Large quasiparticle thermal Hall conductivity in the superconductor $\rm Ba_{1-x}K_xFe_2As_2$*

We have measured the thermal conductivity $\kappa_{xx}$ and thermal Hall conductivity $\kappa_{xy}$ in single-crystal $\rm Ba_{1-x}K_xFe_2As_2$. Below the superconducting transition temperature $T_{c}$ ($\sim$ 30 K), we observe a large peak in the weak-field $\kappa_{xy}$ . A corresponding peak in the zero-field thermal conductivity $\kappa_{xx}$ is also observed. Together, these imply the existence of a large population of hole-like quasiparticles below $T_{c}$ . In magnetic fields $H$ approaching 35 T, the peaks in $\kappa_{xx}$ are strongly suppressed. A fit of the $\kappa_{xx}$ vs. $H$ curves shows that the data are consistent with the scattering of long-lived quasiparticles by vortices. Using these fits, we have extracted estimates of the quasiparticle mean-free-path, and separated the zero-field electronic and phononic terms $\kappa_{e}$ and $\kappa_{ph}$, respectively. We discuss the origin of the large quasiparticle population in terms a strongly anisotropic gap parameter or a gap with nodes.