Electron–phonon coupling and superconductivity in an alkaline earth hydride CaH6 at high pressures

Abstract

Recently, an alkaline earth hydride CaH _6 having a sodalitelike clathrate structure has been experimentally synthesized at megabar pressures with a maximum T _c of 215 K, comparable to that of a rare earth hydride LaH _10 . Here, based on first-principles calculations, we find that CaH _6 exhibits a huge peak in the Eliashberg spectral function α ^2 F around the low-frequency region of H-derived phonon modes, in contrast to LaH _10 having a widely spreading spectrum of α ^2 F over the whole frequencies of H-derived phonon modes. It is revealed that the huge peak of α ^2 F in CaH _6 is associated with an effective electron–phonon coupling (EPC) between low-frequency optical phonons and hybridized H 1 s and Ca 3 d states near the Fermi energy. As pressure increases, the strengthened H–H covalent bonding not only induces a hardening of optical phonon modes but also reduces the electron–phonon matrix elements related to the low-frequency optical modes, thereby leading to a lowering of the EPC constant. It is thus demonstrated that H-derived low-frequency phonon modes play an important role in the pressure-induced variation of T _c in CaH _6 . Furthermore, unlike the presence of two distinct superconducting gaps in LaH _10 , CaH _6 is found to exhibit a single isotropic superconducting gap.

Keywords

• superconductivity
• alkaline earth
• superhydride