Nonequivalence of phonon modes in the sine-Gordon equation - art. no. 056608

We study the resonances in the sine-Gordon equation driven by an ac force u sing a linear perturbation theory. We show that resonances take place when the driving frequency delta is equal to half of the phonon modes' frequenci es as has been shown numerically in our earlier work [N. R. Quintero, A. Sa nchez, and F. G. Mertens, Phys. Rev. E 62, R60 (2000)], however, we find th at the ac force is able to excite not all the phonon modes, but rather only the odd phonons (i.e., the ones with odd eigenfunctions).