Title: Electronic characteristics of ultrathin SrRuO$_3$ films and their relationship with the metal$-$insulator transition,
Abstract: SrRuO$_3$ (SRO) films are known to exhibit insulating behavior as their
thickness approaches four unit cells. We employ electron energy$-$loss (EEL)
spectroscopy to probe the spatially resolved electronic structures of both
insulating and conducting SRO to correlate them with the metal$-$insulator
transition (MIT). Importantly, the central layer of the ultrathin insulating
film exhibits distinct features from the metallic SRO. Moreover, EEL near edge
spectra adjacent to the SrTiO$_3$ (STO) substrate or to the capping layer are
remarkably similar to those of STO. The site$-$projected density of states
based on density functional theory (DFT) partially reflects the characteristics
of the spectra of these layers. These results may provide important information
on the possible influence of STO on the electronic states of ultrathin SRO.