MAGNETIC-RESONANCE IN HIGH-TEMPERATURE SUPERCONDUCTORS

Since the discovery of high-temperature superconductors a new microwave absorption has been found which is reviewed here in detail. The microwaves are absorbed in the flux-quantized eigenstates so that the absorption is proportional to the Josephson current and hence it varies as the gap of the superconductor. This absorption is found in the electron paramagnetic resonance, in the parallel pumping and in the ordinary microwave absorption experiments. The flux-quantized fields are found in small domains of the size of 10(-4) cm. A giant moment is found to occur. The necessary theory as well as experiments in YBa2Cu3O7-delta type of compounds are described. The Cu2+ electron paramagnetic resonance gives an anisotropic exchange narrowed line with anisotropic g-values with g-shifts proportional to the susceptibility. The symmetry of the g-value also reflects the symmetry of the superconducting gap. The nuclear magnetic resonance of various nuclei has been reviewed. It is pointed out that the NMR line widths are determined by the vortex-lattice spacing and the London penetration depth. Nuclear relaxation rates measured in the superconducting state give the gap energy of the superconductor. The ratio of the gap energy to the transition temperature found from the Cu-63 nuclear relaxation rate in La1.83Sr0.17CuO4 is in reasonable agreement with the BCS theory confirming the pair structure of superconductors. The phase diagrams of La2-delta-Ba-delta-CuO4 and YBa2Cu3O7-delta have been described.