DIRECT MEASUREMENT OF ELECTRON SPIN-LATTICE RELAXATION-TIME OF PARAMAGNETIC CENTERS AND NONLINEAR MODULATED RESPONSES IN HTSC

The report consists of two parts. 1. The method is described to measure directly the elec tron spin-lattice relaxation time T-1 in high-T-c superconductors (HTSC). The technique is based on registering the oscillating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. The T-1 values of the Cu2+ centers in YBa2Cu3O6+x (x = 0.24-0.9) and Y2BaCuO5 materials are measured and found to be about 1.4 . 10(-9) s independently of x and temperature in the range 77-300 K. Besides much longer relaxation is displayed in degradated malerials. The data obtained can be considered as an argument in favor of the ''green-phase'' origin of the ''high-temperature'' EPR spectra in the YBaCuO HTSCs. 2. Non-linear responses of the microwave (10(10) Hz) and rf (10(5)-10(7) Hz) absorption in HTSC materials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at microwaves the effect is caused by non-linear interaction with shielding supercurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the Josephson weak links and the thermo-assisted nux creep.