# Raman and fluorescence contributions to resonant inelastic soft x-ray scattering on LaAlO$_3$/SrTiO$_3$ heterostructures

## TL;DR

This study uses high-resolution RIXS to analyze Ti 3d carriers at LaAlO3/SrTiO3 interfaces, revealing the roles of overlayer thickness and oxygen vacancies in charge carrier behavior and challenging previous interpretations of spectral features.

## Contribution

It provides new insights into the nature of Ti 3d electrons and clarifies the interpretation of RIXS spectral features in complex oxide interfaces.

## Key findings

- Interfacial Ti 3d electrons exist below the conductivity threshold.
- Oxygen vacancies significantly dope Ti 3d charge carriers.
- Localized and itinerant Ti 3d electrons coexist at the interface.

## Abstract

We present a detailed study of the Ti 3$d$ carriers at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures by high-resolution resonant inelastic soft x-ray scattering (RIXS), with special focus on the roles of overlayer thickness and oxygen vacancies. Our measurements show the existence of interfacial Ti 3$d$ electrons already below the critical thickness for conductivity and an increase of the total interface charge up to a LaAlO$_3$ overlayer thickness of 6 unit cells before it levels out. By comparing stoichiometric and oxygen deficient samples we observe strong Ti 3$d$ charge carrier doping by oxygen vacancies. The RIXS data combined with photoelectron spectroscopy and transport measurements indicate the simultaneous presence of localized and itinerant charge carriers. However, it is demonstrated that the relative amount of localized and itinerant Ti $3d$ electrons in the ground state cannot be deduced from the relative intensities of the Raman and fluorescence peaks in excitation energy dependent RIXS measurements, in contrast to previous interpretations. Rather, we attribute the observation of either the Raman or the fluorescence signal to the spatial extension of the intermediate state reached in the RIXS excitation process.