Theoretical Evidences for Enhanced Superconducting Transition Temperature of CaSi_2 in a High-Pressure AlB_2 Phase(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi_2 at high pressure. Our simulation shows stability of the AlB_2 structure in a pressure range above 17GPa. 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_c is expected in the AlB_2 structure than the trigonal structures which are known so far.
社団法人日本物理学会の論文
2008-10-15