Superconducting Gap Structure of LaFePO Studied by Thermal Conductivity

TL;DR

This study investigates the superconducting gap structure of LaFePO using thermal conductivity measurements, revealing evidence for multigap superconductivity involving both nodal and nodeless bands with extended s-wave symmetry.

Contribution

It provides the first detailed thermal conductivity analysis indicating multigap superconductivity with contrasting gap values in LaFePO, supporting an extended s-wave pairing symmetry.

Findings

Identification of a two-step field dependence of thermal conductivity

Evidence for multigap superconductivity involving 2D and 3D bands

Support for extended s-wave symmetry with nodal and nodeless gaps

Abstract

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 $H_s(\sim 350$ Oe) much lower than the upper critical field $H_{c2}$ is observed. In spite of 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 the multigap superconductivity with active two-dimensional (2D) and passive 3D 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 the extended s-wave symmetry.