Direct observation of two-dimensional small polarons at correlated oxide interface

TL;DR

This study directly observes 2D small polarons at the LaAlO3/SrTiO3 interface, revealing their formation due to strong charge-lattice interactions, which explains the discrepancy in carrier density and relates to superconductivity phenomena.

Contribution

First direct spectroscopic observation of small polarons at a 2D oxide interface, linking polaron formation to charge-lattice interactions and superconductivity.

Findings

Detection of small polarons at the LAO/STO interface.

Strong electron-lattice coupling causes localized polarons.

Lattice distortions relate to superconductive states.

Abstract

Two-dimensional (2D) perovskite oxide interfaces are ideal systems where diverse emergent properties can be uncovered.The formation and modification of polaronic properties due to short-range strong charge-lattice interactions of 2D interfaces remains hugely intriguing.Here, we report the direct observation of small-polarons at the LaAlO3/SrTiO3 (LAO/STO) conducting interface using high-resolution spectroscopic ellipsometry.First-principles investigations further reveals that strong coupling between the interfacial electrons and the Ti-lattice result in the formation of localized 2D small polarons.These findings resolve the longstanding issue where the excess experimentally measured interfacial carrier density is significantly lower than theoretically predicted values.The charge-phonon induced lattice distortion further provides an analogue to the superconductive states in magic-angle twisted bilayer graphene attributed to the many-body correlations induced by broken periodic lattice symmetry.Our study sheds light on the multifaceted complexity of broken periodic lattice induced quasi-particle effects and its relationship with superconductivity.