# High-energy spin fluctuation in low-$T_{\rm c}$ iron-based superconductor LaFePO$_{0.9}$

## TL;DR

This study reveals high-energy spin fluctuations in LaFePO$_{0.9}$, a low-$T_c$ iron-based superconductor, providing evidence that supports spin-fluctuation-mediated pairing despite previous absence of low-energy spin fluctuation observations.

## Contribution

The paper demonstrates the presence of high-energy spin fluctuations in LaFePO$_{0.9}$, resolving a long-standing puzzle about its superconducting pairing mechanism.

## Key findings

- High-energy spin fluctuations observed at 30-50 meV.
- Spin fluctuations have comparable intensity to optimally doped LaFeAs(O,F).
- Line-node symmetry explained by spin-fluctuation-mediated pairing.

## Abstract

Spin fluctuations are widely believed to play an important role in the superconducting mechanisms of unconventional high-temperature superconductors. Spin fluctuations have been observed in iron-based superconductors as well. However, in some iron-based superconductors such as LaFePO$_{0.9}$, they have not been observed by inelastic neutron scattering (INS). LaFePO$_{0.9}$ is an iron-based superconductor with a low superconducting transition temperature ($T_{\rm c}$= 5 K), where line nodes are observed in the superconducting gap function. The line-node symmetry typically originates from sign reversal of the order parameter in spin-fluctuation-mediated superconductivity. This contradiction has been a long-standing mystery of this superconductor. Herein, spin fluctuations were found at high energies such as 30$-$50 meV with comparable intensities to an optimally doped LaFeAs(O,F). Based on this finding, the line-node symmetry can be explained naturally as spin-fluctuation-mediated superconductivity.