One-sign order parameter in iron based superconductor

TL;DR

This paper investigates the nature of the superconducting order parameter in iron-based superconductors, providing evidence against spin fluctuation-driven pairing and supporting an orbital fluctuation mechanism.

Contribution

It presents experimental data on the momentum dependence of the energy gap, challenging existing models and proposing orbital fluctuations as the pairing mechanism.

Findings

Energy gap anisotropy rules out spin fluctuation coupling

Results support orbital fluctuations assisted by phonons

Contradicts models predicting sign-changing order parameter

Abstract

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s\pm) order parameter [1-6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7-9] and spin fluctuations are not necessary [7,10], contrary to the models [1-6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. Here we report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity.