  The experimental realization of the quantum anomalous Hall (QAH) effect in
magnetically-doped (Bi, Sb)2Te3 films stands out as a landmark of modern
condensed matter physics. However, ultra-low temperatures down to few tens of
mK are needed to reach the quantization of Hall resistance, which is two orders
of magnitude lower than the ferromagnetic phase transition temperature of the
films. Here, we systematically study the band structure of V-doped (Bi, Sb)2Te3
thin films by angle-resolved photoemission spectroscopy (ARPES) and show
unambiguously that the bulk valence band (BVB) maximum lies higher in energy
than the surface state Dirac point. Our results demonstrate clear evidence that
localization of BVB carriers plays an active role and can account for the
temperature discrepancy.
