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Evidence of the molecular state of correlated electrons in a quantum dot

01 June 2008

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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 dot1-3 (QD) where at vanishing density the Coulomb correlation among electrons rigidly fixes their relative position as that of the nuclei in a molecule4-14. In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relativemotion character15. In the weak-correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here we report evidence of roto-vibrational modes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by inelastic light scattering16- 18 in QDs containing four electrons19. Spectra of low-lying excitations associated to changes of the relative-motion wave function -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 via the configuration-interaction (CI) method20 are in agreement with the observed rotovibrational modes and indicate that such molecular excitations develop at the onset of shortrange correlation.