Evidence of Strong-Coupled Superconductivity in CaC6 from Tunneling Spectroscopy

TL;DR

This study uses tunneling spectroscopy to reveal that CaC6 exhibits strong-coupled superconductivity with larger gaps than previously reported, indicating significant involvement of calcium phonons in its superconducting mechanism.

Contribution

It provides the first direct tunneling evidence of strong-coupled superconductivity in CaC6 with larger superconducting gaps, highlighting the role of calcium phonons.

Findings

Superconducting gap of 2.3 meV observed

Gap size is ~40% larger than earlier reports

Ca phonons are primarily involved in superconductivity

Abstract

Point-contact tunneling on CaC$_6$ crystals reproducibly reveals superconducting gaps, $\Delta$, of 2.3$\pm$0.2 meV which are $\sim$~40% larger than earlier reports. That puts CaC$_6$ into the class of very strong-coupled superconductors since 2$\Delta$/kT$_c\sim$~4.6. Thus soft Ca phonons will be primarily involved in the superconductivity, a conclusion that explains the large Ca isotope effect found recently for CaC$_6$. Consistency among superconductor-insulator-normal metal (SIN), SIS and Andreev reflection (SN) junctions reinforces the intrinsic nature of this result.