  It has been experimentally shown that negative exchange interactions can
arise in a linear three-dot system when a two-electron double quantum dot is
exchange 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-like rather than singlet-like ground
state. Here, we show using a microscopic model based on the configuration
interaction (CI) method that both triplet-like and singlet-like ground states
are realized depending on the number of electrons. In the case of only four
electrons, a full CI calculation reveals that triplet-like 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.
