Effect of Pressure on Superconducting Ca-intercalated Graphite CaC$_6$

TL;DR

This study investigates how applying pressure up to 16 kbar increases the superconducting transition temperature in CaC$_6$, explained by electron-phonon interactions involving soft phonon modes.

Contribution

It provides first-principles calculations showing pressure enhances $T_c$ via electron-phonon coupling, clarifying the superconducting mechanism in CaC$_6$.

Findings

$T_c$ increases with pressure at +0.4%/kbar.

Soft phonon modes related to Ca vibrations are key to $T_c$ enhancement.

Results support electron-phonon theory for superconductivity in CaC$_6$.

Abstract

The pressure effect on the superconducting transition temperature ($T_c$) of the newly-discovered Ca-intercalated graphite compound CaC$_6$ has been investigated up to $\sim$ 16 kbar. $T_c$ is found to increase under pressure with a large relative ratio $\Delta$$T_c$/$T_c$ of $\approx$ +0.4 %/kbar. Using first-principles calculations, we show that the large and positive effect of pressure on $T_c$ can be explained in the scope of electron-phonon theory due to the presence of a soft phonon branch associated to in-plane vibrations of Ca atoms. Implications of the present findings on the current debate about the superconducting mechanism in graphite intercalation compounds are discussed.