1. G. Aeppli, Z. Fisk, Comments Condens. Matter Phys. 16, 155 (1992)
2. L. Degiorgi, Rev. Mod. Phys. 71, 687 (1999)
3. P. Wachter, in Handbook on the Physics and Chemistry of Rare Earths, edited by K.A. Gschneidner Jr, L. Eyring, G.H. Lander, G.R. Choppin (Elsevier, Amsterdam 1994), Vol. 19
4. P.S. Riseborough, Adv. Phys. 49, 257 (2000)
5. C.H. Park, Z.X. Shen, A.G. Loeser, D.S. Dessau, D.G. Mandrus, A. Migliori, J. Sarrao, Z. Fisk, Phys. Rev. B. 52, 16981(R) (1995)