Thermal conductivity measurements of the energy-gap anisotropy of superconducting LaFePO at low temperatures

The superconducting gap structure of LaFePO (T(c)=7.4 K) is studied by thermal conductivity (kappa) at low temperatures in fields H parallel and perpendicular to the c axis. A clear two-step field dependence of kappa(H) with a characteristic field H(s)(similar to 350 Oe) much lower than the upper critical field H(c2) is observed. In spite of the large anisotropy of H(c2), kappa(H) in both H directions is nearly identical below H(s). Above H(s), kappa(H) grows gradually with H with a convex curvature, followed by a steep increase with strong upward curvature near H(c2). These results indicate multigap superconductivity with active two-dimensional (2D) and passive three-dimensional bands having contrasting gap values. Together with the recent penetration depth results, we suggest that the 2D bands consist of nodal and nodeless ones, consistent with extended s-wave symmetry.