High-energy spin fluctuation in low-Tc iron-based superconductor LaFePO0.9

Motoyuki Ishikado,Katsuaki Kodama,Ryoichi Kajimoto,Hannu Mutka,Tao Hong,Mitsutaka Nakamura,Shin-Ichi Shamoto

doi:10.1038/s41598-018-33878-x

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 LaFePO0.9, they have not been observed by inelastic neutron scattering (INS). LaFePO0.9 is an iron-based superconductor with a low superconducting transition temperature (Tc = 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.

Highlights

In iron-based superconductors, superconductivity appears in the vicinity of an antiferromagnetic (AF) phase and a structural phase transition from tetragonal to orthorhombic phases
We demonstrate that spin fluctuation has been clearly observed in low-Tc LaFePO0.9 (Tc = 5 K) in the range of 30–50 meV with similar intensities to the optimally doped LaFeAsO0.918F0.082 at the normal state, suggesting the universality of the correlation between line-node symmetry and spin fluctuations
The magnetic resonance mode is expected to be at 2.4 meV in LaFePO0.9 based on the simple linear dependence of the energy on Tc10–12

Summary

Introduction

In iron-based superconductors, superconductivity appears in the vicinity of an antiferromagnetic (AF) phase and a structural phase transition from tetragonal to orthorhombic phases. Neither the magnetic resonance mode nor the spin fluctuation itself have been observed in the low superconducting transition temperature (Tc) iron-based superconductor LaFePO1−y16. It is quite important to obtain dynamical information on the spin fluctuations of LaFePO0.9 in a wide energy range to examine the effects of the spin fluctuation on the superconductivity with line-node gap symmetry. We demonstrate that spin fluctuation has been clearly observed in low-Tc LaFePO0.9 (Tc = 5 K) in the range of 30–50 meV with similar intensities to the optimally doped LaFeAsO0.918F0.082 at the normal state (non-superconducting state), suggesting the universality of the correlation between line-node symmetry and spin fluctuations

Methods

Results

Conclusion