FIELD-DEPENDENCE OF THE RESISTIVE TRANSITION IN TL2BA2CUO6+DELTA

We report measurements of the magnetic field dependence of the resisti ve transition in Tl2Ba2CuO6+delta for several crystals with transition s in the range 15K less than or equal to T-c less than or equal to 62K . The transition of the most overdoped sample (with the lowest transit ion temperature) shifts monotonically to lower temperature with increa sing field with no appreciable change in shape. The temperature depend ence of the upper critical field (H-c2) inferred from these measuremen ts does not have the classical BCS form [1]. The samples with higher t ransition temperatures however do not show such simple behaviour, with the transitions becoming systematically broader in a field as the tra nsition temperature increases. These higher T-c samples develop a knee -like structure in the transition at sufficiently high field. We discu ss the similarities and differences between this knee and the sharp dr op in resistance from an apparent normal state value seen for the lowe st T-c sample, and the interpretation of this feature as a measure of H-c2.