JIAO Yuning, LIU Qingmin, YANG Yuangsheng, HU Zhuangqi,Institute of Meta] Research, Chinese Academy of Sciences, Shenyang, ChinaGAO Yunyan, JIA Guanglin, ZHANG Guozhi, QIAO YuanjunNortheast University, Shenyang, China Manuscript received 12 May, 1994