1. Hunklinger, S., Raychaudhuri, A.K.: In: Progress in low-temperature physics. Vol. IX, Brewer, D.F. (ed.). Amsterdam: Elsevier 1986
2. Anderson, P.W., Halperin, B.I., Varma, C.M.: Philos. Mag.25, 1 (1972); Philips, W.A.: J. Low Temp. Phys.7, 351 (1972)
3. Harrison, J.P., Peressini, P.P., Pohl, R.O.: Phys. Rev. B171, 1037 (1968); Wang, X., Bridges, F.: Phys. Rev. B46, 5122 (1992)
4. Enss, C.: Ph.D. Thesis, Heidelberg 1991 (unpublished)
5. Tornow, M.: Diplomarbeit Heidelberg 1993 (unpublished)