Impurity effects and symmetry of the order parameter in high-temperature oxide superconductors
Author
Bayındır, Mehmet
Advisor
Gedik, Zafer
Date
1997Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Identification of the symmetry of the order parameter (OP) of high-l'c cuprates is
important because it helps to understand possible mechanism that govern the physics
of these materials. However, up to now, there is no consensus on the symmetry
of the OP. On the other hand, nonmagnetic impurity substitutions would test the
symmetry of OP. Present theoretical calculations overestimate suppression of the
critical temperature (by a factor of 2 or more) in comparison to the experimental data.
So far, differences between zinc [Zn) and nickel {Ni) substitutions have not been
well understood. Considering the above arguments, effects of nonmagnetic impurities
on the high-temperature cuprates are investigated by solving the Bogoliulpv-de
Gennes (BdG) equations on two-dimensional square lattice. The critical impurity
concentration is found to be very close to the experimental values. Possibility
of extracting the symmetry of OP from the obtained results is discussed. Finiteranged
impurity potentials and different potential strengths for different impurities
are proposed to explain differences between Zn and Ni substitutions. Finally, it
is concluded that our results are in favor of d-wave symmetry for tetragonal and s+d-wave symmetry for orthorhombic phase, and explain quite well the effects of
nonmagnetic impurity substitutions in the high-7'c oxide superconductors. Beside
these, the physical properties of the high-temperature oxide superconductors, the
BdG equations and effects of nonmagnetic impurities on isotropic and anisotropic
superconductors are reviewed.
Keywords
Superconductivityorder parameter
impurity substitution
hightemperature superconductivity
critical temperature suppression
s-wave
d-wave
Zn or Ni-Cu substitution
d-|-s-wave