[1] Jozef Stefan Int Postgrad Sch, SI-1000 Ljubljana, Slovenia
[2] Univ Ljubljana, Fac Elect Engn, Ljubljana, Slovenia
[3] Jozef Stefan Inst, Dept Condensed Matter Phys, Ljubljana, Slovenia
[4] Univ Maribor, Fac Nat Sci & Math, SLO-2000 Maribor, Slovenia