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.
@article{arxiv.1206.0278,
title = {Dirac point spectral weight suppression and surface "gaps" in nonmagnetic and magnetic topological insulators},
author = {Su-Yang Xu and L. A. Wray and N. Alidoust and Y. Xia and M. Neupane and Chang Liu and H. -W. Ji and S. Jia and R. J. Cava and M. Z. Hasan},
journal= {arXiv preprint arXiv:1206.0278},
year = {2012}
}