Lifting of spin blockade by hyperfine interaction in vertically coupled double quantum dots

The ability of the hyperfine interaction to lift spin blockade was studied for vertically coupled double quantum dots in two different regimes of potential detuning between the two dots. The double dot has a triplet state, which is a sufficiently long-lived excited state to block current flow by the Pauli effect. This blockade is lifted by a spin flip transition to the singlet state, generating a leakage current. The singlet-triplet separation or exchange energy decreases with increasing detuning. For small detuning, the leakage current shows a step when the Zeeman energy equals the exchange energy thus turning on the flip-flop interaction. The threshold magnetic field gradually increases with decreasing detuning. It increases more abruptly near the resonance of two singlet states reflecting the increased exchange energy. For large detuning, the leakage current is caused by singlet-triplet mixing due to the fluctuating nuclear field, and it decreases when the Zeeman energy exceeds the fluctuating nuclear field energy. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.