1. N. Arpiarian and D. Curie, C.R. Acad. Sci. Paris,234, 75, 1952.
2. G.F.J. Garlick and M.J. Dumbleton, Proc. Phys. Soc.,B67, 442, 1954.
3. W. Hoogenstraaten, Philips Res. Repts.13, 515, 1958.
4. I. Broser, H.E. Cumblich and R. Moser, Physik Verhandl.,3, 227, 1963; H.E. Gumlich Meeting.
5. H.E. Gumlich and H.J. Schulz, J. Phys. Chem. Solids.,27, 187, 1966.