  Electron-boson interaction is fundamental to a thorough understanding of
various exotic properties emerging in many-body physics. In photoemission
spectroscopy, photoelectron emission due to photon absorption would trigger
diverse collective excitations in solids, including the emergence of phonons,
magnons, electron-hole pairs, and plasmons, which naturally provides a reliable
pathway to study electron-boson couplings. While fingerprints of
electron-phonon/-magnon interactions in this state-of-the-art technique have
been well investigated, much less is known about electron-plasmon coupling, and
direct observation of the band renormalization solely due to electron-plasmon
interactions is extremely challenging. Here by utilizing integrated oxide
molecular-beam epitaxy and angle-resolved photoemission spectroscopy, we
discover the long sought-after pure electron-plasmon coupling-induced low-lying
plasmonic-polaron replica bands in epitaxial semimetallic SrIrO$_3$ films, in
which the characteristic low carrier concentration and narrow bandwidth combine
to provide a unique platform where the electron-plasmon interaction can be
investigated kinematically in photoemission spectroscopy. This finding enriches
the forms of electron band normalization on collective modes in solids and
demonstrates that, to obtain a complete understanding of the quasiparticle
dynamics in 5d electron systems, the electron-plasmon interaction should be
considered on equal footing with the acknowledged electron-electron interaction
and spin-orbit coupling.
