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delocalized Ti 3d electrons, ferroelectricity, freestanding SrTiO3, RIXS, Electronic structure, Ferroelectric materials, Ruthenium compounds, Single crystals, Strontium titanates, Titanium compounds, Ultrathin films, 3d electron, Delocalized ti 3d electron, Emergent property, Freestanding SrTiO3, RIXS, Strain-free, Strongly correlated materials, Titanate films, Ultra-thin, Wide spectrum, Ferroelectricity
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. ©
