1. Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-8656
2. Division of Materials Physics, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531
3. Département de la Recherche Fondamentale sur la Matière Condenseé, SPSMS, CEA Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France