DOPING EFFECTS ON THE ELECTRONIC-STRUCTURE OF CUO2 PLANES

We have exploited angle-resolved photoelectron spectroscopy to study c hanges in the quasiparticle excitations of CuO2 planes as a function o f the carrier density, in optimally doped and underdoped Bi2Sr2CaCuO8x and in Sr2CuO2Cl2. We have observed large changes in the E versus k dispersion of the quasiparticle bands, and in the topology of the Ferm i surface away from optimal doping. We have also found that the carrie r density has a nontrivial effect on the symmetry properties of these states.