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3. J.Powell, G.Danby: Cryogenics and Industrial Gases4, 19 (1969)
4. P.L.Richards, M.Tinkham: J. Appl. Phys.43, 2680 (1972)
5. L.Urankar, J.Miericke: J. Appl. Phys.44, 1907 (1973)