  Boson-Fermion mixture confined on an optical lattice is numerically studied.
Mott-plateau like behavior is observed under certain conditions in the density
profile of the bosons and fermions as one sees in bosonic atoms in a trap
potential. In the Mott plateau area, the total number of the bosons and
fermions at each site is kept to an integer value but both local density of the
bosons and that of the fermions take some intermediate values. Unlike bosonic
or fermionic Mott insulators, the bosons and fermions can move even in the Mott
plateau area by exchanging their positions. With Monte Carlo simulations we
demonstrate the formation of the Mott plateau, examine the property of the
bosons and fermions by measuring the densities and local density fluctuations,
and give discussion on this rather new state.
