1. School of Mechanical Engineering; Yonsei University; Seoul 120-749 South Korea
2. Guangxi Key Laboratory of Information Materials; Guilin University of Electronic Technology; Guilin 541004 P. R. China
3. School of Mathematics; University of East Anglia; Norwich NR4 7TJ UK
4. Institute of Applied Physics and Materials Engineering; University of Macau; Avenida da Universidade; Taipa Macau 999078 China