Antiferromagnetic Spin Fluctuation above the Superconducting Dome and the Full-Gaps Superconducting State in LaFeAsO1-xFx Revealed by 75As-Nuclear Quadrupole Resonance

TL;DR

This study reveals that antiferromagnetic spin fluctuations persist above the superconducting dome in LaFeAsO1-xFx and that the superconducting state exhibits full energy gaps, with Tc peaking where spin fluctuations are significant.

Contribution

It demonstrates the coexistence of antiferromagnetic spin fluctuations with superconductivity and confirms the fully-opened energy gaps in LaFeAsO1-xFx using 75As-NQR.

Findings

AFSF persists above TN in the superconducting regime.

Maximum Tc occurs at x=0.06 with significant AFSF.

Energy gaps are fully open as shown by exponential 1/T1 decrease.

Abstract

We report a systematic study by 75As nuclear-quadrupole resonance in LaFeAsO1-xFx. The antiferromagnetic spin fluctuation (AFSF) found above the magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime 0.04 < x < 0.08 where superconductivity sets in. A dome-shaped x-dependence of the superconducting transition temperature Tc is found, with the highest Tc = 27 K at x = 0.06 which is realized under significant AFSF. With increasing x further, the AFSF decreases, and so does Tc. These features resemble closely the cuprates La2-xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1) below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates that the energy gaps are fully-opened. The temperature variation of 1/T1 below Tc is rendered nonexponential for other x by impurity scattering.