Two regimes for effects of surface disorder on the zero-bias conductance peak of tunnel junctions involving d-wave superconductors

Abstract

Impurity-induced quasiparticle bound states on a pair-breaking surface of a d-wave superconductor are theoretically described, taking into account hybridization of impurity- and surface-induced Andreev states. Further a theory for effects of surface disorder (of the thin impurity surface layer) on the low-bias conductance of tunnel junctions is developed. We find a threshold, nc, for the surface-impurity concentration nS, which separates the two regimes for surface-impurity effects on the zero-bias conductance peak (ZBCP). Below the threshold, surface impurities do not broaden the ZBCP, but effectively reduce its weight and generate impurity bands. For low nS impurity bands can be, in principle, resolved experimentally, being centered at energies of bound states induced by an isolated impurity on the surface. For larger nS impurity bands are distorted, move to lower energies and, beginning with the threshold concentration nS=nc, become centered at zero energy. With increasing nS above the threshold, the ZBCP is quickly destroyed in the case of strong scatterers, while it is gradually suppressed and broadens in the presence of weak impurity potentials. More realistic cases, taking into account additional broadening, not related to the surface disorder, are also considered.

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