Quasiparticle Transport in the Vortex State ofYBa2Cu3O6.9

The effect of vortices on quasiparticle transport in cuprate superconductors was investigated by measuring the low temperature thermal conductivity of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$ in magnetic fields up to 8 T. The residual linear term (as $T\to 0$) is found to increase with field, directly reflecting the occupation of extended quasiparticle states. A study for different Zn impurity concentrations reveals good agreement with recent calculations for a $d$-wave superconductor. It also provides a quantitative measure of the gap near the nodes.