  We theoretically examine the magnetoluminescence from a trion and a biexciton
in a type-II semiconductor quantum dot, in which holes are confined inside the
quantum dot and electrons are in a ring-shaped region surrounding the quantum
dot. First, we show that two electrons in the trion and biexciton are strongly
correlated to each other, forming a Wigner molecule: Since the relative motion
of electrons is frozen, they behave as a composite particle whose mass and
charge are twice those of a single electron. As a result, the energy of the
trion and biexciton oscillates as a function of magnetic field with half the
period of the single-electron Aharonov-Bohm oscillation. Next, we evaluate the
photoluminescence. Both the peak position and peak height change
discontinuously at the transition of the many-body ground state, implying a
possible observation of the Wigner molecule by the optical experiment.
