High-energy Spin Fluctuation in Iron-based Superconductor LaFePO<sub>0.9</sub>

Abstract

Spin fluctuations are widely believed to play an important role in the superconducting mechanisms of unconventional high-temperature superconductors, and they have been observed in various iron-based superconductors so far. However, in LaFePO0.9, they have not been observed by inelastic neutron scattering. LaFePO0.9 is an iron-based superconductor with a low superconducting transition temperature (Tc = 5 K), where line nodes are observed in the superconducting order parameter. The line-node symmetry typically originates from sign reversal of the order parameter in spin-fluctuation-mediated superconductivity. This contradiction has been a longstanding mystery of this superconductor. Here, we report spin fluctuations were found at high energies such as 30-50 meV. The line-node symmetry can be explained naturally as spin-fluctuation-mediated superconductivity.