Phonon-assisted spin-polarized tunneling through an interacting quantum dot

Using the nonequilibrium Green function technique we study theoretically spin-polarized transport in double-barrier tunneling junctions based on a single-level quantum dot interacting with a local phonon mode. Phonon emission and absorption spectra have been calculated for arbitrary Coulomb correlations on the dot and for different temperatures. It is shown that in the nonlinear response regime the electron-phonon interaction gives rise to current suppression in symmetric junctions as well as to oscillations of the tunnel magnetoresistance (TMR). In asymmetric junctions, the same mechanism may lead effectively to enhancement of the diode-like characteristics. We have also found that at sufficiently low temperatures additional phonon-induced resonance peaks appear in the linear spectral function on both sides of the main resonance peaks corresponding to the quantum dot energy levels. The case of negative effective charging energy is also analyzed numerically. A significant enhancement of electric current (or suppression of TMR) above the threshold bias voltages at which the dot energy level enters the tunneling window is observed. The gate voltage-controlled rectification effect of the tunneling current in asymmetric junctions with positive and negative effective Coulomb correlations is also discussed.