Magnetic quantum ratchet in graphene and ratchet effects in graphene with a lateral superlattice

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Ratchet effects describe how the periodic motion of particles in a potential with broken symmetry, and subjected to periodic excitation, is converted to directed motion. Here we plan to get insight into the origin of ratchet effects in graphene and to apply them to study the electron dynamics in graphene. This will be studied by considering two model systems. First, the magnetic quantum ratchet, where the combined action of broken symmetry perpendicular to the graphene plane, an in plane magnetic field and excitation with terahertz radiation leads to a dc electric current, as we have shown in recent experiments. While this phenomenon is unambiguously demonstrated experimentally, it is not yet fully understood. We therefore plan to investigate in detail the origin of magnetic quantum ratchet effects in graphene and to apply them to study the symmetry, anisotropy and electron dynamics in this material. The second model system is graphene superimposed by a non-symmetric periodic lateral potential, i.e., breaking the symmetry in the graphene plane. Our recent experiments on THz radiation induced ratchet effect in graphene and semiconductor heterostructures with a one-dimensional lateral potential suggest that lateral superlattices should also lead to ratchet effects in graphene. We plan to investigate terahertz radiation induced ratchet effects in graphene with a periodic modulation systematically and to develop a microscopic picture of this effect. Our investigations should also provide a scientific basis for novel optoelectronic devices for THz radiation.