  We introduce time-reversal-symmetry breaking by doping Cr atoms into the
topmost quintuple layer or into the bulk of Sb2Te3 thin films. We find that
even at a high Cr-doping level the Landau level spectrum keeps a good quality,
enabling the first demonstration of deviation of the zero-mode 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 novel magnetoelectric effects.
