1. J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, CZ-18223 Prague 8, Czech Republic
2. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
3. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
4. National Institute for Materials Science, Namiki, Tsukuba 305-0044, Japan
5. Department of Physics, Tohoku University, Sendai 980-8578, Japan
6. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139