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 measur e directly the elec tron spin-lattice relaxation time T-1 in high-T-c superconductors (HTSC). The technique is based on registering the osci llating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. The T-1 values of the Cu2centers in YBa2Cu3O6+x (x = 0.24-0.9) and Y2BaCuO5 materials are measu red and found to be about 1.4 . 10(-9) s independently of x and temper ature in the range 77-300 K. Besides much longer relaxation is display ed in degradated malerials. The data obtained can be considered as an argument in favor of the ''green-phase'' origin of the ''high-temperat ure'' 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 mater ials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at micro waves the effect is caused by non-linear interaction with shielding su percurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the J osephson weak links and the thermo-assisted nux creep.