1211.7042 (Anders Mathias Lunde et al.)

Temperature dependent dynamical nuclear polarization bistabilities in
double quantum dots in the spin-blockade regime    [PDF]

Anders Mathias Lunde, Carlos López-Monís, Ioanna A. Vasiliadou, Luis L. Bonilla, Gloria Platero

The interplay of dynamical nuclear polarization (DNP) and leakage current through a double quantum dot in the spin-blockade (SB) regime is analyzed. A finite DNP is build up due to a competition between hyperfine (HF) spin-flip transitions and another inelastic escape mechanism from the triplets blocking transport. We focus on the temperature dependence of the DNP in the zero energy-detuning configuration (i.e. zero electrostatic energy difference between having one electron in each dot and a singlet in the right dot). We show that below a certain transition temperature, the DNP becomes bistable, which produces a hysteretic current versus external magnetic field B. The bistability and its consequences are studied in two cases. (i) Close to the crossing of the three triplet energy levels near B=0. In this case, the SB of the triplet with zero angular momentum is lifted by the inhomogeneity of the effective magnetic field from the nuclei. (ii) At higher B, where the two spin-polarized triplets simultaneously cross two different singlet energy levels. We develop simplified models leading to different transition temperatures T_TT and T_ST for the crossing of the triplets and the singlet-triplet crossings, respectively. We find T_TT to be given only by the HF couplings, whereas T_ST depends on various parameters. Moreover, T_ST is larger than T_TT for experimental parameters. For both kinds of crossings, the transition temperatures stem from the fact that the HF transitions are phonon mediated. Specifically, we point out that the exponential suppression of phonon absorption compared to emission in a HF transition is behind the existence of transition temperatures at zero energy-detuning. Finally, by comparing the rate equation results with Monte Carlo simulations, we discuss the importance of having both HF interactions and another escape mechanism from the triplets to induce finite DNP.

View original: http://arxiv.org/abs/1211.7042