Band-Order Anomaly at the γ-Al₂O₃/SrTiO₃ Interface Drives the Electron-Mobility Boost

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 γ-Al 2 O 3 /SrTiO 3 interface compared to the paradigm perovskite/perovskite LaAlO 3 /SrTiO 3 . 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 γ-Al 2 O 3 /SrTiO 3 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 γ-Al 2 O 3 and perovskite SrTiO 3 . The band-order engineering exploiting the fundamental symmetry properties emerges as another route to boost the performance of oxide devices.

Large Linear Magnetoresistance of High‐Mobility 2D Electron System at Nonisostructural γ‐Al2O3/SrTiO3 Heterointerfaces

“…Recently, combining theoretical and experimental results, it is found a nontrivial orbital occupation occurs in GAO/STO interface. [ 21–24 ] This novel electronic structure and the resultant interfacial reconstruction could be due to the symmetry breaking at the interface between the nonisostructural GAO and STO. As for the recorded highest mobility of GAO/STO, underlying mechanism of spatial separation between interfacial oxygen vacancies and carriers in deeper STO reduced scattering is recently proposed.…”

Direct visualization of Rashba-split bands and spin/orbital-charge interconversion at KTaO$_3$ interfaces

“…To suppress the second compensation effect, it may be attractive to generate 2DEGs in oxide materials in which crystal field splitting causes the d xz/yz bands to lie below the d xy bands. Recent ARPES measurements suggested that such a band order inversion occurs at interfaces between STO and γ-Al 2 O 3 31 . It would be interesting to see if the same inversion could be engineered in KTO 2DEGs.…”

Robust Electronic Structure of Manganite-Buffered Oxide Interfaces with Extreme Mobility Enhancement

“…Identifying the electronic structure as well as the orbital configuration becomes even more interesting upon the recent finding that the high mobility 2DEG at γ-Al 2 O 3 /STO (GAO/STO) interface shows an anomaly in orbital ordering as well as a weak electron−phonon interaction (EPI) strength of the carriers. 42,43 In this paper, we identify the electronic band structure of the buried oxide interfaces by soft X-ray angle-resolved photoemission spectroscopy (ARPES) for both bare and LMObuffered a-LAO/STO. Besides a slight decrease in the k F and carrier density as well as a strong suppression in the concentration of OVs, 39 the buffered interface exhibits two significant properties: (1) an unexpected irradiation-robust band structure, which reflects the detection of FS of intrinsic interface states, and (2) a significantly reduced EPI strength, which is absent in the nonbuffered 42,43 as well as the diluted crystalline LAO/STO interfaces.…”