1. Laboratory
for Micro- and Nanotechnology, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
2. Physikalisch-Chemisches Institut, University of Zurich, Winterthurerstrasse 190, CH-8057
Zurich, Switzerland
3. Physik-Institut, University of Zurich, CH-8057 Zürich, Switzerland
4. Department of Physics, University of Central Florida, Orlando, Florida 32816,
United States