Markus Scholz vom Helmholtz-Zentrum Berlin
Topological surface states (TSS) are protected by time reversal symmetry (TRS) [1]. If we remove this protection by violating the TRS we expect drastic changes in the electronic structure as was shown in transport measurements in the 2D topological insulator case of HgTe quantum wells [2]. For three-dimensional topological insulators, much more efficient than an external magnetic field is the use of ferromagnetic impurities. Besides, many proposals already exist that suggest interesting effects connected to interfaces of ferromagnets and topological insulators.
Our angle resolved photoemission studies are focussed on the in situ surface doping of three-dimensional topological insulators Bi2Te3 and Bi2Se3 with Fe[3]. We give direct evidence that the TSS can  withstand magnetic impurities. In contrast to a recent study [4], where a gap of 100 meV was reported, we show that the TSS remains intact and no gap opens even for Fe coverages in the monolayer regime. Moreover, we show that Fe can increase or decrease the Dirac point's binding energy depending on the temperature during deposition.
Our analysis of the photoemission line widths reveals an enhancement of scattering events for both hole and electron doping. This implies that the origin of the enhanced scattering is not a gain in warping as reported recently [5].
We also report on a strong circular dichroism effect in angle-resolved photoemission of Bi2Te3[6]. Our measurements reveal a strong variation of the strength and the sign of the effect, depending on the photon energy. By comparing the results with spin resolved measurements and with one-step-photoemission calculations we want to answer the question whether or not the strong spin momentum locking in topological insulators allows a spin detection without spin sensitive equipment.
[1] Kane and Mele, Phy. Rev. Lett 95, 146802 (2005); Liang Fu, Kane and Mele, Phys. Rev. Lett. 98, 106803 (2007).
[2] Koenig et al., Science 318, 5851 (2007)
[3] Scholz et al., arxiv:1108.1037v1 (2011)
[4] Wray et al., Nature Physics 7 (1), 32-37 (2010)
[5] Pan et al., arXiv:1104.0966
[6] Scholz et al. arXiv:1108.1053v1 (2011)