MEASUREMENTS OF HC1 AND JC IN BI1.95SR1.65LA0.4CUO6-HALL PROBE(DELTA WITH A MICRO)

Using a Hall-probe technique, the magnetisation of Bi1.95Sr1.65La0.4Cu O6+delta is measured, yielding values of the field of first penetratio n H(p)(T) along with the critical current density J(c)(T) in low field s. Due to reduced surface barriers and a very small critical current d ensity H(p) is close to H(cl). From this, and published values of xi(a b), an upper-bound estimate of the in-plane penetration depth lambda(a b) = 10 000 +/- 1000 angstrom is obtained. Also a reduction of the Mei ssner screening is observed for 9 < T< T(c) = 13 K and is discussed. A quantitative comparison of these results is made with some experiment s and recent theories which predict vortex fluctuations in such materi als.