1. Theoretical Physics Department,
Faculty of Physics, Vilnius University,
Saulėtekio al. 9, LT-10222 Vilnius, Lithuania
2. Center for Physical Sciences and Technology, Savanorių 231, LT-02300
Vilnius, Lithuania
3. School
of Biological and Chemical
Sciences, Queen Mary University of London, Mile End Road, London E1 4TN, U.K.