Evidence for gap anisotropy in CaC6 from directional point-contact spectroscopy

TL;DR

This study uses directional point-contact spectroscopy and ab-initio calculations to reveal anisotropic superconducting gap features in CaC6, confirming theoretical predictions of gap variation across different crystal directions.

Contribution

First experimental demonstration of gap anisotropy in CaC6 using directional spectroscopy and theoretical modeling.

Findings

Superconducting gap peaks at 1.35 meV along the ab plane.

Superconducting gap peaks at 1.71 meV along the c-axis.

Results agree with recent theoretical predictions of gap anisotropy.

Abstract

We present the first results of directional point-contact spectroscopy in high quality CaC6 samples both along the ab plane and in the c-axis direction. The superconducting order parameter \Delta(0), obtained by fitting the Andreev-reflection (AR) conductance curves at temperatures down to 400 mK with the single-band 3D Blonder-Tinkham-Klapwijk model, presents two different distributions in the two directions of the main current injection, peaked at 1.35 and 1.71 meV, respectively. By ab-initio calculations of the AR conductance spectra, we show that the experimental results are in good agreement with the recent predictions of gap anisotropy in CaC6.