  The excitation spectrum of a two-electron quantum dot is investigated by
tunneling spectroscopy in conjuction with theoretical calculations. The dot
made from a material with negligible Zeeman splitting has a moderate spatial
anisotropy leading to a splitting of the two lowest triplet states at zero
magnetic field. In addition to the well-known triplet excitation at zero
magnetic field, two additional excited states are found at finite magnetic
field. The lower one is identified as the second excited singlet state on the
basis of an avoided crossing with the first excited singlet state at finite
fields. The measured spectra are in remarkable agreement with exact
diagonalization calculations. The results prove the significance of electron
correlations and suggest the formation of a state with Wigner-molecular
properties at low magnetic fields.
