1. BSRF, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, Bin 2-7, Beijing 100039, P. R. China, Centre for Materials Research, School of Physical Sciences, University of Kent, Canterbury, CT2 7NR, U.K., and Beijing Laboratory of Electron Microscopy, Institute of Physics & Centre for Condensed Matter of Physics, Chinese Academy of Sciences, Beijing 100080, P. R. China