Discontinuities in the level density of small quantum dots under strong magnetic fields

Exact diagonalization studies of the level density in a six-electron quantum dot under magnetic fields around 7 T (“filling factor” around $\frac{1}{2})$ are reported. In any spin-polarization channel, two regimes are visible in the dot excitation spectrum: one corresponding to interacting quasiparticles (i.e., composite fermions) for excitation energies below 0.4 meV, and a second one for energies above 0.4 meV, in which the level density (exponentially) increases at the same rate as in the noninteracting composite-fermion model.