1. P. Coleman, C. Pepin, Q. Si, and R. Ramazashvili, J. Phys.: Condens. Matter 13, R723 (2001).
2. J. W. Clark, V. A. Khodel, and M. V. Zverev, Phys. Rev. B 71, 012401 (2004).
3. Z. Hossain, C. Geibel, F. Weickert, et al., Phys. Rev. B 72, 094411 (2005).
4. H. Q. Yuan, M. Nicklas, Z. Hossain, et al., Phys. Rev. B 74, 212403 (2006).
5. V. A. Khodel and V. R. Shaginyan, Pis’ma Zh. Éksp. Teor. Fiz. 51, 488 (1990) [JETP Lett. 51, 553 (1990)]; Condens. Matter Theor. 12, 221 (1997); V. A. Khodel, V. V. Khodel, and V. R. Shaginyan, Phys. Rep. 249, 1 (1994).