Twisted Singlet in Semiconductor Artificial Molecules

The spin-configuration of semiconductor artificial molecules consisting of quantum dots and spin field effect transistors is studied theoretically. We find that the antiferromagnetic spin configuration can be changed to the ferromagnetic one by applying a gate voltage to the spin field effect transistors. We show that the square-norm of the total spin of an artificial molecule oscillates with the twist angle $\theta$ and has maxima at $\theta=$ an odd integer times $\pi$. We also show that the square-norm of the total spin of the ring-shaped artificial molecule changes drastically at certain values of $\theta$ where the lowest two energy levels cross each other.