1. H. Kallman and K. Luchner, Phys. Rev.,123, 2013 (1961);129, 593 (1963).
2. V. V. Antonov-Romanovskii, Izv. Akad. Nauk SSSR, Ser. Fiz.,9, 369 (1945).
3. K.-S. K. Rebane, Trudy IFA Akad. Nauk ÉSSR, No. 4, 81 (1956)7
4. A. K. Aidla and Ya. Ya. Kirs, Izv. Akad. Nauk SSSR, Ser. Fiz.,33, 958 (1969).
5. V. L. Levshin, V. V. Mikhailin, and V. V. Nizovtsev, Izv. Akad. Nauk SSSR, Ser. Fiz.,33, 963 (1969).