title: Excluded volume causes integer and fractional plateaus in colloidal ratchet currents.
abstract: We study the collective transport of paramagnetic colloids driven above a magnetic bubble lattice by an external rotating magnetic field. We measure a direct ratchet current which rises in integer and fractional steps with the field amplitude. The stepwise increase is caused by excluded volume interactions between the particles, which form composite clusters above the bubbles with mobile and immobile occupation sites. Transient energy minima located at the interstitials between the bubbles cause the colloids to hop from one composite cluster to the next with synchronous and period doubled modes of transport. The colloidal current may be polarized to make selective use of type up or type down interstitials.
authors.affiliation: Estructura i Constituents de la Matèria, Universitat de Barcelona, Avenida Diagonal 647, 08028 Barcelona, Spain and Institut de Nanociència i Nanotecnologia IN2UB, Universitat de Barcelona, 08028 Barcelona, Spain.
authors.forename: Pietro
authors.initials: P
authors.lastname: Tierno
authors.affiliation: Institute of Physics, Universität Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Fischer