Coexistence of two order parameters and a pseudogaplike feature in the iron-based superconductor LaFeAsO_(1-x)F_x

TL;DR

This study reveals multiple energy scales, including a pseudogap-like feature, in LaFeAsO_(1-x)F_x, indicating complex, unconventional superconductivity possibly linked to antiferromagnetic spin fluctuations.

Contribution

First direct evidence of three distinct energy scales, including a pseudogap, in LaFeAsO_(1-x)F_x, highlighting the complex nature of its superconductivity.

Findings

Presence of a nodeless superconducting gap (Delta1) that scales with Tc

Observation of an unconventional larger energy scale with peaks at 9.8-12 meV

Detection of a pseudogap-like feature persisting up to 140 K, associated with antiferromagnetic fluctuations

Abstract

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides REFeAsO_(1-x)F_x (RE = rare earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on LaFeAsO_(1-x)F_x gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, Delta1 = 2.8-4.6 meV, which scales with the local Tc of the contact; a larger unconventional OP that gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases on increasing the Tc of the contact; a pseudogaplike feature (i.e. a depression in the conductance around zero bias), that survives in the normal state up to T* ~ 140 K (close to the Neel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations (AF SF) coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in LaFeAsO_(1-x)F_x.