1. Department of Materials Engineering Science, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan
2. Department of Mechanical Engineering, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan
3. Crystal Growth Laboratory, University of Victoria, Victoria, British Columbia V8W 3P6, Canada