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

The superconducting gap structure of LaFePO (Tc =7.4K) is studied by thermal conductivity (κ) at low temperatures in fields H parallel and perpendicular to the c axis. A clear two-step field dependence of κ (H) with a characteristic field Hs (∼350Oe) much lower than the upper critical field Hc2 is observed. In spite of the large anisotropy of Hc2, κ (H) in both H directions is nearly identical below Hs. Above Hs, κ (H) grows gradually with H with a convex curvature, followed by a steep increase with strong upward curvature near Hc2. 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.