Low- and zero-energy quasiparticle heat transport in high-T_c superconductors

A review of our recent measurements on subkelvin thermal conductivity in single crystals of high-T_c cuprates is presented. At optimal doping, both Bi₂Sr₂CaCu₂O₈ and YBa₂Cu₃O₇ exhibit a finite linear term in the temperature-dependence of their thermal conductivity. Moreover, in the former compound the size of the linear term does not change after the introduction of disorder by electron irradiation. Both these observations are in agreement with the standard theory of heat transport by nodal quasiparticles in d-wave superconductors. Surprisingly in the two underdoped systems studied here, the linear term is either too small (La_1-xSR_xCuO₄, x = 0.8) or absent (YBa₂Cu₄O₈). The results point out the limits of our current theoretical understanding of transport by low-energy quasiparticles in the underdoped regime.