1. T. Nattermann and J. Villain, Phase Transit. 11, 5 (1988).
2. E. B. Kolomeisky, Ferroelectrics 105, 131 (1990).
3. E. B. Kolomeiskii, A. P. Levanyuk, and A. S. Sigov, Izv. Akad. Nauk SSSR, Ser. Fiz. 54, 618 (1990).
4. S. V. Maleev, Zh. Éksp. Teor. Fiz. 70, 2375 (1976) [Sov. Phys. JETP 43, 1240 (1976)].
5. S. A. Gridnev, V. V. Gorbatenko, and B. N. Prasolov, Kristallografiya 39, 106 (1994) [Crystallogr. Rep. 39, 94 (1994)].