1. R. C. Zeller, R. O. Pohl: Phys. Rev. B 4, 2029 (1971)
2. see for example: S. Hunklinger, A.K. Raychaudhur i: In Progress in Low-Temperature Physics, de. by D. F. Brewer, Vol. IX, p. 265 (Amsterdam, Oxford, New York, Tokyo: North-Holland 1986)
3. see for example: Amorphous Solids — Low-Temperature Properties, Topics in Current Physics, ed. by.W. A. Phillips, Vol. 24, p. 81, (Berlin, Heidelberg, New York: Springer 1981 )
4. 3. W. A. Phillips: J. Low Temp. Phys. 7 351 (1972)
5. 4. P. W. Anderson, B. I. Halperin, C. Varma: Phil. Mag. 1 (1972) V. G. Karpov, M. I. Klinger, F. N. Ignatev: Sov. Phys. JETP 57, 439 (1983);