Comment on "pi kinks in strongly ac driven sine-Gordon systems".
V. Zharnitsky, I. Mitkov, and N. Gronbech-Jensen [Phys. Rev. E 58, 1, R52 (1998)] found that pi kinks can propagate in strongly perturbed, directly driven rescaled sine-Gordon system provided that the parameters are chosen to make 2pi kink localization vanish. In this paper we would like to note that beside pi and 2pi kinks there can exist other kinklike solutions due to the fact that two unstable equilibria in the sine-Gordon phase emerging at a critical value of the drive amplitude are not necessarily separated by pi, to the contrary with the result of Zharnitsky, Mitkov, and Gronbech-Jensen. As a result, for the nondissipative system two one-parameter families of kink solutions exist that in the degenerate case become a one-parameter family of pi-kink solutions obtained in Zharnitsky, Mitkov, and Gronbech-Jensen. In the dissipative case velocity is selected for each of the two families of kink solutions by the balance between perturbations.
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