  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.
