Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li_0.84Fe_0.16)OHFe_0.98Se

Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. We investigate the ultrafast quasiparticle (QP) dynamics of an intercalated superconductor (Li_1-xFe_x)OHFe_1-ySe, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO_3. We obtain the electron-phonon coupling (EPC) constant \lambda_A_\rm 1g (0.22 \pm 0.04), which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO_3. Significantly, we find that such a positive correlation between \lambda_A_\rm 1g and superconducting T_\rm c holds among all known FeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors.