FIELD-DEPENDENCE OF THE RESISTIVE TRANSITION IN TL2BA2CUO6+DELTA

We report measurements of the magnetic-field dependence of the resistive transition in Tl2Ba2CuO6+delta for several crystals with transition temperatures in the range 15 K less-than-or-equal-to T(c) less-than-or-equal-to 62 K. 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. As reported previously [A. P. Mackenzie et al., Phys. Rev. Lett. 71, 1238 (1993)], the behavior of the upper critcal field (H(c2)) inferred from these measurements does not, however, have the classical BCS form. The samples with higher transition temperatures show a gradual and systematic crossover to field-broadened transitions, with a kneelike structure developing in sufficiently high fields. Some possible interpretations of these observations are discussed.