Excitation spectroscopy of few-electron states in artificial diatomic molecules

We study the excitation spectroscopy of few-electron parallel coupled double quantum dots (DQDs). By applying a finite source drain voltage to a DQD, the first excited states observed in nonequilibrium charging diagrams can be classified into two kinds in terms of the total effective electron number in the DQD, assuming a core filling. When there are an odd (even) number of electrons, a one- (two-) electron antibonding (triplet) state is observed as the first excited state. On the other hand, at a larger source drain voltage, we observe higher excited states where additional single-particle excited levels are involved. Eventually, we identify the excited states with a calculation using the Hubbard model and, in particular, we elucidate the quadruplet state, which is normally forbidden by the spin blockade caused by the selection rule.