Possibility of critical field enhancement due to field penetration in high-Tc sponges and thin films

Magnetic susceptibility measurements of a sample of sintered high- T_c ceramic superconductor of nominal composition Y ₁Ba₂Cu₃O_7-y were conducted as a function of temperature from liquid-He temperatures to T _c. The fitted form of the susceptibility temperature dependence yielded a sample-particle size that was only a few times larger than the field-penetration depth. The particle size was much less than the grain size and commensurate with the thickness of the optical twins. The results of the enhancement studies are also discussed in the light of C.P. Bean´s (1964) early experiments on Pb sponges (in this case λ≫ particle size) which exhibited spectacular enhancements of H_c in association with flux trapping at or between the Pb filaments. It is predicted that it should be possible, using presently available film-deposition techniques, to produce high-T_c films possessing severalfold enhancements of H_c1 beyond the bulk value, and that, as with the Pb sponges, the magnetization loops, even when taken within what passes for the Meissner state in such materials, will be hysteretic