Pressure dependence of the superconducting transition temperature of compressed LaH${}_{10}$

Two recent experiments [M. Somayazulu $et$ $al$., Phys. Rev. Lett. ${\bf 122}$,027001(2019)andA\mathrm{.}P\mathrm{.}Drozdov$et$ $al$., Nature $\mathbf{569}$, 528 (2019)] reported the discovery of superconductivity in the fcc phase of LaH${}_{10}$ at a critical temperature $T_{\mathrm{c}}$ between 250$\sim$260 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 $\sim$275 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 fcc-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 $et$ $al$.