Thermal conductivity across the phase diagram of cuprates: Low-energy quasiparticles and doping dependence of the superconducting gap

Heat transport in the cuprate superconductors ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ and ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ was measured at low temperatures as a function of doping. A residual linear term ${\ensuremath{\kappa}}_{0}/T$ is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from ${\ensuremath{\kappa}}_{0}/T$ is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component $(\mathrm{ix})$ appearing at a putative quantum phase transition and argues against a nonsuperconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.