# Orbital-selective confinement effect of Ru $4d$ orbitals in SrRuO$_3$ ultrathin film

## TL;DR

This study uses RIXS to reveal how quantum confinement in ultrathin SrRuO$_3$ films causes orbital splitting and drives a metal-insulator transition, highlighting the orbital-selective nature of the electronic structure.

## Contribution

It demonstrates the orbital-selective quantum confinement effect causing Mott insulating behavior in ultrathin SrRuO$_3$ films, supported by RIXS and theoretical calculations.

## Key findings

- Orbital splitting due to quantum confinement in ultrathin films
- Suppression of electron-hole continuum at the metal-insulator transition
- QCE induces Mott insulating phase in SrRuO$_3$

## Abstract

The electronic structure of SrRuO$_3$ thin film with thickness from 50 to 1 unit cell (u.c.) is investigated via the resonant inelastic x-ray scattering (RIXS) technique at the O K-edge to unravel the intriguing interplay of orbital and charge degrees of freedom. We found that orbital-selective quantum confinement effect (QCE) induces the splitting of Ru $4d$ orbitals. At the same time, we observed a clear suppression of the electron-hole continuum across the metal-to-insulator transition (MIT) occurring at the 4 u.c. sample. From these two clear observations we conclude that QCE gives rise to a Mott insulating phase in ultrathin SrRuO$_3$ films. Our interpretation of the RIXS spectra is supported by the configuration interaction calculations of RuO$_6$ clusters.