Competing Phases, Strong Electron-Phonon Interaction and Superconductivity in Elemental Calcium under High Pressure

TL;DR

This study investigates the complex phase stability and strong electron-phonon interactions in elemental calcium under high pressure, revealing competing structures, local order, and superconductivity with critical temperatures around 20-25 K.

Contribution

It provides first-principles insights into the stable and competing phases of calcium under high pressure and links these to observed superconductivity without soft phonon modes.

Findings

Multiple structures compete energetically in 40-130 GPa range.

Locally stable structures can explain experimental diffraction patterns.

Superconductivity with T_c of 20-25 K is predicted without soft phonon modes.

Abstract

The observed "simple cubic" (sc) phase of elemental Ca at room temperature in the 32-109 GPa range is, from linear response calculations, dynamically unstable. By comparing first principle calculations of the enthalpy for five sc-related (non-close-packed) structures, we find that all five structures compete energetically at room temperature in the 40-90 GPa range, and three do so in the 100-130 GPa range. Some competing structures below 90 GPa are dynamically stable, i.e., no imaginary frequency, suggesting that these sc-derived short-range-order local structures exist locally and can account for the observed (average) "sc" diffraction pattern. In the dynamically stable phases below 90 GPa, some low frequency phonon modes are present, contributing to strong electron-phonon (EP) coupling as well as arising from the strong coupling. Linear response calculations for two of the structures over 120 GPa lead to critical temperatures in the 20-25 K range as is observed, and do so without unusually soft modes.