Thickness-dependent orbital hybridization in ultrathin SrRuO3 epitaxial films

Abstract

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 t2g - 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.

Phase Instability amid Dimensional Crossover in Artificial Oxide Crystal

“…That is, it is known to become insulating as the film thickness decreases below ~4 perovskite unit cells (u.c.) in most cases [4][5][6][7]. Experimentally, growth-induced disorder has been predominantly proposed as the origin of the MIT [4,8], while other intrinsic mechanisms, including quantum confinement and orbital ordering, have also been suggested theoretically [9].…”

“…Experimentally, growth-induced disorder has been predominantly proposed as the origin of the MIT [4,8], while other intrinsic mechanisms, including quantum confinement and orbital ordering, have also been suggested theoretically [9]. In addition, the close relationship between the spin state and electronic structure has been intensively studied [4,[7][8][9][10]]. Yet, a coherent observation and interpretation of the thickness-dependent MIT is lacking, in part because of the absence of a low dimensional sample with intrinsic SRO layers.…”

Asymmetric interfaces and high-TC ferromagnetic phase in La0.67Ca0.33MnO3/SrRuO3 superlattices

“…Based on the above analyses, we further examined the different electronic structures of Samples A and B by XPS. Since the probing depths for both manganite and ruthenate are more than 3 nm [53,54]. XPS can effectively determine the interfacerelated valence change of Mn cations averaged over the entire LCMO layer.…”

Tunable Magnetic Anisotropy in Patterned SrRuO3 Quantum Structures: Competition between Lattice Anisotropy and Oxygen Octahedral Rotation

“…[25] The growth of a single-crystalline SRO thin film on appropriated substrates, e.g., STO, can be achieved routinely. [26,27] Structural modulations and magnetic behavior of epitaxial SRO thin films on various substrates have been well-documented, [28][29][30] which helps to understand the magnetic properties of SRO nanodots. High-quality SRO thin films exhibit a strong magnetocrystalline uniaxial anisotropy, which is highly sensitive to slight variations in stoichiometry, lattice structure, and temperatures.…”