Electron-pair resonance in the Coulomb blockade

We study many-body corrections to the cotunneling current via a localized state with energy epsilon(d) at large bias voltages V. We show that the transfer of electron pairs, enabled by the Coulomb repulsion in the localized level, results in ionization resonance peaks in the third derivative of the current with respect to V, centered at eV = +/- 2 epsilon(d)/3. Our results predict the existence of previously unnoticed structure within Coulomb-blockade diamonds.