[1] Ben Gurion Univ Negev, Dept Mech Engn, IL-84105 Beer Sheva, Israel
[2] Technion Israel Inst Technol, Dept Phys, IL-32000 Haifa, Israel
[3] Northwestern Univ, Dept Phys & Astron, Evanston, IL 60208 USA
[4] Latvian State Univ, Inst Solid State Phys, LV-1063 Riga, Latvia
[5] UCL, Dept Phys & Astron, Ctr Mat Sci, London WC1E 6BT, England
[6] Max Planck Inst Festkorperforsch, D-70569 Stuttgart, Germany