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 temperatures in the range 15 K less-than-or-equal-to T(c) less-than-o r-equal-to 62 K. The transition of the most overdoped sample (with the lowest transition temperature) shifts monotonically to lower temperat ure with increasing field with no appreciable change in shape. As repo rted previously [A. P. Mackenzie et al., Phys. Rev. Lett. 71, 1238 (19 93)], the behavior of the upper critcal field (H(c2)) inferred from th ese measurements does not, however, have the classical BCS form. The s amples with higher transition temperatures show a gradual and systemat ic crossover to field-broadened transitions, with a kneelike structure developing in sufficiently high fields. Some possible interpretations of these observations are discussed.