ORIGIN OF THE EXTENDED VAN-HOVE REGION IN CUPRATE SUPERCONDUCTORS

We study the evolution of the quasiparticle dispersion with doping for the two-dimensional t-t'-t''-J model using the exact diagonalization technique and present a consistent theoretical analysis. Both calculat ions show that the flat region around (pi, 0) does not exist at half f illing, however, it appears in the underdoped regime and remains almos t unchanged upon additional doping. The enhancement of the incoherent motion of holes induced especially by the t' and t'' terms accounts fo r the formation of the extended Van Hove region. Our results are in ex cellent agreement with the recent photoemission experiments on Bi2Sr2C a1-xDyxCu2O8+delta [Marshall et al., Phys. Rev. Lett. 76, 4841 (1996)] . [S0031-9007(98)07129-4].