We investigate Pauli spin blockade and the influence of hyperfine interaction for a system of six-electrons in a device composed of two weakly coupled vertical quantum dots. As a consequence of a large built-in offset energy between the dots, the relevant charge configurations consist of one (zero) and five (six) electrons on the upstream dot and downstream dot respectively. On sweeping the out-of-dot-plane magnetic field up and down, abrupt steps and hysteresis in the leakage current are observed in the Pauli spin blockade region that evolve systematically with bias. We propose a model to explain our observations in terms of hyperfine induced singlet-triplet mixing. © 2011 The Physical Society of Japan.
We use a vertical double quantum dot (QD) to study spin blockade (SB) for the two-to five-electron states. SB observed for the two- and four-electron states is both assigned to Pauli exclusion with formation of a spin triplet state, and lifted by singlet-triplet admixing due to fluctuating nuclear field. SB observed for the five-electron state is caused by combined Pauli effect and Hund's rule. We observe a hysteretic behavior of the SB leakage current for up and down sweep of magnetic field, and argue that SB and its lifting by hyperfine interaction are subtle with the spin configuration and modified depending on the inter-dot detuning and number of electrons.
We use Pauli-spin blockade in two-electron vertical double quantum dots to quantitatively estimate the exchange energy
J in a wide range of interdot level detuning $\ensuremath{\Delta}$ and fully compare it with calculations. Pauli-spin blockade is lifted via a singlet- (
S-)triplet (
T) transition mediated by hyperfine coupling, which abruptly occurs in our devices when the $S\mathrm{\text{\ensuremath{-}}}T$ transition energy or
J is compensated by the Zeeman energy. We use this feature to derive
J depending on $\ensuremath{\Delta}$ between the $S\mathrm{\text{\ensuremath{-}}}S$ and $T\mathrm{\text{\ensuremath{-}}}T$ resonances. The obtained
J versus $\ensuremath{\Delta}$ including the resonance effect is perfectly reproduced by Hubbard model calculations.