Co-tunneling effects on transport in the spin blockade regime

Using non-equilibrium Green´s function technique with auxiliary-particle operators, we study co-tunneling effects on transport through double quantum dots in the spin blockade regime. We construct the Luttinger-Ward functional by summing infinite number of Feynman diagrams under the non-crossing approximation and, thereby, tunneling currents are calculated in conservative ways for a finite Coulomb potential U. We find that co-tunneling results in Kondo resonance peaks along the chemical potential even in high bias voltages and leads to relatively large occupation probabilities of states which would be prohibited in sequential tunneling cases. As a result, conductance peaks are broadened and large leakage currents are found in the spin blockade regime, which may lead to errors in the spin read-out.