Coexistence of two order parameters and a pseudogaplike feature in the iron-based superconductorLaFeAsO1−xFx

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides ${\text{REFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ ($\text{RE}=\text{rare}$ earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, ${\ensuremath{\Delta}}_{1}=2.8--4.6\text{ }\text{meV}$, which scales with the local ${T}_{c}$ of the contact; a larger unconventional OP that gives conductance peaks at 9.8--12 meV, apparently closes below ${T}_{c}$ and decreases on increasing the ${T}_{c}$ of the contact; a pseudogaplike feature (i.e., a depression in the conductance around zero bias) that survives in the normal state up to ${T}^{\ensuremath{\ast}}\ensuremath{\sim}140\text{ }\text{K}$ (close to the N\'eel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$.