Thickness-dependent orbital hybridization in ultrathin SrRuO₃ epitaxial films

We investigated the electronic structures of ultrathin SrRuO₃ (SRO) films with n = 1, 2, 3, 4, and 8 monolayers (MLs) on SrTiO₃ substrates using O K-edge X-ray absorption spectroscopy (XAS). The intensities of the low-energy features reflect the strengths of the Ru 4d-O 2p orbital hybridization. The Ru 4d orbital state evolves with the increasing SRO thickness, exhibiting a crossover at approximately n = 2. For thick SRO films (n ≥ 3), this constitutes a metallic band, while for the 1 or 2 ML film, the band features shift to a higher energy to form a bandgap (> 0.2 eV), reflecting the emergent insulating nature. The polarization dependence of the peak intensities further shows that in the metallic films (n ≥ 3), Ru t_2g - O 2p hybridizations are strong and anisotropic with stronger (weaker) equatorial (apical) hybridizations, possibly owing to compressive strain effects from the SrTiO₃ substrate, while in thinner films (n ≤ 2), the hybridization effects become weak and rather isotropic because of the localization of Ru 4d orbitals. Thus, the evolution of anisotropic hybridizations in SRO films in the vicinity of the thickness-driven metal-insulator transition was substantiated.