1. Department of Physics and Astronomy, Ångströmlaboratoriet, Uppsala University, Box 516, 751 20 Uppsala, Sweden
2. Department of Cell and Molecular Biology, BMC, Uppsala University, Box 596, 75124 Uppsala, Sweden
3. Department of Physical and Chemical Sciences, University of L’Aquila, Via Vetoio 10, 67100 Coppito, L’Aquila, Italy