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  Nature Physics 8, 616 (2012)
Phys, Rev. B 86, 205127 (2012)
 
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One of the goals of condensed matter physics is to find novel materials with useful properties and to apply quantum mechanics to the study of them. Among the many of successes of condensed matter physics, one well-known example is the better understanding of and the utilization of magnets, which are crucial for hard disk data storage, computer displays and countless other technologies. Recently, the discovery of topological insulators have attracted huge interest worldwide. Interestingly, it has been proposed that the interplay between ferromagnetism and the topological insulator state is predicted to realize a range of exotic quantum magnetic phenomena that are not possible in any "non-topological" magnet, including the quantized anomalous Hall effect, the Axion electrodynamics. These phenomena are of interest in both fundamental physics and device applications. A major difficulty in this topic is that none of the topological insulator materials are naturally ferromagnets and it is challenging to experimentally demonstrate that a ferromagnetic ordering is indeed induced in the topological surface states. Utilizing spin-resolved angle-resolved photoemission spectroscopy, we systematically study the electronic and spin groundstate of magnetically doped topological insulators. We show that a gap is opened at the surface state Dirac point upon magnetic Mn doping. Moreover, our spin-resolved measurements show that the spin polarization of the electrons at the energies near the magnetic gap is significantly reoriented. The resulting configuration of the spin polarization near the magnetic gap edge realizes a novel hedgehog spin texture, which directly demonstrates that time-reversal symmetry is broken on the surface of our magnetic topological insulator. Our observed magnetic spin texture phenomena and the control of their Berry's phase lay the foundation for the realization of the proposed novel phenomena associated with a magnetic topological insulator.

 
 
Magnetic Topological Insulators: Hedgehog spin texture and Berry's phase tuning in a magnetic topological insulator
Published in S.-Y. Xu, M. Neupane, C. Liu, et al.,
Nature Physics 8, 616 (2012).

 
 
Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator
Published in D. Zhang, A. Richardella, D. W. Rench, et al.,
Phys, Rev. B 86, 205127 (2012).