Supported by NSFC(No.10971125);the Specialized Research Fund for the Doctoral Program of Higher Education of China(No.20094402110001);Hanshan Normal University Start-up Project for Ph.D.(No.QD20091202)
The first author was supported by the Special Fund of Shaanxi Provincial Education Department(No.05JK226);The second author was supported by the NSFC(No.10471084);by Guangdong Provincial Natural Science Foundation(No.04010985).