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

Two-dimensional (2D) perovskite oxide interfaces are condensed-matter systems where intriguing emergent properties can be observed. The onset of polaronic properties at such interfaces is a scientific discipline where both theoretical and experimental investigations can uncover interesting insights pertaining to many-body charge dynamics. Here, we report the direct observation of small-polarons at the LaAlO₃/SrTiO₃ (LAO/STO) conducting interface using high-resolution spectroscopic ellipsometry. The experimental observations are complemented by first-principles investigations further reveals a strong coupling between the interfacial electrons and the Ti-lattice that results in the formation of localized 2D small polarons. These findings resolve the longstanding issue where the experimentally derived interfacial carrier density is significantly lower than theoretically predicted values.