Extended dx2-y2-wave superconductivity.: Flat nodes in the gap and the low-temperature asymptotic properties of high-Tc superconductors

Remarkable anisotropic structures have been recently observed in the order parameter Delta(k) of the underdoped superconductor Bi2Sr2CaCu2O8+delta. Such findings are strongly suggestive of deviations from a simple d(x2-y2)-wave picture of high-T-c superconductivity, i.e. Delta(k) similar to cos k(x), - cos k(y). In particular,flatter nodes in Delta(k) are observed along the k(x) = +/- k(y) directions in k-space, than within this simple model for a d-wave gap. We argue that nonlinear corrections in the k-dependence of nk near the nodes introduce new energy scales, which would lead to deviations in the predicted power-law asymptotic behaviour of several measurable quantities, at low or intermediate temperatures. We evaluate such deviations, either analytically or numerically, within the interlayer pair-tunneling model, and within yet another phenomenological model for a d-wave order parameter. We find that such deviations are expected to be of different sign in the two cases. Moreover, the doping dependence of the flatness of the gap near the nodes is also attributable to Fermi surface effects, in addition to possible screening effects modifying the in-plane pairing kernel, as recently proposed.