Evolution of the SrTiO$_3$ surface electronic state as a function of LaAlO$_3$ overlayer thickness

TL;DR

This study investigates how the surface electronic state of SrTiO$_3$ evolves with LaAlO$_3$ overlayer thickness, revealing an instability at 4 unit cells that distinguishes LAO/STO interfaces from bare STO.

Contribution

It provides in situ ARPES and RIXS analysis showing the evolution of electronic properties and a new instability at the critical LAO thickness in LAO/STO heterostructures.

Findings

Universal metallic state on STO surface observed.

Instability towards 2x1 Fermi surface folding at 4 u.c. LAO thickness.

Similarities in conduction band character across samples.

Abstract

The novel electronic properties emerging at interfaces between transition metal oxides, and in particular the discovery of conductivity in heterostructures composed of LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) band insulators, have generated new challenges and opportunities in condensed matter physics. Although the interface conductivity is stabilized when LAO matches or exceeds a critical thickness of 4 unit cells (u.c.), other phenomena such as a universal metallic state found on the bare surface of STO single crystals and persistent photon-triggered conductivity in otherwise insulating STO-based interfaces raise important questions about the role of the LAO overlayer and the possible relations between vacuum/STO and LAO/STO interfaces. Here, we study how the metallic STO surface state evolves using angle-resolved photoemission spectroscopy (ARPES) in situ prepared samples complemented by resonant inelastic X-ray scattering (RIXS) during the growth of a crystalline LAO overlayer. In all the studied samples, the character of the conduction bands, their carrier densities, the Ti3+ crystal field, and the response to photon irradiation bear strong similarities. Nevertheless, we report here that studied LAO/STO interfaces exhibit an instability toward an apparent 2 x 1 folding of the Fermi surface at and above 4 u.c. thickness threshold, which distinguishes these heterostructures from bare STO and sub-critical-thickness LAO/STO.