Direct measurement of electron spin-lattice relaxation time of paramagnetic centers and non-linear modulated responses in HTSC

Abstract

The report consists of two parts. 1. The method is described to measure directly the electron spin-lattice relaxation time T t in high-Tr 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 l values of the Cu 2+ centers in YBa2Cu30~+~ (x = 0.24-0.9) and Y2BaCuO5 materials ate measured and found to be about 1.4.10 9 s independemly of x and temperature in the range 77-300 K. Besides much longer relaxation is displayed in degradated materials. The data obtained can be considered as an arglmaent 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 ~o Hz) and rf (10~-107 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 accotmt the Josephson weak links and the thermo-assisted flux creep. Eleetron Spin-Lattiee RelaxationStudying the electron spinqattice relaxation (ESRR) of paramagnetic centers in high-T c superconductors (HTSC) could yield valuable information about the origin of unpaired spins, internal fluctuating fields, phase separation and other properties of these materials. Contrary to NMR, however, little or nothing is known at present of ESRR in HTSC. The point is that the longitudinal and transverse relaxation times T 1 and T 2 of electron spins in high-Tr superconductors are expected to be extremely short (about 10 9s and less). This makes practically impossible to perform substantial saturation of the EPR line with a reasonable level of microwave power; so the standard cw and pulse methods of T t measuring become inapplicable.