We report on the mass acquisition of Dirac fermions by doping Cr into the topmost quintuple layer
or into the bulk of Sb2Te3 topological insulator films. By careful investigation of the scanning tunneling
microscopy/spectroscopy on the films, we find that the Landau level spectrum keeps a good quality even at a high
Cr-doping level, enabling a demonstration of deviation of the zeroth Landau level, induced by the acquisition of
a mass term in the surface states in the presence of surface or bulk magnetic doping. The magnitude of the mass
term in the surface states increases with increasing Cr-doping level. Our observation suggests Cr-doped Sb2Te3
is a promising candidate for the realization of the proposed magnetoelectric effects.