1. Institut for Energiteknikk, P.O.Box 40, N-2007 Kjeller, Norway
2. Institute of Physics, Warsaw University Branch, Lipowa 41, 15-424 Bialystok, Poland
3. Brookhaven National Laboratory, Physics Department, Upton, New York 11973-5000, U.S.A.
4. Laboratoire Leon Brillouin, CEN-Saclay, 91191 Gif-sur-Yvette, France
5. Department of Applied Physics, Nagoya University, Nagoya 464