  Polonium is known as the only simple metal that has the simple cubic (SC)
lattice in three dimension. There is a debate about whether the stabilized SC
structure is attributed to the scalar relativistic effect or the spin-orbit
coupling (SOC). Here, we study another phase, two-dimensional (2D) polonium
(poloniumene), by performing density-functional theory calculations. We show
that the 2D polonium has the square lattice structure as its ground state and
demonstrate that the SOC (beyond the scalar relativistic approximation)
suppresses the Peierls instability and is necessary to obtain no imaginary
phonon frequencies over the Brillouin zone.
