Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li0.84Fe0.16)OHFe0.98Se

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<INF>1 -x</INF>Fe<INF>x</INF>)OHFe<INF>1 -y</INF>Se, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO<INF>3</INF>. We obtain the electron-phonon coupling (EPC) constant lambda<INF>A<INF>1g</INF></INF>(0.22 +/- 0.04), which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO<INF>3</INF>. Significantly, we find that such a positive correlation between lambda<INF>A<INF>1g</INF></INF>and superconductingT<INF>c</INF>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.