ultrathin film*

The electronic structure of SrRuO₃ thin film with the 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 peaks in RIXS spectra of thin films, which corresponds to the charge transfer from O 2p to Ru 4d orbitals. At the same time, we observed a spectral weight transfer from electron-hole continuum to intersite d-d excitation across the metal-to-insulator transition (MIT) occurring between 5 and 4 u.c. samples. From these two clear observations, we conclude that QCE gives rise to a Mott insulating phase in ultrathin SrRuO₃ films. Our interpretation of the RIXS spectra is supported by configuration interaction calculations of RuO₆ cluster models.