We report measurements of the magnetic-field dependence of the resistive transition in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$ for several crystals with transition temperatures in the range 15 K≤${\mathit{T}}_{\mathit{c}}$≤62 K. 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 critical field (${\mathit{H}}_{\mathit{c}2}$) 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.

• Received 23 December 1993

DOI:https://doi.org/10.1103/PhysRevB.49.13243