1. S. V. Pan’kova, V. V. Poborchii, and V. G. Solov’ev, J. Phys.: Condens. Matter 8, L203 (1996).
2. E. Colla, A. V. Fokin, and Yu. A. Kumzerov, Solid State Commun. 103, 127 (1997).
3. C. Tien, E. V. Charnaya, S. V. Baryshnikov, M. K. Lee, S. Y. Sun, D. Michel, and W. Böhlmann, Fiz. Tverd. Tela (St. Petersburg) 46(12), 2224 (2004) [Phys. Solid State 46 (12), 2301 (2004)].
4. S. V. Baryshnikov, E. V. Charnaya, Cheng Tien, D. Michel, N. P. Andriyanova, and E. V. Stukova, Fiz. Tverd. Tela (St. Petersburg) 49(4), 751 (2007) [Phys. Solid State 49 (4), 791 (2007)].
5. I. V. Golosovsky, A. V. Fokin, Yu. A. Kumzerov, A. I. Kurbakov, A. A. Naberezhnov, N. M. Okuneva, and S. B. Vakhrushev, Phys. Rev. Lett. 89, 175503–1 (2002).