Band-Order Anomaly at the {\gamma}-Al2O3/SrTiO3 Interface Drives the Electron-Mobility Boost

TL;DR

This study uncovers how a band-order anomaly at the { extgamma}-Al2O3/SrTiO3 interface enhances electron mobility by altering orbital ordering and electron distribution, offering new pathways for oxide device optimization.

Contribution

It reveals the fundamental physics behind the mobility boost at the { extgamma}-Al2O3/SrTiO3 interface through direct electronic structure measurements and band-order analysis.

Findings

Orbital ordering anomaly depopulates top STO electron states.

Electron density shifts away from the interface, reducing defect overlap.

Band order changes due to symmetry breaking between materials.

Abstract

Rich functionalities of transition-metal oxides and their interfaces bear an enormous technological potential. Its realization in practical devices requires, however, a significant improvement of yet relatively low electron mobility in oxide materials. Recently, a mobility boost of about two orders of magnitude has been demonstrated at the spinel/perovskite {\gamma}-Al2O3/SrTiO3 interface compared to the paradigm perovskite/perovskite LaAlO3/SrTiO3. We explore the fundamental physics behind this phenomenon from direct measurements of the momentum-resolved electronic structure of this interface using resonant soft-X-ray angle-resolved photoemission. We find an anomaly in orbital ordering of the mobile electrons in {\gamma}-Al2O3/SrTiO3 which depopulates electron states in the top STO layer. This rearrangement of the mobile electron system pushes the electron density away from the interface that reduces its overlap with the interfacial defects and weakens the electron-phonon interaction, both effects contributing to the mobility boost. A crystal-field analysis shows that the band order alters owing to the symmetry breaking between the spinel {\gamma}-Al2O3 and perovskite SrTiO3. The band-order engineering exploiting the fundamental symmetry properties emerges as another route to boost the performance of oxide devices.