1. Toyota Riken – Kyoto University Research Center (TRiKUC), Kyoto 606-8501, Japan
2. Department of Electronic Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
3. Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
4. Condensed Matter Theory Group, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland