Impurity States in D-wave Superconductors

Author(s):D.-C. Chen, D. Rainer and J.A. Sauls
Comments: Chapter in the Proceedings of the Verditz Workshop on Quasiclassical Methods in Superconductvity and Superfluidity, ed. D. Rainer and J.A. Sauls (1998); 9 pages with 3 figures.
The structure of the order parameter and the excitation spectrum are investigated for isolated impurities in d-wave superconductors. Atomic scale impurities, or defects, scatter quasiparticles and lead to local suppression (pair-breaking) near the impurity. The pair-breaking effect arises from the formation of quasiparticle states bound to the impurity. The corresponding reduction in spectral weight in the pair condensate is responsible for pair-breaking. The formation of the bound state is due to multiple Andreev scattering by the combined effects of potential scattering, which leads to changes in momentum of the scattered quasiparticle, and the anisotropy of the d-wave order on the Fermi surface. The spectral weight of the bound state decays exponentially away from the impurity on a length scale ξ* = ℏvf/√(Δ(pf)2 - ε*2) where ε* is the energy of the impurity bound state. The continuum spectrum exhibits Tomasch oscillations due to the interference between Andreev reflected particle- and hole-like quasiparticles.

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