1. J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague 8, Czech Republic, Institute of Solid State and Materials Research, Helmholtzstrasse 20, D-01069 Dresden, Germany, Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan, and Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan