Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films

TL;DR

This study uses point-contact Andreev-reflection measurements to analyze the superconducting gap in ReFeAsO_{1-x}F_x films, revealing a consistent gap size around 6 meV and discussing spectral features and measurement reliability.

Contribution

It provides the first detailed point-contact AR analysis of ReFeAsO_{1-x}F_x films, highlighting the gap distribution and challenges in detecting multiple gaps.

Findings

Superconducting gap ranges from 5 to 10 meV, averaging around 6 meV.

The temperature dependence fits BCS theory with a reduced gap ratio of about 3.5.

Detection of a second larger gap was inconclusive due to spectral features.

Abstract

Point-contact (PC) Andreev-reflection (AR) measurements of the superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have been carried out. The value of the gap is distributed in the range 2\Delta \simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV. Temperature dependence of the gap \Delta(T) can be fitted well by BCS curve giving reduced gap ratio 2\Delta /kT_c^*\simeq 3.5 (here T_c^* is the critical temperature from the BCS fit). At the same time, an expected second larger gap feature was difficult to resolve distinctly on the AR spectra making determination reliability of the second gap detection questionable. Possible reasons for this and the origin of other features like clear-cut asymmetry in the AR spectra and current regime in PCs are discussed.