Superconducting gap anisotropy and quasiparticle interactions: A doping dependent photoemission study

Comparing photoemission measurements on Bi2212 with penetration depth data, we show that a description of the nodal excitations of the d-wave superconducting state in terms of noninteracting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction parameter which renormalizes the linear T contribution to the superfluid density. Furthermore, although consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cosk(x) - cosk(y) form, which suggests an increasing importance of long range interactions as the insulator is approached.