(1) Kamerlingh Onnes Laboratory, Leiden University,
P.O. Box 9506, 2300 RA Leiden, The Netherlands.
(2) Van der Waals-Zeeman Laboratory, University of
Amsterdam,
Valkenierstraat 65-57, 1018 XE Amsterdam.
(3) Verkin Institute for Low Temp. Physics, 47 Lenin Avenue,
Kharkov 310164 Ukraine.