Charge correlation and spin coupling in double quantum dots

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. 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 a magnitude of the antiferromagnetic spin coupling decreases in two stages, from a strong coupling to a weak coupling, As the magnetic field increases, both the antiferromagnetic spin coupling constant and the two-electron occupation probability in the same dot are suppressed. In the case of weak lateral confinement, transitions of the ground state can be also induced by a magnetic field and they accompany changes of the spin coupling between antiferromagnetic and ferromagnetic couplings.