1. The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01
2. Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka 560
3. Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka 560
4. The Institute of Inorganic Chemistry, University of Erlangen-Nürnberg, 91058 Erlangen, Germany