[1] Czech Tech Univ, Inst Expt & Appl Phys, CZ-12800 Prague 2, Czech Republic
[2] Inst Max Von Laue Paul Langevin, F-38042 Grenoble 9, France
[3] Tech Univ Munich, Phys Dept E18, D-85748 Garching, Germany
[4] Czech Metrol Inst, CZ-10227 Prague 10, Czech Republic