Phonon softening and superconductivity triggered by spin-orbit coupling in simple-cubic α-polonium crystals

We have investigated the mechanism of stabilizing the simple-cubic (sc) structure in polonium (alpha-Po), based on the phonon dispersion calculations using the first-principles all-electron band method. We have demonstrated that the stable sc structure results from the suppression of the Peierls instability due to the strong spin-orbit coupling (SOC) in alpha-Po. We have also discussed the structural chirality realized in beta-Po, as a consequence of the phonon instability. Further, we have explored the possible superconductivity in alpha-Po, and predicted that it becomes a superconductor with T-c similar to 4 K. The transverse soft phonon mode at q approximate to 2/3 R, which is greatly influenced by the SOC, plays an important role both in the structural stability and the superconductivity in alpha-Po.