Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO$_3$ embedded in GdTiO$_3$

TL;DR

This study uses resonant angle-resolved photoemission to identify a critical SrTiO$_3$ thickness for 2DEG formation at GdTiO$_3$/SrTiO$_3$ interfaces, revealing spectral features linked to the 2DEG's emergence.

Contribution

It demonstrates the use of resonant ARPES to detect the critical thickness for 2DEG formation in GdTiO$_3$/SrTiO$_3$ multilayers, providing new spectroscopic insights.

Findings

2DEG spectral feature appears at 1.5 unit cells of SrTiO$_3$

No 2DEG feature at 1.0 unit cell of SrTiO$_3$

Additional Ti and Sr states linked to 2DEG emergence

Abstract

For certain conditions of layer thickness, the interface between GdTiO$_3$ (GTO) and SrTiO$_3$ (STO) in multilayer samples has been found to form a two-dimensional electron gas (2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ (LSAT), with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved (ARPES) and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening.