Electronic structure and small-hole polarons in YTiO 3

As a prototypical Mott insulator with ferromagnetic ordering, ${\mathrm{YTiO}}_{3}$ (YTO) is of great interest in the study of strong electron correlation effects and orbital ordering. Here we report the first molecular beam epitaxy (MBE) growth of YTO films, combined with theoretical and experimental characterizations of the electronic structure and charge transport properties. The obstacles of YTO MBE growth are discussed and potential routes to overcome them are proposed. DC transport and Seebeck measurements on thin films and bulk single crystals identify $p$-type Arrhenius transport behavior with an activation energy of $\ensuremath{\sim}0.17\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in thin films, consistent with the energy barrier for small hole polaron migration from hybrid density functional theory calculations. Hard x-ray photoelectron spectroscopy measurements show the lower Hubbard band at 1.1 eV below the Fermi level, whereas a Mott-Hubbard band gap of $\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ is determined from photoluminescence measurements. These findings provide critical insight into the electronic band structure of YTO and related materials.