The BCS-like gap in superconductor SmFeAsO_0.85F_0.15

TL;DR

This study reports the observation of a BCS-like superconducting gap in SmFeAsO_0.85F_0.15, revealing a nodeless, isotropic gap consistent with conventional BCS theory, contrasting with cuprate behaviors.

Contribution

First direct measurement of a single, BCS-like superconducting gap in SmFeAsO_0.85F_0.15 using Andreev spectroscopy, providing insights into pairing mechanisms in oxypnictide superconductors.

Findings

Superconducting gap of 13.34 meV observed.

Gap closes at T_C following BCS temperature dependence.

Results suggest a nodeless, isotropic gap incompatible with cuprate models.

Abstract

Since the discovery of superconductivity in the cuprates two decades ago, it has been firmly established that the CuO_2 plane is consequential for high T_C superconductivity and a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO_(1-x)F_x has recently been discovered but with the conspicuous lacking of the CuO_2 planes, thus raising the tantalizing questions of the different pairing mechanisms in these oxypnictide superconductors. Intimately related to pairing in a superconductor are the superconducting gap, its value, structure, and temperature dependence. Here we report the observation of a single gap in the superconductor SmFeAsO_0.85F_0.15 with T_C = 42 K as measured by Andreev spectroscopy. The gap value of 2Delta = 13.34+/-0.3 meV gives 2Delta/k_BT_C = 3.68, close to the BCS prediction of 3.53. The gap decreases with temperature and vanishes at T_C in a manner consistent with the Bardeen-Cooper-Schrieffer (BCS) prediction but dramatically different from that of the pseudogap behavior in the cuprate superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, t-J model, and the like, originally designed for cuprates.