Dirac point spectral weight suppression and surface "gaps" in   nonmagnetic and magnetic topological insulators
  It is predicted that electrons on the surface of a topological insulator can acquire a mass (massive Dirac fermion) by opening up a gap at the Dirac point when time-reversal symmetry is broken via the out-of-plane magnetization. We report photoemission studies on a series of topological insulator materials focusing on the spectral behavior in the vicinity of the Dirac node. Our results show that the spectral intensity is suppressed resulting in a "gap"-like feature in materials with or without any magnetic impurity or doping. The Zeeman gap in magnetically doped samples, expected to be rather small, is likely masked by the non-magnetic strong spectral weight suppression involving a large energy scale we report. The photoemission spectral weight suppression observed around the Dirac node thus cannot be taken as the sole evidence for a time-reversal symmetry breaking magnetic gap. We discuss a few possible extrinsic and kinematic origins of the Dirac point spectral weight suppression ("gap") observed in many commonly studied topological materials. 
Su-Yang Xu, L. A. Wray, N. Alidoust, Y. Xia, M. Neupane, Chang Liu,   H.-W. Ji, S. Jia, R. J. Cava, M. Z. Hasan
cond-mat.mes-hall cond-mat.mtrl-sci