[1] Swiss Fed Inst Technol, Inst Theoret Phys, Wolfgang Pauli Str 27, CH-8093 Zurich, Switzerland
[2] Univ Geneva, Dept Theoret Phys, Quai Ernest Ansermet 30, CH-1205 Geneva, Switzerland
[3] Univ Cologne, Inst Theoret Phys, Zulpicher Str 77, D-50937 Cologne, Germany