Electron-phonon coupling and superconductivity in an alkaline earth hydride CaH at high pressures
arXiv:2111.10797 · doi:10.1088/1367-2630/ac8a0c
Abstract
Recently, an alkaline earth hydride CaH having a sodalitelike clathrate structure has been experimentally synthesized at megabar pressures with a maximum of 215 K, comparable to that of a rare earth hydride LaH. Here, based on first-principles calculations, we find that CaH exhibits a huge peak in the Eliashberg spectral function around the low-frequency region of H-derived phonon modes, in contrast to LaH having a widely spreading spectrum of over the whole frequencies of H-derived phonon modes. It is revealed that the huge peak of in CaH is associated with an effective electron-phonon coupling (EPC) between low-frequency optical phonons and hybridized H 1 and Ca 3 states near the Fermi energy. As pressure increases, the strengthened HH 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 in CaH. Furthermore, unlike the presence of two distinct superconducting gaps in LaH, CaH is found to exhibit a single isotropic superconducting gap.
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