(1) Department of Physics, Faculty of Science & Technology, Science University of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan (Fax: +81-471/23-9361, E-mail: morinaga@ph.noda.sut.ac.jp), JP
Abstract:
Received: 24 March 1998/Revised version: 2 June 1998