Why is d-wave pairing in HTS robust in the presence of impurities?

In the recent theory of strong correlations by [1] - [3] it was shown that by lowering doping concentration a forward peak is developed in the charge scattering channel. Physically, this means that short-range impurity scattering potential is transformed into long-range one. Accordingly, near the optimal doping the nonmagnetic scattering is pronounced in the d-channel and d-wave pairing is robust against defects and impurities. It is reflected in a decrease of the slope of the critical temperature T-c(T-s) at Gamma(s) --> 0 by factor (1 - beta), where beta is the scattering anisotropy parameter and Gamma(s) is an average scattering rate. For large doping, beta is small [1], [2] and d-wave pairing loses its robustness. The theory is generally formulated for the bi-layer model by including: (1) intra-and inter-plane pairing; (2) intra-and inter-plane impurities. (C) 1997 Elsevier Science Ltd.