1. Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA.
2. Institut für Theoretische Festkörperphysik, Universität Karlsruhe, 76128 Karlsruhe, Germany; Institute for Materials Research, Tohoku University, Sendai 980–8577, Japan; and Institute of Molecular Physics, Polish Academy of Sciences, 60–179 Poznań, Poland.