DISCONTINUITY IN THE MAGNETIZATION OF SINGLE-CRYSTAL TL2BA2CUO6+DELTAWITH H /AB/

Using a low-field dc SQUID susceptometer, we have measured the zero-fi eld-cooled magnetization M(H) of a single crystal of T1-2201 (T-c = 82 .5K) with the field applied parallel to the CuO2 planes. For field swe eps carried out below T = 50K, a large jump in magnetization is observ ed at H = H-d, close to the field of first flux penetration. It is tho ught that the discontinuity in the magnetization arises from the destr uction of Josephson screening currents between the CuO2 planes at a fi eld close to the lower critical field H-c1.