THE LOW MAGNETIC-FIELD PENETRATION INTO CERAMIC HTSC - LOW-FIELD ELECTRODYNAMICS

The paper is devoted for the theoretical and experimental study of the picture of the low magnetic field penetration into HTSC ceramics, the field being much smaller than the critical one of grains. Within the critical state theory for the low-field electrodynamics of the random Josephson medium the expressions are obtained for the harmonics of the induction B(t) as a function of the external parameters in two asymptotic cases of a low field and thin sample. In particular, for a thin sample the harmonics are shown to be an oscillation function of the ac field amplitude. The behavior of the linear and nonlinear susceptibility in the ac field, hysteresis loops and dc magnetization curve are studied experimentally in a wide temperature and field range. A satisfactory agreement in the experimental and theoretical results is outlined. A number of the magnitudes characterizing the low-field electrodynamics in HTSC and their temperature dependences are obtained. The explicit expression for the function J(c)(H, T) = = j0H0(2)(T)/(H-2 + H0(2)(T)) is determined in the experiment.