[1] Univ Penn, David Rittenhouse Lab, Dept Phys, Philadelphia, PA 19104 USA
[2] Univ Penn, David Rittenhouse Lab, Dept Math, Philadelphia, PA 19104 USA
[3] Univ Hamburg, Zentrum Math Phys, D-20146 Hamburg, Germany
[4] Univ Hamburg, Inst Theoret Phys 2, D-22761 Hamburg, Germany