Impurity-induced gap renormalization in anisotropic superconductors: Mixed-state specific heat of La 2− xSr x(Cu 1− yZn y)O 4 and Y(Ni 1−xPt x) 2B 2C

Abstract

The effect of nonmagnetic impurity on the superconducting gap is examined on La 2− x Sr x (Cu 1− y Zn y )O 4 and Y(Ni 1− x Pt x ) 2B 2C from the T and H dependence of the electronic specific heat C e ( T, H) at low temperatures. In La 2− x Sr x CuO 4, impurity scattering leads to an appearance of a finite density of states (DOS) at the Fermi energy, consistent with d-wave pairing state with nonmagnetic impurity scattering in the unitarity limit. In contrast, nonmagnetic impurity scattering in YNi 2B 2C leads to a smearing of the gap anisotropy and the opening of a gap in the DOS over the whole Fermi surface, though C e ( T, H) for pure compound suggests the presence of anisotropic gap. This indicates YNi 2B 2C is a highly anisotropic s-wave superconductor.