Structural and electronic properties of Mn doped topological insulators Bi2 Te3 and Bi2 Se3 O. Caha, J. Růžička, V. Holý G. Springholz, V. Volobuev, H. Steiner, S. Wimmer, A. Ney, G. Bauer O. Rader, J. Sanchez-Barriga, P. Mandal, E. Rienks, A. Varykhalov M. Albu – Graz center electron microscopy J. Minár, S. Khan - ZČU Plzeň H. Ebert - Uni Mnichov Outline ● Motivation ● Mn doped topological insulators thin films – sample preparation – structure: XRD, XAFS, HRTEM – magnetic properties – elektronic structure: ARPES ● Conclusion Topological insulators Band structure of topological insulator: Large spin orbit splitting and time reversal symmetry →spin polarized surface states with Dirac-cone dispersion Prototypical materials: narrow band gap semiconductors Bi2Se3, Bi2Te3 Ferromagnetic ordering brakes time reversal symmetry →band gap within surface states, Quantum anomalous Hall effect Bulk conduction band Bulk valence band Surface states Sample preparation Substrate BaF2 (111) Mn doped Bi2 X3 thickness 300 to 500nm G. Springholz group, JKU Linz Deposition technique: Molecular beam epitaxy Compound sources: Bi2 Te3 / Bi2 Se3 , additional Te/Se cell to achieve correct stoichiometry Sample series: Bi2 Te3 up to 11% of Mn doping Bi2 Se3 up to 10% of Mn doping Crystal structure of Bi2 X3 (X=Se,Te) Te/Se Bi Possible incorporation position of Mn atoms Electron microscopy JKU Linz, Graz HAADF STEM10% Mn Bi2 Te3 6% MnBi2 Se3 QL SL QL SL QL SL QL QL SL QL QL QL QL SL QL SL QL QL QL SL QL QL SL XRD structure analysis Symmetric scan with scattering vector perpendicular to the surface Higher Mn content leads to disturbed structure Fitted with a paracrystal model: Random sequence of Bi2 X3 (quintuple layers – QL) and Bi2 MnX4 (septuple layer – SL) XRD structure analysis Paracrystal model parameters: average length and RMSD of QL and SL segments =5 relative RMSD =0.5 XRD structure analysis In-plane lattice parameter dependence X-ray absorption spectroscopy Experiment at BM23, ESRF Grenoble Very weak Mn concentration dependence X-ray absorption spectroscopy Simulations of various Mn positions X-ray absorption spectroscopy Fitted distances of Mn nearest neighbors Magnetic properties SQUID (JKU Linz) Telluride easy axis out-of-plane TC ≈ 10 K Selenide easy axis In-plane TC ≈ 6 K Transport measurements Hall effect in van der Pauw geometry Electronic structure ARPES BESSYII, HZB Berlin Mn 0% 2% 4% 8% Bi2 Se3 , 12K Mn 8% J. Sanchez-Barriga et al,. Nature Comm. 7:10559 (2015). Electronic structure 6% Mn doped Bi2Se3 at 1K Temperature independent nonmagnetic gap 200 meV Electronic structure ARPES BESSYII, HZB Berlin, Bi2Te3 6% Mn doped samples Magnetic gap 90 meV DFT theoretical prediction ≈16 meV for 10% Mn Henk et al., Phys. Rev. Lett 109, 076801 (2011). ≈40-80 meV for heterostructure Otrokov et al., 2D mater. 4, 025082 (2017). Conclusion ● Mn doped topological insulators form natural heterostructure of alternating QL and SL segments ● Mn atoms are mostly positioned in the central position of septuple layer ● Ferromagnetic ordering has been observed with Curie temperature in range of 6K to 15K for Mn concentration above 3% ● Easy magnetization axis is: – Out-of-plane for bismuth telluride – In-plane for bismuth selenide ● Bismuth telluride shows large magnetic band gap of (90±10) meV opened bellow Curie temperature ● Bismuth selenide does show temperature independent band gap of ≈200 meV Transport measurements Hall effect in van der Pauw geometry