1. Department of Physics, University of California, Berkeley, CA 94720, USA.
2. U.S. Army Research Laboratory, Adelphi, MD 20783, USA.
3. Center for High Technology Materials and Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87106, USA.
4. Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany.
5. Helmholtz-Institut, GSI Helmholtzzentrum für Schwerionenforschung, 55128 Mainz, Germany.