Doping dependence of the superconducting gap in Tl_2Ba_2CuO_{6+delta}   from heat transport
  We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_{6+delta} throughout the overdoped regime. In the T -> 0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap, Delta. We find Delta to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentration of 0.1 hole/Cu. As hole concentration is decreased, the gap-to-Tc ratio increases, showing that the suppression of the superconducting transition temperature Tc (relative to the gap) begins in the overdoped regime. 
D. G. Hawthorn, S. Y. Li, M. Sutherland, Etienne Boaknin, R. W. Hill,   C. Proust, F. Ronning, M. A. Tanatar, Johnpierre Pag...