Modification of the superconducting order parameter Delta(k) by long-range interactions

It is demonstrated that the inclusion of long-range intersite interactions qualitatively modifies the dependence of a superconducting gap on quasimomentum for both s- and d-symmetry types. In particular, the order parameter of a superconducting phase with the symmetry type depends on two amplitudes and has the form Delta(k) = Delta(1)(cosk(x)-cosk(y)) + Delta(2)(cos2k(x)-cos2k(y)). In this case, the theoretical dependence of the critical temperature on the degree of doping agrees with the experimental dependence. (C) 2003 MAIK Nauka/Interperiodica.