[1] Charles Univ Prague, Fac Math & Phys, CZ-12116 Prague, Czech Republic
[2] ASCR, Vvi, Inst Phys, CZ-16253 Prague 6, Czech Republic
[3] SUNY Buffalo, Dept Phys, Buffalo, NY 14260 USA
[4] Univ Sao Paulo, Inst Fis Sao Carlos, BR-13560970 Sao Carlos, SP, Brazil
[5] Texas A&M Univ, Dept Phys, College Stn, TX 77843 USA
[6] Tech Univ Ostrava, Dept Phys, CZ-70833 Ostrava, Czech Republic
[7] Tech Univ Ostrava, Nanotechnol Ctr, CZ-70833 Ostrava, Czech Republic
[8] Univ Nottingham, Sch Phys & Astron, Nottingham NG7 2RD, England