Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany; udo.schwingenschloegl(at)physik.uni-augsburg.de
Received 18 March 2004, accepted 26 March 2004 by U. Eckern
Institut für Theoretische Physik, Universität Zürich, 8057 Zürich, Switzerland; grigore(at)physik.unizh.ch, scharf(at)physik.unizh.ch
Received 26 September 2003, accepted 19 March 2004 by K. Fredenhagen
Center for Optical and Electromagnetic Research, Zhejiang University, Hangzhou 310027, P. R. China; jqshen(at)coer.zju.edu.cn / jqshencn(at)yahoo.com.cn
Received 3 March 2004, revised 13 April 2004, accepted 22 April 2004 by F. W. Hehl