1. Department of Physics, University of the Free State, QwaQwa Campus, Private Bag X13, Phuthaditjhaba 9866; South Africa
2. Department of Physics, Addis Ababa University, P. O. Box 1176, Addis Ababa; Ethiopia
3. R and D core (Energy initiatives), Council for Scientific and Industrial Research, PO Box 395, Pretoria; South Africa