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

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 Δ(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.

arXiv comment4 pages, 4 color eps figures. New results at 400 mK added; figures and discussion modified accordingly. Phys. Rev. Lett., accepted

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