Spectroscopic Observation of Plasmonic Polarons in a Doped Ferromagnetic Semiconductor

Since its discovery in the 1960s, europium monoxide (EuO) has been shown to host a rich array of physical phenomena including the giant magneto-optic Kerr and Faraday effects, anomalous Hall effect, colossal magnetoresistance under doping [1], and a massive, tuneable ferromagnetic metal-insulator transition [2]. The Curie temperature responds sensitively to carrier doping, which can be readily controlled via substitution of Gd for Eu in epitaxial thin films. Here, we use in-situ synchrotron-based angle-resolved photoemission spectroscopy to study the corresponding electronic structure evolution in Eu1-xGdxO grown by molecular beam epitaxy. At low carrier densities, our measured spectral function exhibits signatures of polaron formation due to strong coupling to a bosonic mode. Similar spectral features have recently attracted great attention in polar oxides such as TiO2 [3] and SrTiO3 [4-6] and interfacial systems such as FeSe/SrTiO3 [7]. Unlike these systems, however, we show that it is not electron-phonon, but, rather, electron-plasmon coupling that is the dominant driver of this effect, providing a rare observation of plasmonic polarons.[1] Phys. Rev. B 8, 2316 (1973); [2] Phys. Rev. Lett. 100, 046404 (2008); [3] Phys. Rev. Lett. 110, 196403 (2013); [4] Nat. Comms. 6, 8585 (2015); [5] Nat. Comms. 7, 10386 (2016); [6] Nat. Mat. 15, 835 (2016); [7] Nature 515, 245 (2014)