最近拓樸絕緣體的一個議題是如何改變拓樸的狀態，可以藉由參雜的成分或是場效元件來達成。本實驗先藉由成長〖(〖Bi〗_(1-x) 〖Sb〗_x)〗_2 〖Se〗_3高比例的成分在sapphire(0001)上面，在確認晶格結構沒問題後，我們針對接近拓樸態轉換Sb成分的樣品做電性上和ARPES的分析。我們確認高Sb參雜比例和改變厚度即便是增加樣品的電阻值使樣品更接近絕緣的狀態，其在低溫下的磁導表現仍然保有拓樸的行為，更進一步我們調查和觀察到參雜Sb雖然降低樣品的自旋軌道藕荷行為，卻不會改變樣品的bulk band gap。最後我雖然在材料上我們無法改變樣品的狀態，我們成長薄膜在SrTiO3基板上，最後使用場效的方式使樣品的拓樸金屬態更皆近強局籲的絕緣態。

Ultrathin topological insulator film are promising for use in field effect device. We expect that increasing the Sb doping level and reducing the film thickness cause a crossover of transport phenomena from the quantum diffusive regime to the intermediate insulating regime. The crossover of different quantum transport under an electric field may form the basis for spin transistor in the future.
Magneto-transport measurements have been carried out on (Bi1-xSbx)2Se3 (x=0.26, 0.37, 0.52, 0.63, 0.7) film at 2K temperature in the magnetic field range of 0 to 9T. Weak anti-localization (WAL) were observed in samples with Sb concentration up to x=0.26~0.7. The analyses of observed in magneto-resistance data using HLN equation reveal all sample show WAL phenomenon. All sample show metallic temperature dependent in R-T measurement. We also report thickness-independent magneto-transport properties from 4.6nm to 8nm. We get Kfl≥3 in all sample at 2K temperature, where KF is the 3d Fermi wave vector and l is mean free path. The lowest K_f l we getting is 3.4, this means that all sample locate in quantum diffusive regime.
Sb doping cause decreasing spin orbit coupling (SOC), so that the bulk band gap may decrease when inducing doping level. We then check sample’s band structure by ARPES analysis. The magnitude of bulk band gap are 0.5eV、0.44eV for X=0、0.63, respectively, indicating slight change upon doping. We suggest that the microscopic mechanism of topological phase transition (TPT) could not be simply elucidated by SOC effect.
Finally, the field-effect properties of ultrathin (Bi1-xSbx)2Se3 that is grown on SrTiO3(111) using molecular beam epitaxy (MBE) are investigated. The 15nm (Bi0.68Sb0.32)2Se3 film exhibits the large field effect (FE), as the transport is turn into intermediate disorder regime (Kf l=1.2). The crossover of different quantum transport under electric field may form the basis for topological insulator-base spin transition in the future.

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