Pressure dependence of the superconducting transition temperature of compressed LaH10

Abstract

Two recent experiments [M. Somayazulu et al. Phys. Rev. Lett. 122, 027001 (2019) and A. P. Drozdov et al., Nature (London) 569, 528 (2019)] reported the discovery of superconductivity in the fcc phase of ${\mathrm{LaH}}_{10}$ at a critical temperature ${T}_{\mathrm{c}}$ between $250\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}260\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ under a pressure of about 170 GPa. However, the dependence of ${T}_{\mathrm{c}}$ on pressure showed different patterns, i.e., the former experiment observed a continuous increase of ${T}_{\mathrm{c}}$ up to $\ensuremath{\sim}275\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ on further increase of pressure to 202 GPa, while the latter one observed an abrupt decrease of ${T}_{\mathrm{c}}$ with increasing pressure. Here, based on first-principles calculations, we reveal that for the $\mathrm{fcc}\ensuremath{-}{\mathrm{LaH}}_{10}$ phase, softening of the low-frequency optical phonon modes of H atoms dramatically occurs as pressure decreases, giving rise to a significant increase of the electron-phonon coupling (EPC) constant. Meanwhile, the electronic band structure near the Fermi energy is insensitive to change with respect to pressure. These results indicate that the pressure-dependent phonon softening is unlikely associated with Fermi-surface nesting, but driven by effective screening with the electronic states near the Fermi energy. It is thus demonstrated that the strong variation of EPC with respect to pressure plays a dominant role in the decrease of ${T}_{\mathrm{c}}$ with increasing pressure, supporting the measurements of Drozdov et al.