Absence of residual quasiparticle conductivity in the underdoped cuprate   YBa2Cu4O8
  We report here measurements of the in-plane thermal conductivity K(T) of the underdoped cuprate YBa2Cu4O8 (Y124) below 1K. K(T) is shown to follow a simple, phononic T^3 dependence at the lowest temperatures for both current directions, with a negligible linear, quasiparticle contribution. This observation is in marked contrast with behavior reported in optimally doped cuprates, and implies that extended zero-energy (or low energy) low-energy quasiparticles are absent in YBa2Cu4O8 at low temperatures. 
N.E. Hussey, S. Nakamae, K. Behnia, H. Takagi, C. Urano, S. Adachi and   S. Tajima