The electronic structure of ${\mathrm{SrRuO}}_3$ thin film with a thickness from 1 to 50 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 the 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 occurring in the 4-u.c. sample. From these two clear observations we conclude that the QCE gives rise to a Mott insulating phase in ultrathin ${\mathrm{SrRuO}}_3$ films. Our interpretation of the RIXS spectra is supported by the configuration interaction calculations of ${\mathrm{RuO}}_6$ clusters.

• Received 2 August 2018

DOI:https://doi.org/10.1103/PhysRevB.99.045113