Doping evolution of nodal quasiparticles in the cuprate superconductor YBCO via low-temperature thermal conductivity

The thermal conductivity of the cuprate superconductor YBa$_2$Cu$_3$O$_y$ was measured at temperatures down to $T \sim 50$~mK in magnetic fields up to $H=15$~T on high-quality single crystals with a hole doping ranging from $p = 0.08$ to $p = 0.18$. The residual linear term at $T \to 0$, a direct measure of the nodal quasiparticle velocities [1], is tracked as a function of doping, and compared to recent, high-resolution ARPES measurements of the Fermi velocity and gap magnitude as a function of doping, in the related cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ [2]. \\[4pt] [1] D.G. Hawthorn {\it et al.}, Phys. Rev. B {\bf 75}, 104518 (2007). \\[0pt] [2] I.M. Vishik {\it et al.}, Phys. Rev. Lett. {\bf 104}, 207002 (2010).