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Preparation and Properties of Thin Films of Magnetically Doped Topological Insulators

Author: ChangCuiZu
Tutor: XueQiKun
School: Tsinghua University
Course: Physics
Keywords: Topological insulator Magnetic doping Quantized anomalousHall effect (QAHE) Electronic band structure Band engineering
CLC: O484.1
Type: PhD thesis
Year: 2012
Downloads: 22
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Thin films of magnetically doped topological insulators (TI) are of greatimportance to realize theoretically predicted exotic quantum phenomena. In thisdissertation, growth, electronic structure, and electric and magnetic properties of thinfilms of magnetically doped Bi2Se3family TIs have been systematically investigatedby combining molecular beam epitaxy (MBE), scanning tunneling microscopy (STM),angle-resolved photoemission spectroscopy (ARPES), and electron transport.(1) High-quality thin films of intrinsic and magnetically doped TIs (includingBi2Se3, Bi2Te3, Sb2Se3,(Bi1-xSbx)2Te3and Bi2(Te1-xSex)3) with well controlledthickness and chemical potential have been realized on sapphire (0001) and SrTiO3(111). The work lays the material foundation for transport study and for exploringapplications of TIs and magnetic TIs.(2) The electronic structures and transport and magnetic properties of thin filmsof chromium (Cr) doped Bi2Se3, Bi2Te3, Sb2Se3,(BixSb1-x)2Te3and Bi2(SexTe1-x)3have been systematically studied. Long range ferromagnetic order has been found inCr-doped Te-based TIs (Bi2Te3, Sb2Se3,(BixSb1-x)2Te3), which is insensitive to thecarrier type and density, either induced by chemical doping or by electrical fieldeffect. The result demonstrates that ferromagnetic insulator phase, which promises therealization of quantized anomalous Hall effect (QAHE) can be realized in thesematerials. It is found that the anomalous Hall resistance of Cr-doped (BixSb1-x)2Te3increases abruptly with decreasing carrier density and reaches0.6quantum resistance(h/e2,25.8kΩ) for x=0.2, the signature of QAHE. On the other hand, no longrange ferromagnetic order has been observed in Cr-doped Bi2Se3. By ARPES, STM,and first principle calculations, we show that the absence of long-range ferromagneticorder is either due to aggregation of Cr dopants (at low doping level), or due to veryweak spin-orbit coupling at high Cr doping level so that band inversion can’t occurand the van Vleck mechanism no longer works. This study experimentallydemonstrates that ferromagnetic insulator phases can be obtained in magnetically doped TIs with van Vleck mechanism, and provides the conditions for realizinglong-range ferromagnetic order in which various topological magneto-electricphenomena in TIs can be investigated.(3) The gapless Dirac surface states (DSSs) in TIs are determined by the bulkband topology and are very robust against non-magnetic surface modification. In thethird part of the dissertation, we have attempted to engineer the band and spinstructures of the DSS of TIs by growing different over-layers on the surface of TIswithout interrupting their topological characters. The result provides a method forindependent controlling the bulk and surface electronic structures of TIs.

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CLC: > Mathematical sciences and chemical > Physics > Solid State Physics > Thin Film Physics > Film growth,structure and epitaxy
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