INTRASUBLATTICE HOPPING IN THE EXTENDED T-J MODEL AND T-C(MAX) IN THECUPRATES

It is shown that BCS-like scenarios for high-T-c superconductivity bas ed on the extended t-J model yield a characteristic prediction for T-c (max), the critical temperature at optimum doping. A pivotal role is p layed by t(-) equivalent to t' - 2t '' (t' is the next-nearest-neighbo r hopping, t '' is the third-neighbor hopping). Because t(-) determine s the shape of the Fermi surface and the nature of saddle points, it s trongly affects T-c(max). Owing to symmetry, structural differences ou tside the CuO2 planes are described exclusively by t(-). This explains semiquantitatively the actual variation of T-c(max) among the various cuprates, with estimated parameters that are consistent with the obse rved Fermi surfaces.