Dipartimento di Fisica, Universita di Bologna, Via Irnerio 46, 40126 Bologna, Italy; appignani(at)bo.infn.it
Received 23 July 2003, accepted 12 March 2004 by G. Röpke
Department of Physics, Faculty of Science, King Kalid University, Abha, P.O. Box 9003, Saudi Arabia; sbougouffa(at)kku.edu.sa
Received 1 October 2003, accepted 12 March 2004 by G. Röpke
Center for Optical and Electromagnetic Research, Zhejiang University, Hangzhou 310027, P. R. China; jqshen(at)coer.zju.edu.cn
Received 12 November 2003, accepted 13 February 2004 by U. Eckern
Centre for Liquid Crystal Research and Education, Faculty of Physical Sciences, Nagarjuna University, Nagarjuna Nagar-522 510, A. P., India; durga_ojha(at)hotmail.com
Received 4 November 2003, accepted 23 March 2004 by U. Eckern