Superconductivityelectron-phonon interaction
magnetic impurity
weak localization
Mooij rule