We present experimental and theoretical results on a new regime in quantum dots in which the filling factor two-singlet state is replaced by new spin polarized phases. We make use of spin blockade spectroscopy to identify the transition to this new regime as a function of the number of electrons. The key experimental observation is a reversal of the phase in the systematic oscillation of the amplitude of Coulomb blockade peaks as the number of electrons is increased above a critical number. It is found theoretically that correlations are crucial to the existence of the new phases.

We present ground-state calculations for laterally coupled quantum dots containing 2, 4, and 8 electrons. As our emphasis is on spin effects our results are obtained by applying spin-density functional theory (SDFT). By varying the distance between the centers of the coupled quantum dots, the transition from weak to strong coupling situation is realized. For the 2-electron system we also apply the Heitler-London approximation and analytical concepts to check the reliability of SDFT calculations in this case. In addition we discuss the features of the Coulomb staircase of laterally coupled quantum dots in the weak and strong coupling regimes in comparison to that of a circular parabolic quantum dot.Comment: 13 pages, 9 figures on 2 page

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A theory of electronic properties of a spin-singlet quantum Hall droplet at filling factor ν = 2 in a parabolic quantum dot is developed. The excitation spectrum and the stability of the droplet due to the transfer of electrons into the second Landau level at low magnetic fields and due to spin flip at the edge at higher magnetic fields is determined using Hartree-Fock, exact diagonalisation, and spin-density functional methods. We show that above a critical number of electrons N c the unpolarised ν = 2 quantum Hall droplet ceases to be a ground state in favor of spin-polarised phases. We determine the characteristic pattern in the addition and current-amplitude Coulomb blockade spectra associated with the stable ν = 2 droplet. We show that the spin transition of the droplet at a critical number of electrons is accompanied by the reversal of the current amplitude modulation at the ν = 2 line, as observed in recent experiments.