Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications1-8. The edge states are hosted by a magnetic energy gap at the Dirac point2, but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temperatures that are one to two orders of magnitude below the ferromagnetic Curie temperature, TC (ref. 8). Here we use low-temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our analysis reveals large gap sizes at 1 kelvin of up to 90 millielectronvolts, which is five times larger than theoretically predicted9. Using multiscale analysis we show that this enhancement is due to a remarkable structure modification induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap10. Mn-doped Bi2Se3 (ref. 11) and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin-orbit interaction by comparison with Mn-doped Bi2Te3. Our findings provide insights that will be crucial in pushing lossless transport in topological insulators towards room-temperature applications.

Central European Institute of Technology Brno University of Technology Brno Czech Republic

Christian Doppler Laboratory for Nanoscale Phase Transformations Zentrum für Oberflächen und Nanoanalytik Johannes Kepler Universität Linz Austria

Department Chemie Ludwig Maximilians Universität München Germany

Department of Condensed Matter Physics Masaryk University Brno Czech Republic

Graz Center for Electron Microscopy Institute of Electron Microscopy and Nanoanalysis Graz University of Technology Graz Austria

Helmholtz Zentrum Berlin für Materialien und Energie Elektronenspeicherring BESSY 2 Berlin Germany

Institut für Festkörperphysik Technische Universität Dresden Dresden Germany

Institut für Halbleiter und Festkörperphysik Johannes Kepler Universität Linz Austria

Institut für Physik und Astronomie Universität Potsdam Potsdam Germany

International Research Centre MagTop and Institute of Physics Polish Academy of Sciences Warsaw Poland

Leibniz Institut für Festkörper und Werkstoffforschung Dresden Dresden Germany

National Technical University 'Kharkiv Polytechnic Institute' Kharkiv Ukraine

New Technologies Research Centre University of West Bohemia Pilsen Czech Republic

Nature. 2019 Dec;576(7787):390-392. doi: 10.1038/d41586-019-03831-7. PubMed

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Mn-Rich MnSb2 Te4 : A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45-50 K

Twin Domain Structure in Magnetically Doped Bi2Se3 Topological Insulator