First-principles investigation of the origin of superconductivity in TlBi2

The intermetallic compound TlBi2 crystallizes in the MgB2 structure and becomes superconducting below 6.2 K. Considering that both Tl and Bi have heavy atomic masses, it is puzzling why TlBi2 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 TlBi2. The 6p orbitals of bismuth dominate over the states near the Fermi level, forming strong intralayer px/y and interlayer pz sigma bonds which are known to have strong electron-phonon coupling. In addition, the large spin-orbit coupling interaction in TlBi2 increases significantly its electron-phonon coupling constant. As a result, TlBi2, with a logarithmic phonon frequency average one-tenth that of MgB2, is a phonon-mediated superconductor.