Impurity effects in few-electron quantum dots: Incipient Wigner molecule regime

Numerically exact path-integral Monte Carlo data are presented for $N\leq 10$ strongly interacting electrons confined in a 2D parabolic quantum dot, including a defect to break rotational symmetry. Low densities are studied, where an incipient Wigner molecule forms. A single impurity is found to cause drastic effects: 1) The standard shell-filling sequence with magic numbers N=4, 6, 9, corresponding to peaks in the addition energy $\Delta(N)$, is destroyed, with a new peak at N=8; 2) spin gaps decrease; 3) for N=8, sub-Hund's rule spin S=0 is induced; and 4) spatial ordering of the electrons becomes rather sensitive to spin. We also comment on the recently observed bunching phenomenon