A molecular state of correlated electrons in a quantum dot

Correlation among particles in finite quantum systems leads to complex behaviour and novel states of matter. One remarkable example is predicted to occur in a semiconductor quantum dot(1-3), where at vanishing electron density the Coulomb interaction between electrons rigidly fixes their relative positions as those of the nuclei in a molecule(4-14). In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character(15). In the weak correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here, we report evidence for roto-vibrationalmodes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by using inelastic light scattering(16-18) in quantum dots containing four electrons(19). Spectra of low-lying excitations associated with changes of the relative-motion wavefunction-the analogues of the vibration modes of a conventional molecule-do not depend on the rotational state represented by the total angular momentum. Theoretical simulations by the configuration-interaction method(20) are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation.