Presence of Delocalized Ti 3d Electrons in Ultrathin Single-Crystal SrTiO₃

Abstract

Strontium titanate (STO), with a wide spectrum of emergent properties such as ferroelectricity and superconductivity, has received significant attention in the community of strongly correlated materials. In the strain-free STO film grown on the SrRuO3 buffer layer, the existing polar nanoregions can facilitate room-temperature ferroelectricity when the STO film thickness approaches 10 nm. Here we show that around this thickness scale, the freestanding STO films without the influence of a substrate show the tetragonal structure at room temperature, contrasting with the cubic structure seen in bulk form. The spectroscopic measurements reveal the modified Ti–O orbital hybridization that causes the Ti ion to deviate from its nominal 4+ valency (3d0 configuration) with excess delocalized 3d electrons. Additionally, the Ti ion in TiO6 octahedron exhibits an off-center displacement. The inherent symmetry lowering in ultrathin freestanding films offers an alternative way to achieve tunable electronic structures that are of paramount importance for future technological applications.

Freestanding Perovskite Oxide Films: Synthesis, Challenges, and Properties

“…Enhanced tetragonality was also observed in ultrathin freestanding STO thin films by Chiu et al. [ 119 ] In their work, the authors showed that STO thin films below 10 nm undergo through a transformation from cubic to tetragonal structure when released from the substrate. This phase is characterized by a displacement of Ti atoms from the TiO 6 octahedron center and by the appearance of delocalized electrons.…”

Confinement-Induced Isosymmetric Metal–Insulator Transition in Ultrathin Epitaxial V2O3 Films

“…Similarly, many other physical phenomena in ultrathin films and heterostructures can be explained as, for example, the enhanced tetragonality in various perovskites, such as PbTiO 3 , BaTiO 3 , and SrTiO 3 . 49 This involves the electron localization in the out-of-plane π-bonding orbitals which favors the hybridization with the O 2p orbitals strengthening the tetragonal distortion. 50,51 Similarly, it can be expected that the suppression/enhancement of octahedral rotations in perovskite-based heterostructures can be explained by the same hybridization preference upon confinement.…”