Resolving interfacial charge transfer in titanate superlattices using resonant X-ray reflectometry

TL;DR

This study uses resonant X-ray reflectometry to visualize and quantify charge transfer at buried oxide interfaces in titanate superlattices with atomic-scale precision, revealing electrostatic discontinuities and electronic reconstruction.

Contribution

It introduces high-resolution depth profiling of charge transfer in oxide heterostructures using RXR, demonstrating its effectiveness for atomic-scale interface analysis.

Findings

Approximately half an electron per unit cell transferred at the interface

Suppression of t2g absorption peaks indicating charge transfer

RXR sensitivity to electronic reconstruction at buried interfaces

Abstract

Charge transfer in oxide heterostructures can be tuned to promote emergent interfacial states, and accordingly, has been the subject of intense study in recent years. However, accessing the physics at these interfaces, which are often buried deep below the sample surface, remains difficult. Addressing this challenge requires techniques capable of measuring the local electronic structure with high-resolution depth dependence. Here, we have used linearly-polarized resonant X-ray reflectometry (RXR) as a means to visualize charge transfer in oxide superlattices with atomic layer precision. From our RXR measurements, we extract valence depth profiles of SmTiO$_3$ (SmTO)/SrTiO$_3$ (STO) heterostructures with STO quantum wells varying in thickness from 5 SrO planes down to a single, atomically thin SrO plane. At the polar-nonpolar SmTO/STO interface, an electrostatic discontinuity leads to approximately half an electron per areal unit cell transferred from the interfacial SmO layer into the neighboring STO quantum well. We observe this charge transfer as a suppression of the t$_{2g}$ absorption peaks that minimizes contrast with the neighboring SmTO layers at those energies and leads to a pronounced absence of superlattice peaks in the reflectivity data. Our results demonstrate the sensitivity of RXR to electronic reconstruction at the atomic scale, and establish RXR as a powerful means of characterizing charge transfer at buried oxide interfaces.