1. Low Temperature Physics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198
2. Department of Physics, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522
3. Institute of Low Temperature Physics and Engineering, 47 Lenin Avenue, 61103 Kharkov, Ukraine