三維拓撲絕緣體是一種新的物質相，其獨特的物理性質近年來得到了廣泛的研究。其中一個最顯著的特徵便是非平庸的可導電表面態，其由螺旋狀自旋的無質量二維狄拉克費米子所組成，並且在空間上被絕緣的體態所隔開。
為了研究與三維拓撲絕緣體的狄拉克表面態相關的傳輸特性和量子效應，許多團隊開發了雙閘極元件的製程，這種先進的製程技術使得同時調控拓樸絕緣體薄膜不同表面態的傳輸性質成為可能。因為該元件具有可以分別獨立控制的閘極，進而能夠同時控制上表面和下表面的化學位能。但是，由於元件幾何結構的限制，來自於不同表面的電訊號通常在測量的過程中會混合在一起，因此仍難以區分上表面和下表面的貢獻。
能夠區分三維拓撲絕緣體薄膜上不同表面的訊號貢獻是至關重要的。為了能夠區分被耦合的訊號，我們提出了一種改良的製程，該方法可以製造具有頂部及底部金屬觸點的四元拓撲絕緣體BiSbTeSe2 薄膜元件，使我們能夠辨認來自上表面和下表面的訊號。在我們的量測結果中，我們成功地分辨了來自不同表面態的訊號。透過這種新型元件，我們可以直接操控且分析拓樸絕緣體上不同表面的表面態，因此其成為了一種重要且實用的量測工具。
在能夠同時獨立的分辨和控制上和下表面的表面態之後，為了能使不同表面態的自旋電流能夠相互影響，以形成類似巨磁阻或穿隧磁阻機制，我們需要減少體態的貢獻。我們可以通過減少拓樸絕緣體的厚度來滿足這一條件，因此，我們接著開發了另一種新的製程，這種新的製程方法可以用於製造超薄三維拓撲絕緣體薄膜元件。

A three-dimensional topological insulator is a new phase of matter and its unique physical properties are widely investigated recently. A prominent feature of three-dimensional topological insulators is the unusual conducting surface states, which is comprised of spin-helical massless two-dimensional Dirac fermions and spatially separated by the gapped insulating bulk.
For the studies of the transport properties and quantum effects related to Dirac surface states, technical advances have made it possible to address the transport properties of different surfaces of topological insulator thin films at the same time, via tuning the chemical potentials of both top and bottom surfaces by dual-gated devices with independent gate control. However, due to the geometry of the device, the electrical signals are usually mixed together during the measurement process, so the contributions of the top and bottom surfaces are still difficult to be distinguished.
It is crucial to be able to decouple the mixed signals, here we report an improved method to fabricate a quaternary topological insulator, BiSbTeSe2, thin film device, containing top, bottom metal contacts, enabling us to recognize the signals from the top and bottom surfaces respectively. In our measurement results ,we successfully identify and manipulate different surface states individually and simultaneously. Such a device provides a useful tool to analyze the topological protection of the surface states on different surfaces.
After being able to distinguish and manipulate the states of the top and bottom surface states, in order to make the top and bottom surface spin currents affect each other and form the mechanism of giant magnetoresistance or tunnel magnetoresistance, we need to reduce the contribution of bulk. We can achieve this condition by reducing the thickness of topological insulators. Therefore, we have developed a new fabrication for ultrathin three-dimensional topological insulator devices.

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