SPring-8, the large synchrotron radiation facility

Release Date

31 Jan, 2008

Key research achievements
- Clarification of the mystery of "nonconductive" conductive electrons in strontium titanate
- Detection of the orbital components of oxygen atoms for the first time in the world by high-accuracy soft X-ray resonant photoemission spectroscopy
- Confirmation of "nonconductivity" of conductive electrons being dependant on the contributions of the oxygen orbital

Researchers at RIKEN (Ryoji Noyori, President) discovered the cause of the mysterious phenomenon of nonconductive electrons — some electrons are nonconductive even though they are added as conductive electrons to strontium titanate (SrTiO₃), a transparent oxide semiconductor.  This was achieved in joint research by Shik Shin, leader of the Excitation Order Research Team, Quantum Order Research Group, at the Harima Institute, RIKEN SPring-8 Center and jointly appointed as a professor of the Institute for Solid State Physics, University of Tokyo; Yukiaki Ishida, special researcher on basic science in Dr. Shin's team; Haruhiko Ohashi (associate chief scientist) and Yasunori Senba (research scientist) at the Japan Synchrotron Radiation Research Institute (JASRI); and Hiromichi Ohta, associate professor in the School of Engineering, Nagoya University, and his colleagues.

SrTiO₃ is a crystalline transition-metal oxide semiconductor and is expected to be applied as transparent electrodes for driving liquid crystal displays and high-efficiency thermal-electricity conversion materials.  SrTiO₃ crystals present a mystery — electrons, which are added to make the crystal conductive, are conductive some times but not at other times with a certain probability.  The researchers examined the components of the electron orbitals by resonant photoemission spectroscopy in the cases of conductive and nonconductive electrons added to a SrTiO₃ crystal.  As a result, they showed that the dual characteristics of conductive/nonconductive electrons are caused by the electron orbitals of oxygen atoms, which has not been proposed before.  Moreover, they also found that nonconductive electrons can be explained by a model of a transition-metal atom embedded in a semiconductor. 

These achievements are hoped to provide important guidelines for modeling and understanding the electronic states in transition-metal oxides including SrTiO₃ and for realizing new electronic device materials through the control of their various functions.

The results of this research were published in the American scientific journal, Physics Review Letters, and its online version.

Publication:
"Coherent and Incoherent Excitations of Electron-Doped SrTiO₃"
Yukiaki Ishida, Ritsuko Eguchi, Masaharu Matsunami, Koji Horiba, Munetaka Taguchi, Ashish Chainani, Yasunori Senba, Haruhiko Ohashi, Hiromichi Ohta, and Shik Shin
Physical Review Letter 100, 056401 (2008), published 4 February 2008

Models for describing the electronic states of SrTiO₃