Electronic structure, electron-phonon coupling and superconductivity of isotypic noncentrosymmetric crystals Li2Pd3B and Li2Pt3B

Electronic structure of recently discovered isotypic ternary borides Li2Pd3B and Li2Pt3B, with noncentrosymmetric crystal structures, is studied with a view to understanding their superconducting properties. Estimates of the Fermi surface averaged electron-phonon matrix element and Hopfield parameter are obtained in the rigid ion approximation of Gaspari and Gyorffy [G.D. Gaspari, B.L. Gyorffy, Phys. Rev. Lett. 28 (1972) 801]. The contribution of the lithium atoms to the electron-phonon coupling is found to be negligible, while both boron and palladium atoms contribute equally strongly to the Hopfield parameter. There is a significant transfer of charge from lithium, almost the entire valence charge, to the B-Pd(Pt) complex. The electronic structure and superconducting properties of Li2Pd3B, thus, can be understood from the viewpoint of the compound being composed of a connected array of B-Pd tetrahedra decoupled from the backbone of Li atoms, which are connected by relatively short bonds. Our results suggest that conventional s-wave electron-phonon interaction without explicit consideration of SO coupling can explain qualitatively the observed Tc in Li2Pd3B. However, such an approach is likely to fail to describe superconductivity in Li2Pt3B.