Spin-polarized two dimensional electron gas at the (001) EuTiO3 surface

By using in situ synchrotron radiation high-resolution Angle-Resolved photoemission Spectroscopy (ARPES) and ab initio calculations, we demonstrate the formation of a spin-polarized quasi-two--dimensional electron gas (q2DEG) at the surface of $EuTiO_3$ epitaxial films. While sharing some similarities with the $SrTiO_3$ q2DEG, the $EuTiO_3$ surface state shows distinctive features. Besides the presence of a localized $Eu_{4f}$ band about 2 eV below the Fermi level, the $EuTiO_3$ q2DEG is characterized by heavy bands extending deeper in energy below EF, with a more pronounced hybridization with light bands, compared to the $SrTiO_3$ q2DEG. Ab initio calculations on $TiO_2$ terminated ETO and STO surfaces, including one vertically clustered oxygen di-vacancy in a $2\times 2\times 7$ slab, satisfactorily reproduce the main features of the electronic bands as well as the main differences among the two systems. Furthermore, theoretical calculations show that the $Ti_{3d}$ bands are spin-polarized and in the $EuTiO_3$ q2DEG the exchange coupling between $Ti_{3d}$ and $Eu_{4f}$ states plays a crucial role in stabilizing a ferromagnetic ground state in this system.