Excitonic physics in a Dirac quantum dot

We present a description of vacuum polarization in a circular Dirac quantum dot in two spatial dimensions assuming $\alpha$ - the relative strength of the Coulomb interaction small enough to render an approximation with a single electron (hole) lowest energy level relevant. Applying this approximation, we find that for $\alpha_c \approx 1.05$ the lowest level is half-filled irrespective of the number of flavors that are present. The ground state can be represented as a superposition of particular (even number) excitonic states which constitute an excitonic cloud that evolves in a crossover manner. The ground state is degenerate with an intervalley excitonic state at $\alpha_c \approx 1.05$, a critical strength, that in our approximation marks a point with single electron and exciton resonances.