NMR INVESTIGATION OF LOW-ENERGY EXCITATIONS IN HIGH T-C SUPERCONDUCTORS

We review the NMR results in the normal state (T > T-c) of superconducting YBa2Cu3O6+x single crystals as a function of oxygen hole doping. We show that the characteristic energy Gamma = pi omega chi'/chi'' for low-energy excitations can be deduced both from NMR (for antiferromagnetic Q(AF) and zero wave vector), and from inelastic neutron scattering for Q(AF) only. In underdoped (dTc/dx > 0) samples Gamma(0) is found to be constant down to T-c, while Gamma(Q(AF)) starts to increase below similar to 150 K, i.e., well above T-c, reflecting the opening of the pseudogap for magnetic excitations at Q(AF). In overdoped samples (dT(c)/dx < 0) there is only the superconducting gap opening at T-c, supporting the phase diagram based on the spin-charge separation.