Foundation item: Project(10672191)supported by the National Natural Science Foundation of China; Project(06JJ2059)supported by the Provincial Natural Science Foundation of Hunan, China; Project(KF0607) supported by the Key Laboratory of Low Dimensional Materials and Application Technology (Xiangtan University), Ministry of Education, China Received date: 2008−06−25; Accepted date: 2008−08−05 Corresponding author: LUO Xing-she, Professor, PhD; Tel: +86−731−5623376; E-mail: lys0258@sina-com-s.stat.lib.xjtu.edu.cn:443