1. Faculty of Physics, Vilnius University, Sauletekio Avenue 9, LT-10222 Vilnius, Lithuania
2. Semiconductor Physics Institute, Center for Physical Sciences and Technology, Sauletekio Avenue 3, LT-10257 Vilnius, Lithuania
3. CNR-SPIN, c/o Dipartimento di Scienze Fisiche e Chimiche—Università degli Studi dell’Aquila, Via Vetoio, 67100 Coppito, L’Aquila, Italy
4. Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box
1410, PL-50-950 Wroclaw 2, Poland