School of Electronics Eletrical and Communication Engineering University of Chinese Academy of Sciences Beijing 100049 China;
Institut für Physik Martin-Luther-Universit(a)t Halle-Wittenberg Nanotechnikum-Weinberg Heinrich-Damerow-Strasse 4 D-06120 Halle(Saale) Germany;
School of Physics University of Chinese Academy of Sciences Beijing 100049 China;