Low-Field Electrodynamics of High-T sub c Superconductors--Theory and Experiment

The penetration of magnetic fields into ceramic HTSC has been theoretically and experimentally investigated for fields below the lower critical field of the grains. In the framework of a critical state theory in the low-field electrodynamics of a random Josephson medium, expressions for the harmonics of the induction B(t) as a function of external parameters were derived for two asymptotic cases--the case of the weak field and the case of the thin sample. The behavior of the magnetic induction B was investigated in ac and dc fields in a wide range of temperature and field amplitudes. Rather good agreement between experimental and theoretical results has been found. From the experimental results carried out on YBCO samples, the values of a number of quantities characterizing the low-field electrodynamics of HTSC and their temperature dependences and an explicit expression for the critical current density j sub c (H,T) = j sub 0 (T)H exp 2 sub 0 (T)/\H exp 2 + H exp 2 sub 0 (T)\ were derived.