title: Comment on "Soliton ratchets induced by excitation of internal modes".
abstract: Very recently Willis [Phys. Rev. E 69, 056612 (2004)] have used a collective variable theory to explain the appearance of a nonzero energy current in an ac-driven, damped sine-Gordon equation. In this Comment, we prove rigorously that the time-averaged energy current in an ac-driven nonlinear Klein-Gordon system is strictly zero.
authors.affiliation: Departamento de Física Aplicada I, E. U. P., Universidad de Sevilla, Virgen de Africa 7, 41011 Sevilla, Spain. niurka@euler.us.es
authors.forename: Niurka R
authors.initials: NR
authors.lastname: Quintero
authors.forename: Bernardo
authors.initials: B
authors.lastname: Sánchez-Rey
authors.forename: Jesús
authors.initials: J
authors.lastname: Casado-Pascual