First-principles investigation of the origin of superconductivity in TlBi2

The intermetallic compound ${\mathrm{TlBi}}_{2}$ crystallizes in the ${\mathrm{MgB}}_{2}$ structure and becomes superconducting below 6.2 K. Considering that both Tl and Bi have heavy atomic masses, it is puzzling why ${\mathrm{TlBi}}_{2}$ 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 ${\mathrm{TlBi}}_{2}$. The $6p$ orbitals of bismuth dominate over the states near the Fermi level, forming strong intralayer ${p}_{x/y}$ and interlayer ${p}_{z} \ensuremath{\sigma}$ bonds which are known to have strong electron-phonon coupling. In addition, the large spin-orbit coupling interaction in ${\mathrm{TlBi}}_{2}$ increases significantly its electron-phonon coupling constant. As a result, ${\mathrm{TlBi}}_{2}$, with a logarithmic phonon frequency average one-tenth that of ${\mathrm{MgB}}_{2}$, is a phonon-mediated superconductor.