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
Topological insulators feature light-like robust surface conduction electrons that are protected by the time-reversal symmetry. It is predicted that the surface electrons can acquire a mass (massive Dirac fermion) by opening up a gap at the Dirac point when time-reversal symmetry is broken. Such a scenario requires out-of-plane magnetic order in the material. Here we report photoemission studies on a series of compounds of topological insulator single crystals, focusing on the electronic structure in the vicinity of the Dirac crossing region. Our results show that the photoemission spectral intensity is suppressed, resulting in a "gap"-like feature even in topological insulator systems without any magnetic impurities or doping, including in nominally stoichiometric systems suggesting similar spectral profiles as in the magnetically doped samples. The photoemission observed "gap" at the Dirac point thus cannot be taken as the sole evidence of a magnetic gap, and the issue of \textbf{time-reversal symmetry breaking gap} (magnetically massive Dirac fermions) on the surface of a topological insulator remains open to this date. In magnetically doped samples the real gap is likely masked by the non-magnetic spectral weight suppression we report. We discuss a few possible origins of the observed Dirac point spectral weight suppression ("gap") where neither bulk nor surface magnetism is present in the material.
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http://arxiv.org/abs/1206.0278
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