1. The School of Biological and
Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, U.K.
2. Theoretical Physics Department,
Faculty of Physics, Vilnius University,
Saulėteko al. 9, LT-10222 Vilnius, Lithuania
3. Institute of Physics, Center for Physical Sciences and Technology, Gostauto
11, LT-01108 Vilnius, Lithuania