Electron, phonon, and superconducting properties of Mg2CuH6 under pressure

Abstract

Recent theoretical prediction of Mg2IrH6 is a significant advance in achieving high-temperature superconductivity under atmospheric pressure, Mg2IrH6 is the hydride superconductor with the highest superconducting transition temperature (Tc ∼ 160 K) under ambient pressure so far. In general, Tc is usually related to the degree of hydrogen enrichment. As a representative of the hydrogen-poor structures, MgHCu3 was also predicted to have a Tc of 43 K under atmospheric pressure. In this work, we try to replace the Ir atom in Mg2IrH6 with the Cu atom to improve the superconductivity of the system. The phonon dispersion curves and the ab initio molecular dynamics (AIMD) simulation indicate that the novel ternary Mg2CuH6 hydride is dynamically and thermodynamically stable. But, regrettably, the Tc and electron-phonon coupling (EPC) parameters λ of Mg2CuH6 are calculated to be 12.5 K and 0.55 at 25 GPa, which is far inferior to the superconductivity of Mg2IrH6. Compared with Fm3¯m Mg2CuH6, Fm3¯m Mg2IrH6 produces obvious phonon softening in the G point near 30 meV, which significantly enhances the EPC and increases the Tc of the system. Moreover, Cu substitution for Ir results in a sharp decrease in the contribution of the electronic states for Mg and H to the total density of states at the Fermi level. The huge difference in predicted Tc between Mg2CuH6 and Mg2IrH6 and their causes may provide insights into the design of high-temperature atmospheric superconductors and help to understand the superconducting mechanisms of related systems.