  Molecular states in a SINGLE PAIR of strongly coupled self-assembled InAs
quantum dots are investigated using a sub-micron sized single electron
transistor containing just a few pairs of coupled InAs dots embedded in a GaAs
matrix. We observe a series of well-formed Coulomb diamonds with charging
energy of less than 5 meV, which are much smaller than those reported
previously. This is because electrons are occupied in molecular states, which
are spread over both dots and occupy a large volume. In the measurement of
ground and excited state single electron transport spectra with magnetic field,
we find that the electrons are sequentially trapped in symmetric and
anti-symmetric states. This result is well-explained by numerical calculation
using an exact diagonalization method.
