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 supercon ducting state in terms of noninteracting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction p arameter which renormalizes the linear T contribution to the superfluid den sity. Furthermore, although consistent with d-wave symmetry, the gap with u nderdoping cannot be fit by the simple cosk(x) - cosk(y) form, which sugges ts an increasing importance of long range interactions as the insulator is approached.