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

The superconducting gap structure of LaFePO $({T}_{c}=7.4\text{ }\text{K})$ is studied by thermal conductivity $(\ensuremath{\kappa})$ at low temperatures in fields $H$ parallel and perpendicular to the $c$ axis. A clear two-step field dependence of $\ensuremath{\kappa}(H)$ with a characteristic field ${H}_{s}(\ensuremath{\sim}350\text{ }\text{Oe})$ much lower than the upper critical field ${H}_{c2}$ is observed. In spite of the large anisotropy of ${H}_{c2}$, $\ensuremath{\kappa}(H)$ in both $H$ directions is nearly identical below ${H}_{s}$. Above ${H}_{s}$, $\ensuremath{\kappa}(H)$ 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.