Field dependence of the resistive transition in Tl2Ba2CuO6+δ

We report measurements of the magnetic field dependence of the resistive transition in Tl2Ba2CuO6+delta for several crystals with transitions 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 transition temperature) shifts monotonically to lower temperature with increasing field with no appreciable change in shape. The temperature dependence of the upper critical field (H-c2) inferred from these measurements does not have the classical BCS form [1]. The samples with higher transition temperatures however do not show such simple behaviour, with the transitions becoming systematically broader in a field as the transition temperature increases. These higher T-c samples develop a knee-like structure in the transition at sufficiently high field. We discuss the similarities and differences between this knee and the sharp drop in resistance from an apparent normal state value seen for the lowest T-c sample, and the interpretation of this feature as a measure of H-c2.