Negative exchange interactions in few-electron double quantum dots

In a recent experiment, it was shown that negative exchange interactions can arise when a single-electron quantum dot is coupled to a larger quantum dot containing on the order of one hundred electrons. The origin of this negative exchange can be traced to the larger quantum dot exhibiting a spin triplet rather than singlet ground state. Here, we show using a microscopic model based on the configuration interaction (CI) method that both triplet and singlet ground states are realized depending on the number of electrons. In the case of only four electrons, a full CI calculation reveals that triplet ground states occur for sufficiently large dots. These results hold for symmetric and asymmetric quantum dots in both Si and GaAs, showing that negative exchange interactions are robust in few-electron double quantum dots and do not require large numbers of electrons.