Superconducting Gap Anisotropy and Quasiparticle Interactions: a Doping   Dependent ARPES Study
  Comparing ARPES 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 non-interacting 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 coskx-cosky form, which suggests an increasing importance of long range interactions as the insulator is approached. 
J. Mesot, M. R. Norman, H. Ding, M. Randeria, J. C. Campuzano, A.   Paramekanti, H. M. Fretwell, A. Kaminski, T. Takeuchi, T...