DISCONTINUITY IN THE LOW-FIELD MAGNETIZATION OF SINGLE-CRYSTAL TL2BA2CUO6 WITH H-PARALLEL-TO-AB

The zero-field-cooled magnetization M(H,T) of a single crystal of Tl2B a2CuO6 (T-c=82.5 K) has been measured with the field applied parallel to the CuO2 planes. For field sweeps carried out below T=50 K and temp erature sweeps carried out in applied fields H greater than or equal t o 25 Oe, a large jump in magnetization is observed at H=H-d, close to the field of first flux penetration. The data provide strong evidence that Tl2Ba2CuO6 behaves as a weak-Josephson-coupled layered supercondu ctor, the discontinuity arising from the destruction of Josephson scre ening currents between the planes at a field close to the lower critic al field H-c1. The value of H-d is an order of magnitude lower than th e predicted surface-barrier. penetration field.