1. Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.
2. Institute of Problems of Chemical Physics RAS, 142432 Chernogolovka, Russia.
3. The Institute for Quantum Matter and the Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA.
4. The Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.