Institution: | a Physics Department, CINVESTAV-IPN, Apdo. Postal 14-740, México D.F. 07000, Mexico b Univ Guadalajara, CUCEI, Dept Fis, Guadalajara 44430, Jalisco, Mexico c Electrical Engineering Department, Solid State Section, CINVESTAV-IPN, Apdo. Postal 14-740, México D.F. 07000, Mexico |