title: Nonequivalence of phonon modes in the sine-Gordon equation.
abstract: We study the resonances in the sine-Gordon equation driven by an ac force using a linear perturbation theory. We show that resonances take place when the driving frequency delta is equal to half of the phonon modes' frequencies as has been shown numerically in our earlier work [N. R. Quintero, A. Sanchez, and F. G. Mertens, Phys. Rev. E 62, R60 (2000)], however, we find that 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).
authors.affiliation: Grupo de Física No Lineal, Departamento de Física Aplicada I, Universidad de Sevilla, Ave. Reina Mercedes s/n, 41012, Sevilla, Spain. niurka@euler.us.es
authors.forename: N R
authors.initials: NR
authors.lastname: Quintero
authors.forename: P G
authors.initials: PG
authors.lastname: Kevrekidis