  By path integral Monte Carlo simulations we study the phase diagram of two -
dimensional mesoscopic clusters formed by electrons in a semiconductor quantum
dot or by indirect magnetoexcitons in double quantum dots. At zero (or
sufficiently small) temperature, as quantum fluctuations of particles increase,
two types of quantum disordering phenomena take place: first, at small values
of quantum de Boer parameter q < 0.01 one can observe a transition from a
completely ordered state to that in which different shells of the cluster,
being internally ordered, are orientationally disordered relative to each
other. At much greater strengths of quantum fluctuations, at q=0.1, the
transition to a disordered (superfluid for the boson system) state takes place.
