Nuclear magnetic resonance studies of the high-temperature superconductors in the normal and superconducting states

Nuclear Magnetic Resonance reveals important information about the correlated electronic system in the high temperature superconductors. In the normal state of YBa2Cu4O8 we find two characteristic temperatures at 215K and 500K describing crossovers between different scaling regimes of the antiferromagnetic correlation length. In the superconducting mixed state of YBa2Cu3O7-delta we find evidence for an unconventional field dependence of the density of states at the Fermi level. This field dependence agrees with a recent prediction by Volovik that a superconductor with nodal lines in the superconducting gap can have quasiparticle excitations outside the vortex cores associated with the Doppler shift of the Cooper pairs involved in the screening supercurrent.