  Spectral properties of the two-dimensional Bose-Hubbard model, which emulates
ultracold gases of atoms confined in optical lattices, are investigated by
means of the variational cluster approach. The phase boundary of the quantum
phase transition from Mott phase to superfluid phase is calculated and compared
to recent work. Moreover the single-particle spectral functions in both the
first and the second Mott lobe are presented and the corresponding densities of
states and momentum distributions are evaluated. A qualitatively similar
intensity distribution of the spectral weight can be observed for spectral
functions in the first and the second Mott lobe.
