The superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy

TL;DR

This study investigates the superconducting gaps in FeSe single crystals using soft point-contact Andreev-reflection spectroscopy, revealing two gaps with BCS-like temperature dependence and weak magnetic field effects.

Contribution

The paper provides detailed measurements of the two superconducting gaps in FeSe and analyzes their temperature and magnetic field dependence using an extended two-gap model.

Findings

Two superconducting gaps with average values of 1.8 meV and 1.0 meV.

Gaps follow BCS temperature dependence.

Weak magnetic field influence on gap magnitudes.

Abstract

FeSe single crystals have been studied by soft point-contact Andreev-reflection spectroscopy. Superconducting gap features in the differential resistance dV/dI(V) of point contacts such as a characteristic Andreev-reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing dV/dI within the extended two-gap Blonder-Tinkham-Klapwijk model allows to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field indicating a weak decrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25 point contacts results in the averaged gap values <Delta_L> = 1.8+/-0.4meV and <Delta_S>=1.0+/-0.2 meV and reduced values 2<Delta_L>/kTc=4.2+/-0.9 and 2<Delta_S>/kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent decreasing with both temperature and magnetic field. No signatures in the dV/dI spectra were observed testifying a gapless superconductivity or presence of even smaller gaps.