1. Carr V M, Chadwick A V and Saghafian R 1978J. Phys. C11 L637
2. Chou Y C and Jaw T S 1988Solid State Commun. 67 753
3. Dekker A J 1986Solid state physics (Madras: Macmillan) pp. 133–148
4. Hutchings M T, Clausen K, Dickens M H, Hayes W, Kjems J K, Schnabel P G and Smith C 1984J. Phys. C17 3903
5. Kosacki I and Zalibowski K 1988Acta Phys. Pol. A73 403