H_3Se in the Im\bar3m Phase: High-Pressure Superconductor with T_\rm c Reaching 200 K at 64 GPa Mediated by Anharmonic Phonons

Hydrogen-based compounds have attracted significant attention in recent years due to the discovery of conventional superconductivity with high critical temperature under high pressure, rekindling hopes for finding room-temperature superconductors. In this study, we investigated the vibrational and superconducting properties of H_3Se in the Im\bar3m phase under pressures of 50--200 GPa. Our approach combines the stochastic self-consistent harmonic approximation and first-principles calculations to account for the quantum and anharmonic effects of ions. According to the results, these effects significantly modify the crystal structure, increasing the inner pressure by approximately 8 GPa compared to situations in which they are ignored. The phonon spectra suggest that when these effects are considered, the crystal stabilizes at pressures as low as approximately 61 GPa, which is significantly lower than the previously predicted value of over 100 GPa. Our calculations also highlight the critical role of quantum and anharmonic effects on the electron--phonon coupling properties. Neglecting these factors can result in a significant overestimation of the superconducting critical temperature (T_\rm c) by approximately 4 K (200 GPa) to 25 K (125 GPa). With anharmonic phonons, the T_\rm c calculated from the Migdal--Eliashberg equations reaches 200 K (\mu^\star=0.1, \lambda=4.1) as the pressure decreases to 64 GPa, indicating that the crystal is a rare high-T_\rm c superconductor at moderate pressures.