[1] Univ Stuttgart, Inst Phys 4, Pfaffenwaldring 57, D-70569 Stuttgart, Germany
[2] Univ Stuttgart, Res Ctr SCoPE, Pfaffenwaldring 57, D-70569 Stuttgart, Germany
[3] Max Planck Inst Intelligent Syst, Heisenbergstr 3, D-70569 Stuttgart, Germany
[4] Heidelberg Univ, Kirchhoff Inst Phys, Im Neuenheimer Feld 227, D-69120 Heidelberg, Germany