Low magnetic field dependence of A.C. susceptibility in superconducting YBa 2Cu 3O 7−δ

Abstract

We have performed high resolution low magnetic field dependent ac susceptibility measurements on a single phase superconducting YBa 2Cu 3O 7−δ sample. Below the onset temperature T c, a knee in the susceptibility and a peak in the dissipation curve were observed upon cooling but not upon warming after field sweeps at low temperatures. It is suggested that weak links may be responsible for this response. For the field dependent diamagnetic curves, the position of a major change in slope at very low fields may be identified as the critical field of the weak links, which may be fit by H 1 = H 1(0) [1 − ( T/ T c ) 2]; the position of a small slope change at higher fields may be identified as H c1 for the grain and which may be fit by H c1 = H c1 (0) [1 − ( T/ T c ) 2], where H c1 (0) = 460 Oe.