Crystal Symmetry, Electron-Phonon Coupling, and Superconducting Tendencies in Li$_2$Pd$_3$B and Li$_2$Pt$_3$B

TL;DR

This paper investigates the electronic structure, phonon modes, and superconducting tendencies of Li$_2$Pd$_3$B and Li$_2$Pt$_3$B, highlighting the role of electron-phonon coupling and spin-orbit effects in their superconductivity.

Contribution

It provides detailed calculations of phonon frequencies, deformation potentials, and the impact of spin-orbit coupling, revealing the mechanisms driving superconductivity in these compounds.

Findings

Li$_2$Pd$_3$B has a dynamic mass enhancement of 0.75.

Superconductivity in Li$_2$Pd$_3$B is likely driven by Pd vibrations.

Spin-orbit coupling significantly affects the band structure of Li$_2$Pt$_3$B.

Abstract

After theoretical determination of the internal structural coordinates in Li$_2$Pd$_3$B, we calculate and analyze its electronic structure and obtain the frequencies of the two $A_g$ phonons (40.6 meV for nearly pure Li mode, 13.0 meV for the strongly mixed Pd-Li mode). Pd can be ascribed a $4d^{10}$ configuration, but strong 4d character remains up to the Fermi level. Small regions of flat bands occur at -70 meV at both the $\Gamma$ and X points. Comparison of the Fermi level density of states to the linear specific heat coefficient gives a dynamic mass enhancement of $\lambda$ = 0.75. Rough Fermi surface averages of the deformation potentials $D$ of individual Pd and Li displacements are obtained. While <$D_{Li}$> is small, <$D_{Pd}> ~ 1.15 eV/\AA is sizable, and a plausible case exists for its superconductivity at 8 K being driven primarily by coupling to Pd vibrations. The larger d bandwidth in Li$_2$Pt$_3$B leads to important differences in the bands near the Fermi surface. The effect on the band structure of spin-orbit coupling plus lack of inversion is striking, and is much larger in the Pt compound.