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

Abstract

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.