1. Department of Materials Science & Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-855, Japan, Departamento de Química Física, Universidade de Vigo, E-36200, Vigo, Spain, and Department of Applied Physics, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan