Institut für Theoretische Physik, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany; paes(at)physik.uni-wuerzburg.de
Received 1 July 2002, revised 31 July 2002, accepted 1 August 2002
by F. W. Hehl
(review article; approx. 25 pages)
(1) Institute of Applied Physics, University of Münster, 48149 Münster, Germany; (2) Institute of Applied Physics, TU Darmstadt, 64289 Darmstadt, Germany; weilnau(at)uni-muenster.de
Received 31 July 2002, accepted 14 August 2002
by U. Eckern
(review article; approx. 63 pages)