Two-dimensional square lattice polonium stabilized by the spin-orbit coupling

TL;DR

This study uses density-functional theory to show that spin-orbit coupling stabilizes the two-dimensional square lattice structure of polonium, preventing phonon instabilities and clarifying its stabilization mechanism.

Contribution

It demonstrates that spin-orbit coupling is essential for stabilizing 2D polonium's square lattice structure, beyond scalar relativistic effects.

Findings

2D polonium has a stable square lattice ground state.

Spin-orbit coupling suppresses phonon instabilities.

Without SOC, imaginary phonon frequencies appear.

Abstract

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.