Magnetic field induced transitions in the few-electron ground states of artificial molecules

Few- and several-electron states in a single quantum dot and vertically quantum mechanically coupled double quantum dot structures are investigated at high magnetic (B-) fields up to 12 T in circular-shaped vertical single electron transistors. The phase diagrams showing the evolution with B-field of the Coulomb oscillation peaks reflecting the electro-chemical potential of the N-electron ground states, μ(N), for N from 0 to about 20 are compared, and the origin of the differences is discussed. The vertical coupling between two dots gives rise to a set of lateral bonding states and a set of lateral anti-bonding states, each set of which is composed of Darwin–Fock-like states arising from a two-dimensional harmonic confining potential. For the most strongly coupled double dot structure, only the bonding-states are occupied for N<12 at 0 T, and the stability of the spin-polarized maximum density droplet (MDD) at filling factor, ν=1, is much reduced compared to the single dot system. This is attributed to the effect of reduced Coulomb interactions and reduced lateral confinement in the larger volume occupied by the electrons in the double dot system. For the weakly coupled double dot structure, both bonding and anti-bonding states can be occupied, and no highly stable MDD is present because of frequent magnetic depopulation of anti-bonding states.