Electronic structure, electron–phonon coupling and superconductivity of isotypic noncentrosymmetric crystals Li 2Pd 3B and Li 2Pt 3B

Abstract

Electronic structure of recently discovered isotypic ternary borides Li 2Pd 3B and Li 2Pt 3B, 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 Li 2Pd 3B, 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 T c in Li 2Pd 3B. However, such an approach is likely to fail to describe superconductivity in Li 2Pt 3B.