heterostructure*

A magnetic gap at the Dirac point of topological insulators has so far never been observed directly. Here, we use low temperature ARPES to reveal the magnetic gap of Mn-doped Bi₂Te₃ films which is present only below T_C of 10-12 K. Surprisingly, the gap is 5 times larger than predicted by density functional theory. We show that this enhancement is due to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, it forms an alternating sequence of septuple and quintuple layer blocks, with Mn in the center of the septuple layers. Mn-doped Bi₂Se₃ forms a similar heterostructure, however, only a large, albeit nonmagnetic gap is formed. We explain both differences based on the higher spin-orbit interaction in Bi₂Te₃. The present findings provide crucial insights for pushing the lossless transport properties of topological insulators towards room temperature.