Electron-phonon superconductivity in CaBi2 and the role of spin-orbit interaction

CaBi2 is a recently discovered type-I superconductor with T-c = 2K and a layered crystal structure. In this work electronic structure, lattice dynamics, and electron-phonon interaction are studied, with special attention paid to the influence of the spin-orbit coupling (SOC) on the above-mentioned quantities. We find that in the scalar-relativistic case (without SOC), electronic structure and electron-phonon interaction show quasi-two-dimensional character. Strong Fermi surface nesting is present, which leads to the appearance of the Kohn anomaly in the phonon spectrum and enhanced electron-phonon coupling for the phonons propagating in the Ca-Bi atomic layers. However, strong spin-orbit coupling in this material changes the topology of the Fermi surface and reduces the nesting, and the electron-phonon coupling becomes weaker and more isotropic. The electron-phonon coupling parameter. is reduced by SOC to almost half, from 0.94 to 0.54, giving an even stronger effect on the superconducting critical temperature T-c, which drops from 5.2 K (without SOC) to 1.3 K (with SOC). Relativistic values of lambda and T-c remain in good agreement with experimental findings, confirming the general need for including SOC in the analysis of the electron-phonon interaction in materials containing heavy elements.