Formation of two-dimensional small polarons at the conducting LaAlO3/SrTiO3 interface

Abstract

A long-lasting puzzle of the conducting $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LAO/STO) interface is that the excess electron density counted in the transport measurements is much less than 0.5 electrons per unit cell as predicted by the polar catastrophe model. In this study, via first-principles calculations, we show that the excess electrons at the LAO/STO interface favor the formation of small polarons. These electrons interact strongly with the interfacial Ti ions, forming localized midgap states, which make an insignificant contribution to the conductivity. We also find that the interaction between the neighboring spin-polarized small polarons is weak and does not lead to long-range magnetic ordering. Compared with bulk STO, the formation of the polarons is more favorable at the LAO/STO interface, which is ascribed to the reduced symmetry of the crystal field and the increased lattice distortion. Our results suggest that a large number of the excess electrons at the LAO/STO interface are localized in the form of small polarons, which can partially explain the unexpected fewer electrons in the transport measurements, and also shed light on understanding various properties of other complex perovskite oxide interfaces.