Electronic structure, electron-phonon coupling and superconductivity of isotypic noncentrosymmetric crystals Li$_2$Pd$_3$B and Li$_2$Pt$_3$B

TL;DR

This study investigates the electronic structure and electron-phonon interactions in noncentrosymmetric superconductors Li$_2$Pd$_3$B and Li$_2$Pt$_3$B, revealing their different superconducting mechanisms and the dominant atomic contributions to pairing.

Contribution

It provides a comparative analysis of the electronic and superconducting properties of Li$_2$Pd$_3$B and Li$_2$Pt$_3$B, highlighting the role of atomic contributions and the limitations of conventional models.

Findings

Li$_2$Pd$_3$B's superconductivity can be explained by conventional electron-phonon coupling.

Li$_2$Pt$_3$B likely requires more complex models beyond simple electron-phonon interactions.

Boron and palladium atoms are the main contributors to electron-phonon coupling.

Abstract

Electronic structure of recently discovered isotypic ternary borides Li$_2$Pd$_3$B and Li$_2$Pt$_3$B, 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 [Phys. Rev. Lett. {\bf 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$_2$Pd$_3$B, 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$_2$Pd$_3$B. However, such an approach is likely to fail to describe superconductivity in Li$_2$Pt$_3$B.