Anharmonic stabilization of the high-pressure simple cubic phase of calcium

TL;DR

This study demonstrates that anharmonic effects stabilize the high-pressure simple cubic phase of calcium, which appears unstable under harmonic approximation, and also predicts superconducting properties consistent with experimental observations.

Contribution

The paper introduces a variational approach incorporating anharmonic effects to accurately predict phonon stability and superconductivity in high-pressure calcium.

Findings

Anharmonic effects stabilize the cubic phase at 50 GPa.

Predicted superconducting transition temperature matches experiments.

Identified anomalous specific heat behavior.

Abstract

The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approx- imation, shows imaginary branches that make it mechanically unstable. In this letter, the phonon spectrum is recalculated using density-functional theory (DFT) ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that perturbation theory cannot be employed with imaginary harmonic frequencies, a variational procedure based on the Gibbs- Bogoliubov inequality is used to estimate the renormalized phonon frequencies. The results show that strong quantum anharmonic effects make the imaginary phonons become positive even at zero temperature so that the simple cubic phase becomes mechanically stable, as experiments suggest. Moreover, our calculations find a superconducting Tc in agreement with experiments and predict an anomalous behavior of the specific heat.