A first-principles investigation of the origin of superconductivity in TlBi$_2$

TL;DR

This study uses first-principles calculations to reveal that TlBi$_2$ is a conventional phonon-mediated superconductor with a critical temperature of 6.2 K, driven by strong electron-phonon interactions influenced by spin-orbit coupling.

Contribution

It provides a detailed first-principles analysis explaining the origin of superconductivity in TlBi$_2$, highlighting the role of electron-phonon coupling and spin-orbit effects.

Findings

TlBi$_2$ is a phonon-mediated superconductor below 6.2 K.

Strong electron-phonon coupling is due to Bi $p$ orbitals and spin-orbit interactions.

The phonon frequency is significantly lower than in MgB$_2$, facilitating superconductivity.

Abstract

The intermetallic compound TlBi$_2$ crystallizes in the MgB$_2$ structure and becomes superconducting below 6.2 K. Considering that both Tl and Bi have heavy atomic masses, it is puzzling why TlBi$_2$ is a conventional phonon-mediated superconductor. We have performed comprehensive first-principles calculations of the electronic structures, the phonon dispersions and the electron-phonon couplings for TlBi$_2$. The $6p$ orbitals of bismuth dominate over the states near the Fermi level, forming strong intra-layer $p_{x/y}$ and inter-layer $p_z$ $\sigma$ bonds which is known to have strong electron-phonon coupling. In addition, the large spin-orbit coupling interaction in TlBi$_2$ increases its electron-phonon coupling constant significantly. As a result, TlBi$_2$, with a logarithmic phonon frequency average one tenth that of MgB$_2$, is a phonon-mediated superconductor.