Charge correlation and spin coupling in double quantum dots: A quantum diffusion Monte Carlo study

We study both the spin and the charge correlations between two electrons in vertically coupled quantum dots with a finite well width, using a diffusion quantum Monte Carlo method. We clarify the mechanism of their dependence on both a barrier thickness and a magnetic field, on the basis of the Hubbard model analysis. When the barrier thickness increases, two-electron occupation probability in the same dot is suppressed drastically by enhanced charge correlation relative to the electron transfer effect, and the magnitude of the antiferromagnetic spin coupling decreases in two stages from a strong coupling region to a weak coupling region. Both the antiferromagnetic spin coupling constant and the two-electron occupation probability in the same dot are suppressed by magnetic field, being caused by enhanced intradot Coulomb interaction. In the case of weak lateral confinement, transition of the ground state can also be induced by a magnetic field and it accompanies changes of the spin coupling between antiferromagnetic and ferromagnetic couplings.