Quasiparticle Mean Free Path in YB${\mathrm{a}}_2$C${\mathrm{u}}_3$${\mathrm{O}}_7$ Measured by the Thermal Hall Conductivity

Asymmetric scattering of quasiparticles by pinned vortices in YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_7$ produces an unusually large thermal Hall conductivity ${\kappa }_{\mathrm{xy}}$ in the mixed state. In high fields, the ${\kappa }_{\mathrm{xy}}$ vs field profile displays strong curvature, which allows the zero-field mean free path $l_0$ of the quasiparticles to be determined. We find that $l_0$ undergoes a remarkable increase from 90 Å near 90 K to 2500 Å at 15 K. The scattering rate changes by a factor of 3 between 90 and 75 K. We discuss implications for the scattering mechanism. From ${\kappa }_{\mathrm{xy}}$ and $l_0$, we obtain a new estimate of the thermal conductivity of in-plane quasiparticles.