拓樸絕緣體具有特殊的表面態能帶結構，使此種材料具有內部絕緣而表面導電的特性。藉由在三維拓樸絕緣體加入磁性物質，將破壞時間返衍對稱性(time reversal symmetry)，使其能夠實現量子異常霍爾效應(Quantum Anomalous Hall Effect, QAHE)，而MnBi2Te4已被證實為一種本徵性磁性拓樸絕緣體，藉由Sb替代其中的Bi元素形成Mn(Bi(1-x)Sbx))2Te4，藉由調整不同的Sb比例來達到改變物理性質的效果，開展關於磁性拓樸絕緣體新的領域。
本實驗以分子束磊晶系統(Molecular beam epitaxy, MBE)透過調控各元素氣體分壓比及成長溫度得到晶向結構穩定的MnBi2Te4薄膜，並透過Beam Flux monitor觀察不同Sb分子束分壓初步認定摻雜比例，使其形成四元磁性拓樸絕緣體，隨後進行磁性質的分析。首先以X光繞射儀(X-ray diffraction, XRD)進行樣品結構的分析，確認樣品沿著Sapphire基板C軸方向磊晶且比對MnBi2Te4晶向隨著摻雜比例提升之特徵峰位移，磊晶後透過in-situ的反射式高能電子繞射儀(Reflection high-energy electron diffraction, RHEED)。表面平整度使用原子力顯微鏡(Atomic force microscope, AFM)觀察薄膜表面型態與平整度。化學分析藉由X射線電子能譜儀(X-ray photoelectron spectrometer, XPS)確認樣品元素之結合能且與能量散佈分析儀(Energy dispersive spectrometer, EDS)分析已定性定量方式推定元素比例。利用穿透式電子顯微鏡(Transmission electron microscope, TEM)量測樣品剖面結構為七層結構(septuple layer)明顯量測到層狀結構並且量測樣品厚度與XRR比對樣品厚度。最後利用超導量子干涉儀(Superconducting Quantum Interference Device, SQUID)確認樣品磁性行為。

Recent research on magnetic topological insulators (MTIs) with quantum anomalous hall effect has created a surge of interest for condensed matter physics. In this study, MnBi2Te4 and Mn(Bi(1-x)Sbx )2Te4 thin films on the sapphire (0001) substrates were grown using molecular beam epitaxy (MBE) system. By adjusting the Bi cell flux and growth temperature, we achieved the high-quality MnBi2Te4 and Mn(Bi(1-x)Sbx)2Te4thin film. The structural quality and structure characterization of these thin films had been verified by reflective high energy electron diffraction (RHEED), X-ray diffraction (XRD) and atomic force microscope (AFM) that indicated the flat surface and the single crystal along c-axis direction. The high-resolution transmission electron microscopy (TEM) showed the layer structure with septuple layers (SLs) and confirmed the thickness, consistent with X-ray reflectometry (XRR). Moreover, the elemental composition was identified by X-Ray photoelectron spectrometer (XPS), .
Furthermore, the magnetic properties of Mn(Bi(1-x)Sbx)2Te4 samples with different Sb/Bi ratio were measured by superconducting quantum interference device (SQUID). we reveal several magnetic properties. As the Sb ratio increased the out-of-plane antiferromagnetic behavior changed to ferromagnetic behavior in Mn(Bi(1-x)Sbx)2Te4, and also the Neel temperature slightly increased.

1. Hasan, M.Z. and C.L. Kane, Colloquium: Topological insulators. Reviews of Modern Physics, 2010. 82(4): p. 3045-3067.
2. 欒丕綱, 蔡.吳., 從量子霍爾效應到拓樸光子學與拓樸聲子學 From Quantum Hall Effect to Topological Photonics/Acoustics. 科儀新知, 2017. 211 2017.06: p. 68-79.
3. Bernevig, B.A., T.L. Hughes, and S.-C. Zhang, Quantum Spin Hall Effect and Topological Phase Transition in HgTe Quantum Wells. Science, 2006. 314(5806): p. 1757-1761.
4. KöNig, M., et al., Quantum Spin Hall Insulator State in HgTe Quantum Wells. Science, 2007. 318(5851): p. 766-770.
5. Fu, L. and C.L. Kane, Topological insulators with inversion symmetry. Physical Review B, 2007. 76(4): p. 045302.
6. Zhang, H., et al., Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface. Nature Physics, 2009. 5(6): p. 438-442.
7. Hsieh, D., et al., Observation of Time-Reversal-Protected Single-Dirac-Cone Topological-Insulator States in Bi2Te3 and Sb2Te3. Physical Review Letters, 2009. 103(14): p. 146401.
8. Chang, C.-Z. and M. Li, Quantum anomalous Hall effect in time-reversal-symmetry breaking topological insulators. Journal of Physics: Condensed Matter, 2016. 28(12): p. 123002.
9. Yan, J.Q., et al., Evolution of structural, magnetic, and transport properties in MnBi2-xSbxTe4. Physical Review B, 2019. 100(10): p. 104409.
10. Otrokov, M.M., et al., Highly-ordered wide bandgap materials for quantized anomalous Hall and magnetoelectric effects. 2D Materials, 2017. 4(2): p. 025082.
11. Yan, J.Q., et al., Crystal growth and magnetic structure of MnBi2Te4. Physical Review Materials, 2019. 3(6).
12. Otrokov, M.M., et al., Prediction and observation of an antiferromagnetic topological insulator. Nature, 2019. 576(7787): p. 416-422.
13. Chang, C.-Z., et al., Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator. Science, 2013. 340(6129): p. 167-170.
14. Chen, B., et al., Intrinsic magnetic topological insulator phases in the Sb doped MnBi2Te4 bulks and thin flakes. Nature Communications, 2019. 10(1): p. 4469.
15. Li, J., et al., Intrinsic magnetic topological insulators in van der Waals layered MnBi2Te4 -family materials. Science Advances, 2019. 5(6): p. eaaw5685.
16. Otrokov, M.M., et al., Unique Thickness-Dependent Properties of the van der Waals Interlayer Antiferromagnet MnBi2Te4. Physical Review Letters, 2019. 122(10): p. 107202.
17. Gong, Y., et al., Experimental Realization of an Intrinsic Magnetic Topological Insulator. Chinese Physics Letters, 2019. 36(7): p. 076801.
18. Lee, S.H., et al., Spin scattering and noncollinear spin structure-induced intrinsic anomalous Hall effect in antiferromagnetic topological insulator MnBi2Te4. Physical Review Research, 2019. 1(1): p. 012011.
19. Zeugner, A., et al., Chemical Aspects of the Candidate Antiferromagnetic Topological Insulator MnBi2Te4. Chemistry of Materials, 2019. 31(8): p. 2795-2806.
20. Litvinov, V.I., Magnetic exchange interaction in topological insulators. Physical Review B, 2014. 89(23): p. 235316.
21. Coey, J.M.D., Magnetism and Magnetic Materials. 2019.
22. Hasegawa, S., Reflection High-Energy Electron Diffraction. Characterization of Materials, 2012: p. 1-14.
23. Stephen T. Thornton, A.R., Modern Physics for Scientists and Engineers, 4th Edition. 2013: Cengage Learning.
24. 黃英碩, 掃描探針顯微術的原理及應用Scanning Probe Microscopy: Principles and Applications. 科儀新知, 2005. 26:4=144 2005.02[民94.02]: p. 7-17.
25. Riberolles, S.X.M., et al., Evolution of magnetic interactions in Sb-substituted MnBi2Te4. Physical Review B, 2021. 104(6).