BASIC ASPECTS AND MAIN RESULTS OF NMR-NQR SPECTROSCOPIES IN HIGH-TEMPERATURE SUPERCONDUCTORS

After a mention of the structural, magnetic and electronic properties of high-temperature superconductors (HTSC), the basic principles of NM R-NQR experiments in these compounds are presented, emphasizing the ma rked differences and the novel aspects of the latter systems in compar ison with metals and conventional superconductors. It follows a review of NMR-NQR spectra and relaxation rates in two-dimensional quantum an tiferromagnets (particularly La2CuO4) driven towards the superconducti ng state by charge doping. The main results obtained in the normal sta te of HTSC are summarized, while the problems of the spin-gap and of t he superconducting fluctuations are discussed to a certain extent, by including the most recent contributions. An overview is given on the m ain conclusions derived from NMR-NQR experiments in the superconductin g state, A section is devoted to the insights into the vortex lattice and the flux lines motion that have been obtained from NMR line narrow ing, T-1 and echo dephasing. This review deals mostly with three syste ms, La2-xSrxCuO4, YBa2Cu3O6+x and YBa2Cu4O8.