[1] Boston Univ, Dept Phys, Boston, MA 02215 USA
[2] Univ Jena, Inst Festkorpertheorie & Opt, D-07743 Jena, Germany
[3] Univ Jena, ETSF, D-07743 Jena, Germany
[4] Tech Univ Darmstadt, Inst Mat Sci, D-64287 Darmstadt, Germany
[5] Univ Oxford, Chem Res Lab, Oxford OX1 3TA, England