Phonon-assisted Andreev reflection in a hybrid junction based on a quantum dot
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Spin-dependent tunneling through a quantum dot coupled to one ferromagnetic and one superconducting electrodes is investigated in the Andreev reflection (AR) regime occurring in the presence of the on-dot electron-phonon interactions. Current-voltage characteristics of the system are evaluated within the nonequilibrium Green function technique. Features of the AR current due to interplay between the electron-phonon interactions, intradot Coulomb correlations and the polarization of the ferromagnetic electrode are analyzed in both linear and nonlinear transport regimes. It is shown that for the case of electron-hole symmetry, the phonon resonances may appear on both sides of the main elastic resonances in spectral function. A phonon-induced renormalization of the Andreev transmission levels is found and splitting of the phonon side bands for nonvanishing intradot Coulomb repulsion is observed. It is shown that linear conductance exhibits the polaron shift and suppression in the presence of the polaron transmission. In nonequilibrium situation the mechanism of the negative differential conductance (NDC) in the presence of competing magnetic electrode polarisation and the Coulomb correlations on the dot is analyzed. An influence of the polaron transmission through the Andreev intradot states on the NDC oscillations is also discussed.