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Effects of electron-phonon coupling in the Kondo regime of a two-orbital molecule

We study the interplay between strong electron-electron and electron-phonon interactions within a two-orbital molecule coupled to metallic leads, taking into account Holstein-like coupling of a local phonon mode to the molecular charge as well as phonon-mediated interorbital tunneling. By combining canonical transformations with numerical renormalization-group calculations to address the interactions nonperturbatively and on equal footing, we obtain a comprehensive description of the system's many-body physics in the anti-adiabatic regime where the phonons adjust rapidly to changes in the orbital occupancies, and are thereby able to strongly affect the Kondo physics. The electron-phonon interactions strongly modify the bare orbital energies and the Coulomb repulsion between electrons in the molecule, and tend to inhibit tunneling of electrons between the molecule and the leads. The consequences of these effects are considerably more pronounced when both molecular orbitals lie near the Fermi energy of the leads than when only one orbital is active. In situations where a local moment forms on the molecule, there is a crossover with increasing electron-phonon coupling from a regime of collective Kondo screening of the moment to a limit of local phonon quenching. At low temperatures, this crossover is associated with a rapid increase in the electronic occupancy of the molecule as well as a marked drop in the linear electrical conductance through the single-molecule junction.