Competing mechanisms for singlet-triplet transition in artificial molecules

We study the magnetic field induced singlet/triplet transition for two electrons in vertically-coupled quantum dots by exact diagonalization of the Coulomb interaction. We identify the different mechanisms occurring in the transition, involving either in-plane correlations or localization in opposite dots, depending on the field direction. Therefore, both spin and orbital degrees of freedom can be manipulated by field strength and direction. The phase diagram of realistic devices is determined.