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  • 學位論文

二維材料之自旋相關現象探討

Spin-Related Phenomena in Two-dimensional Materials

指導教授 : 張慶瑞

摘要


自從石墨烯被實驗驗證之後,這十年來,二維材料變得越來越重要,不只是因為理論上的興趣,還因為它有其高的應用潛力。在理論方面,由於對稱破壞或是局域效應,二維系統會呈現於相較於三維相當不同的性質。在應用方面,二維材料在設計元件上面有更大的優勢,其特性像是輕薄短小,更大彈性,以及在某些材料上面有更高的電子遷移率。每個不同的二維系統都有其獨特性。 其中一個關於二維系統很大的研究面向就是自旋相關現象。電子自旋在目前的奈米科技很少被利用到,主要也是因為其通常較短的自旋同調長度。在二維系統上面,因為較少的碰撞機會,所以有望有更長的自旋同調長度。除此之外,當元件越做越小的現代科技當中,量子現象已經越來越重要,而自旋就能更容易地被使用上。還有一個重要原因是,翻轉自旋所需的能量較低,所以在元件的能量消耗可以更低。因此,在二維材料上面自旋相關現象的探討,在現在很熱門的自 旋電子學領域是極其需要的。 二維材料有很多不同種類,可能是天然存在也可能需要人造。在本論文當中,我們討論了兩種二維材料:二維拓樸絕緣體以及釩二硫化物。雖然兩者結構上相差甚遠,他們都有自旋相關的性質顯現。拓樸絕緣體有其獨特的邊界態,其自旋及動量是互相耦合的。這些邊界態有明確的旋性,即對同一邊界而言,不同的自旋其前進方向是不同的。然而,有限大小效應會將此旋性破壞掉,降低了利用不同邊界而來偵測或傳輸不同自旋的使用性。我們因此研究了在環狀拓樸絕緣體上面,如何降低有限大小效應的方法。第二種我們探討的材料是釩二硫化物,很特別的,此材料是磁性絕緣體。這意味著我們可以利用電壓來調控傳輸行為的同時,也同時調控了自旋的方向。為了要探討釩二硫化物在真實元件中其自旋傳輸行為,緊束縛模型是極其需要的。我們利用了 Koster-Slater 二中心近似模型,來對第一原理計算出來的結果,給出了一個合理的緊束縛模型。利用這個緊束縛模型,我們建構了一個鋸齒型(zigzag)釩二硫化物奈米帶,並探討其傳輸行為。

並列摘要


Two dimensional (2D) material plays more and more important roles in researches of this decade since the experimental realization of graphene not only because of its theoretical interests but also its potentials in applications. In the theoretical part, two dimension can show quit different behaviours to three dimension due to confinement of interactions or emergence/break-down of certain symmetries. In the application part, two-dimensional materials provide us more advantages to design devices, such as more flexibility, lighter weight and even higher mobilities in some materials. Each two-dimensional material has its peculiarity. One of the big aspects to study for two-dimensional material is the spin-dependent quantum phenomena. Spin is seldom utilized in modern nanotechnology, mainly due to the usually short spin coherent length. It is helpful to have longer spin coherent length in lower dimension by less chance of collisions. Besides, with devices shrinking to nano scale nowadays, quantum phenomena has to be taken into account and spin can be utilized more easily. Besides, the low energy required to flip spin also makes spin-related devices low power consumption. Thus, the spin related phenomena study in two-dimensional materials is eagerly needed in the promising spintronics field. There are many varieties of two dimension materials, either existing in nature or by man-made. In this thesis, we study two kinds of 2D materials: the 2D topological insulator and vanadium disulphide. Although quite different in structures, they both manifest spin-dependent properties. Topological insulator has edge/surface states that are spin-momentum locked. The edge states have definite helicity, i.e. different spin orientation at different edges. However, the finite size effect destroys this helicity, diminishing the usage to separate signals by different spins at different edges. We thus study ways to reduce the finite size effect of topological insulator in annulus geometry. The second material is the peculiar vanadium disulphide. Hardly seen, the vanadium disulphide is a magnetic insulator. This means that we can electrically tune its transport phenomena while simultaneously changing the spin orientation. To study how to use vanadium disulphide in real devices, a tight binding model for calculating the spin transport is eagerly needed. Using the Koster-Slater two-center approximation, we then try to find a tight-binding model that fits the ab initio band structure quite reasonably well. A transport result for a zigzag nanoribbon vanadium disulphide based on these fitting paramters is also provided.

參考文獻


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