Crossover from Polarons to Fermi Liquids in Doped Oxides: <i>Ab Initio</i> Many-Body Calculations Using the Cumulant Expansion Approach

The coupling of infrared-active phonons to electrons (Fröhlich interaction) can lead to the formation of polarons, i.e. electrons dressed by a phonon cloud. Conducting oxides represent an ideal playground to investigate this phenomenon. In angle-resolved photoemission spectroscopy (ARPES) the signature of polarons is the appearance of spectral satellites below the conduction band. Recent ARPES experiments reported a transition of the charge carriers from polarons to a weakly-coupled Fermi liquid with increasing doping [1,2]. Here we calculate ARPES spectra from first principles by combining accurate ab initio calculations of the electron-phonon coupling with the cumulant expansion method [3]. For the paradigmatic example anatase TiO₂, we show that the transition observed in the experiments originates from nonadiabatic polar electron-phonon coupling. We show that this is a universal mechanism, and in particular it also applies to the ferromagnetic semiconductor EuO.
[1] S. Moser et al., PRL 110, 196403 (2013).
[2] Z. Wang et al., Nat. Mater. 15, 835 (2016).
[3] C. Verdi et al., Nat. Commun. 8, 15769 (2017).