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In the first part of the talk, I will propose a transport experiment of excitons in 2d transition-metal dichalcogenides. Due to their interactions with unbound electrons in the Fermi sea, excitons are dressed by a cloud of particle-hole excitations forming Fermi polarons. An in-plane electric field cannot directly accelerate neutral excitons, however, due to the polaronic dressing, the unbound electrons will drag the excitons along. This phenomenon is similar to Coulomb drag in bilayer semiconductors. We determine the drag conductivity for a low density of excitons using simple arguments as well as a diagrammatic calculation based on a self-consistent T matrix approximation. Due to the strong nonperturbative coupling, the exciton drag mobility is expected to be of the same order as the electron mobility even at zero temperature. Time permitting, I will discuss a novel probe of quantum Hall ferromagnetism in graphene based on thermal transport in the second part of the talk. Strong interactions lift the high degeneracy of the zero-energy Landau level in graphene mono- and bilayers, giving rise to a rich phase diagram of competing spin and valley order. Here we show that thermal transport at low temperatures can give compelling evidence for spontaneous symmetry breaking and exhibits many other signatures distinguishing the various phases.

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Falko Pientka, Harvard University
Seminar Room 0.03, ETP
Contact: not specified