Disorder effects in pnictides : a tunneling spectroscopy study

TL;DR

This study investigates the superconducting gaps in various pnictides using tunneling spectroscopy, revealing complex multi-gap structures and supporting the 's extpm' pairing model, with disorder influencing gap characteristics.

Contribution

It provides experimental tunneling spectroscopy evidence for multi-gap superconductivity and supports the 's extpm' pairing symmetry in pnictides, highlighting disorder effects.

Findings

FeSe0.5Te0.5 shows a single ~1 meV gap.

SmFeAsO0.85 and SmFeAsO0.9F0.1 exhibit two gaps at ~4 meV and ~10 meV.

Disorder affects the gap structure, favoring an 's'-wave in FeSe0.5Te0.5.

Abstract

We present the synthesis and the tunneling spectroscopy study of superconducting FeSe0.5Te0.5 (Tc = 14 K), SmFeAsO0.85 (Tc = 45 K) and SmFeAsO0.9F0.1 (Tc = 52 K). The samples were characterized by Rietveld refinement of X-ray diffraction patterns and transport measurements. Tunneling experiments on FeSe0.5Te0.5 revealed a single superconducting gap of ~1 meV in BCS-like tunnelling conductance spectra. In SmFeAsO0.85 and SmFeAsO0.9F0.1 however, more complex spectra were observed characterized by two gap-like structures at ~4 meV and ~10 meV. These spectra are qualitatively understood assuming a two-band superconductor with a 's\pm' order parameter. We show that depending on the sign relation between the pairing amplitudes in the two bands, the interband quasiparticle scattering has a crucial effect on the shape of the tunneling spectra. Detailed analysis of the tunneling spectroscopy data supports the 's\pm' model, but single-gap spectra found in FeSe0.5Te0.5 are more compatible with a disorder-induced 's'-wave gap, due to the Se-Te substitution.