Absence of a Dirac gap in ferromagnetic Crx(Bi0.1Sb0.9)2−xTe3

Magnetism breaks the time-reversal symmetry expected to open a Dirac gap in 3D topological insulators that consequently leads to the quantum anomalous Hall effect. The most common approach of inducing a ferromagnetic state is by doping magnetic 3 d elements into the bulk of 3D topological insulators. In Cr 0.15 ( Bi 0.1 Sb 0.9 ) 1.85 Te 3, the material where the quantum anomalous Hall effect was initially discovered at temperatures much lower than the ferromagnetic transition, T C, the scanning tunneling microscopy studies have reported a large Dirac gap of ∼ 20 – 100 meV. The discrepancy between the low temperature of quantum anomalous Hall effect ( ≪ T C) and large spectroscopic Dirac gaps ( ≫ T C) found in magnetic topological insulators remains puzzling. Here, we used angle-resolved photoemission spectroscopy to study the surface electronic structure of the pristine and potassium doped surface of Cr 0.15 ( Bi 0.1 Sb 0.9 ) 1.85 Te 3. Upon potassium deposition, the p-type surface state of the pristine sample was turned into an n-type, allowing the spectroscopic observation of Dirac point. We find a gapless surface state, with no evidence of a large Dirac gap reported in tunneling studies.