[1] Tech Univ Darmstadt, Eduard Zintl Inst, Alarich Weiss Str 8, D-64287 Darmstadt, Germany
[2] Comenius Univ, Fac Nat Sci, Dept Phys & Theoret Chem, Ilkovicova 6, Bratislava 84215, Slovakia
[3] Comenius Univ, Fac Nat Sci, Lab Adv Mat, Ilkovicova 6, Bratislava 84215, Slovakia
[4] Univ Vienna, Fac Phys, Vienna Ctr Quantum Sci & Technol VCQ, Boltzmanngasse 5, A-1090 Vienna, Austria