Theoretical evidences for enhanced superconducting transition temperature of CaSi${}_2$ in a high-pressure 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 $T_{\mathrm{c}}$ is expected in the AlB${}_2$ structure than the trigonal structures which are known so far.

6 pages and 11 figures