dearprof.,herewithinenclosedisourpaperforconsiderationtobepublishedon(journalname).thefurtherinformationaboutthepaperisinthefollowing:thetitle:theauthors:andtheauthorsclaimthatnoneofthematerialinthepaperhasbeenpublishedorisunderconsiderationforpublicationelsewhere.iamthecorrespondingauthorandmyaressandotherinformationisasfollows,aress:departmentof,tsinghuauniversity,beijing,10084,p.r.chinaemail:ma...