A comparative ab initio study of superconductivity in the body centered tetragonal YC2and LaC2

Abstract

Ab initio studies of the electronic band structure and phonon dispersion relations, using the planewave pseudopotential method and the density functional theory, have been made for the superconducting materials YC<inf>2</inf> and LaC<inf>2</inf>. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges between these materials are investigated and discussed. By integrating the Eliashberg spectral function α²F(ω), the average electron-phonon coupling parameter is found to be λ=0.55 for YC<inf>2</inf> and 0.54 for LaC<inf>2</inf>, indicating these to be weak-coupling BCS superconductors. It is established that about 60% of λ is contributed by acoustic phonons in both materials. Using a reasonable value of μ=0.13 for the effective Coulomb repulsion parameter, the superconducting critical temperature T<inf>c</inf> is found to be 3.81K for YC<inf>2</inf> and 2.44K for LaC<inf>2</inf>, in good agreement with values reported from experimental measurements.