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

Chun-Chien Chiu^1*, Sheng-Zhu Ho¹, Jenn-Min Lee², Yu-Cheng Shao³, Yang Shen⁴, Yu-Chen Liu¹, Yao-Wen Chang¹, Yun-Zhe Zheng⁴, Rong Huang⁴, Chun-Fu Chang⁵, Chang-Yang Kuo^6,7, Chun-Gang Duan⁴, Shih-Wen Huang^2,8, Jan-Chi Yang¹, Yi-De Chuang³

¹Department of Physics, National Cheng Kung University, Tainan, Taiwan
²MAX IV Laboratory, Lund University, Lund, Sweden
³Advanced Light Source, Lawrence Berkeley National Laboratory, California, USA
⁴Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronics, East China Normal University, East China Normal University, China
⁵Max-Planck Institute for Chemical Physics of Solids, Dresden, Germany
⁶Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
⁷National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁸Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland

* Presenter:Chun-Chien Chiu, email:l28091025@gs.ncku.edu.tw

In the past decade, Strontium Titanate (STO), known as a quantum paraelectric material with a diverse range of emergent properties like ferroelectricity and superconductivity, has garnered significant attention. Recent research has revealed that in strain-free STO films grown on SrRuO₃ buffer layers, existing polar nanoregions play a pivotal role in promoting room-temperature ferroelectricity, particularly when the STO film thickness approaches 10 nm.
In this talk, we present evidence that within this specific thickness range, freestanding STO films unconstrained by single-crystal substrates, exhibit a tetragonal structure at room temperature, in contrast to the cubic lattice observed in bulk forms. Spectroscopic measurements unveil modified Ti−O orbital hybridization, resulting in deviations in the nominal 4+ valency of Ti ions (3d⁰ configuration) due to excess delocalized 3d electrons. Furthermore, the Ti ion within the TiO₆ octahedron displays an off-center displacement. The reduced inherent symmetry observed in these ultrathin freestanding films provides a unique avenue for manipulating the electronic structures of strongly correlated systems. Such control is pivotal for prospective technologically significant applications.

Keywords: Strontium Titanate, ultrathin freestanding films, strongly correlated systems