Electron spin flip by antiferromagnetic coupling between semiconductor quantum dots

We have investigated antiferromagnetic coupling between semiconductor quantum dots. Electron spin is observed to flip at 80 ps after photoexcitation via the interdot-exchange interaction. The spin relaxation time under the antiferromagnetic order is extended to 10-12 ns, one order of magnitude longer than that in isolated quantum dots. The antiferromagnetic order exists at temperatures lower than 50-80 K. The photoluminescence experiments for various carrier densities show that antiferromagnetic coupling disappears when the electron pairing probability is low. A model calculation based on the Heitler-London approximation supports the finding that the antiferromagnetic order is observable at low temperature.