Thermal hall conductivity as a probe of gap structure in multiband superconductors: The case of Ba _1-xK _xFe ₂As ₂

The sign and profile of the thermal Hall conductivity κ _xy gives important insights into the gap structure of multiband superconductors. With this perspective, we have investigated κ _xy and the thermal conductivity κ _xx in Ba _1-xK _xFe ₂As ₂ which display large peak anomalies in the superconducting state. The anomalies imply that a large holelike quasiparticle (QP) population exists below the critical temperature T _c. We show that the QP mean free path inferred from κ _xx reproduces the observed anomaly in κ _xy, providing a consistent estimate of a large QP population. Further, we demonstrate that the peak structure and holelike signal are consistent with a theoretical scenario where, despite potentially large gap variations on the electron pockets, the minimal homogeneous gap of the superconducting phase resides at a hole pocket. Implications for probing the gap structure in the broader class of pnictide superconductors are discussed.