Coherence Effect of Sign-Reversing s(+-)-wave Cooper Pair State in Heavily Overdoped LaFeAsO-based Superconductor: 75As-Nuclear Quadrupole Resonance

TL;DR

This study investigates the superconducting gap symmetry in heavily overdoped LaFeAsO-based superconductors using 75As-NQR, revealing evidence for an s(+-)-wave state associated with sign-reversing Cooper pairs, especially in regimes with poor Fermi surface nesting.

Contribution

It provides experimental evidence for the s(+-)-wave pairing symmetry in heavily overdoped LaFeAsO superconductors, highlighting the role of interband scattering and Fermi surface nesting.

Findings

Partial recovery of the Hebel-Slichter peak in heavily overdoped regimes.

Suppressed interband scattering due to vanishing hole Fermi surface.

Support for the s(+-)-wave pairing symmetry in LaFeAsO superconductors.

Abstract

We report an 75As-nuclear quadrupole resonance (NQR) study on heavily electron-doped LaFeAsO_1-xF_x(La1111) with Tc=5 K. Nuclear spin relaxation rate (1/T1) measurement has revealed that a Hebel-Slichter (HS) peak partially recovers in heavily electron-overdoped regimes where the nesting condition of hole and electron Fermi surfaces(FSs) becomes significantly worse. This is in contrast to previous results reported in optimally doped La1111 with Tc=28 K where a lack of the HS peak was reported. It indicates that the interband scattering between the hole and electron FSs is strongly suppressed by an almost vanishing hole FS through the heavily electron-overdoping. Our findings strongly suggest that the sign reversal of the gap functions on the different FSs, that is, s(+-)-wave state is realized in La1111 compounds. We remark that interband scattering on well-nested FSs is essential for stabilizing the s(+-)-wave state and enhancing the Tc up to 28 K in LaFeAsO-based superconductors.