Theoretical evidences for enhanced superconducting transition temperature of CaSi$_2$ in a high-pressure AlB$_2$ phase

TL;DR

This study uses first-principles calculations to demonstrate that CaSi$_2$ adopts a stable AlB$_2$ structure under high pressure, which is predicted to have a higher superconducting transition temperature due to strong electron-phonon interactions.

Contribution

The paper provides theoretical evidence that the AlB$_2$ phase of CaSi$_2$ is stable above 17 GPa and likely exhibits enhanced superconductivity compared to known structures.

Findings

AlB$_2$ structure stable above 17 GPa

Strong electron-phonon interactions in specific phonon modes

Higher estimated superconducting transition temperature in AlB$_2$ phase

Abstract

By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi$_2$ at high pressure. Our simulation showed stability of the AlB$_2$ structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron-phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB$_2$ structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher $T_{\rm c}$ is expected in the AlB$_2$ structure than the trigonal structures which are known so far.