1. Cotti, P.: Phys. kondens. Materie3, 40 (1964).
2. Sellmaier, A.: Conf. Rep.: Low Temperatures and Electric Power, 24.?26. 3. 1969 (London).
3. Cairns, D. N. H., et al.: ebenda.
4. Minnich, S. H., Fox, G. R.: Cryogenics, June 1969, 165.
5. Graneau, P.: Cryogenic Eng. News, March 1969, 19.