- Chiu, Chun-Chien;
- Ho, Sheng-Zhu;
- Lee, Jenn-Min;
- Shao, Yu-Cheng;
- Shen, Yang;
- Liu, Yu-Chen;
- Chang, Yao-Wen;
- Zheng, Yun-Zhe;
- Huang, Rong;
- Chang, Chun-Fu;
- Kuo, Chang-Yang;
- Duan, Chun-Gang;
- Huang, Shih-Wen;
- Yang, Jan-Chi;
- Chuang, Yi-De
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.