【摘要】：拓撲晶體絕緣體是一種新的物質(zhì)相,它的拓撲性質(zhì)受晶體對稱性保護,并具有多個狄拉克表面態(tài)�；诘谝恍栽砗土孔虞斶\計算,我們系統(tǒng)研究了拓撲晶體絕緣體SnTe的(111)表面和薄膜中的一些新奇性質(zhì)。由于極性,理想的SnTe(111)表面原則上是不穩(wěn)定的。因此我們首先研究了SnTe(111)表面的穩(wěn)定性,發(fā)現(xiàn)在不同的生長條件下可以形成三種穩(wěn)定的表面結(jié)構(gòu)。表面電子結(jié)構(gòu)計算發(fā)現(xiàn)它們具有兩種定性不同類型的拓撲表面態(tài):未重構(gòu)和(√3×√3)重構(gòu)的表面具有第一種類型的表面態(tài),即四個狄拉克點位于四個時間反演不變動量點;(2×1)的表面重構(gòu)引起表面布里淵區(qū)的折疊,使不同狄拉克谷產(chǎn)生相互作用,產(chǎn)生了新類型的表面態(tài),即兩個狄拉克點偏離了布里淵區(qū)中心的時間反演不變點。我們的研究表明,除了選擇不同的表面方向還可以通過控制生長條件來產(chǎn)生不同類型的拓撲表面態(tài)。拓撲晶體絕緣體在其表面能帶中具有偶數(shù)個狄拉克錐(多個谷)。我們系統(tǒng)研究了SnTe(111)表面狄拉克谷在應(yīng)變下的演化,發(fā)現(xiàn)壓縮應(yīng)變使ˉΓ和ˉM谷的狄拉克錐產(chǎn)生不同程度的移動,甚至相反的移動;拉伸應(yīng)變可以增強上下表面間的耦合,甚至使ˉΓ和ˉM谷產(chǎn)生不同大小的能隙。在SnTe(111)表面上,我們設(shè)計了一種應(yīng)變異質(zhì)結(jié)構(gòu),并發(fā)現(xiàn)通過動態(tài)施加局部壓力,可以實現(xiàn)強的狄拉克費米子的谷過濾效應(yīng)。這些結(jié)果顯示,拓撲晶體絕緣體中可以實現(xiàn)應(yīng)變的功能化應(yīng)用和狄拉克谷電子學應(yīng)用。拓撲材料薄膜的狄拉克費米子具有螺旋自由度。我們以拓撲晶體絕緣體SnTe的(111)薄膜為例,發(fā)現(xiàn)用適當?shù)碾妶隹梢允贡∧さ牡依速M米子產(chǎn)生巨大的螺旋性劈裂�；谶@些結(jié)果,我們對狄拉克費米子透過雙柵極納米結(jié)構(gòu)的輸運進行了計算,發(fā)現(xiàn)了一些螺旋性相關(guān)的特性,包括狄拉克費米子螺旋性的選擇透射,螺旋性切換,螺旋性負折射以及雙負折射。我們的結(jié)果為實現(xiàn)基于螺旋性的電子學應(yīng)用提供了可能。
[Abstract]:Topological crystal insulator is a new material phase. Its topological properties are protected by crystal symmetry and have many Dirac surface states. Based on the first-principles and quantum transport calculations, we have systematically studied some novel properties on the (111) surface and thin films of the topological crystal insulator (SnTe). The ideal SnTe (111) surface is unstable in principle due to its polarity. Therefore, we first studied the stability of SnTe (111) surface and found that three stable surface structures can be formed under different growth conditions. Surface electronic structure calculations show that they have two types of qualitatively different topological surface states: unreconstructed surfaces and (3 脳 m2 3) reconstructed surfaces with the first type of surface states. That is, the four Dirac points are located at four time inversion invariant momentum points. (2 脳 1) the surface reconstruction results in the folding of the Brillouin zone, which results in the interaction of different Dirac valleys and the formation of new types of surface states. That is, two Dirac points deviate from the center of Brillouin zone in time inversion invariant points. Our results show that different types of topological surface states can be generated by controlling growth conditions in addition to choosing different surface directions. A topological crystal insulator has an even number of Dirac cones (multiple valleys) in its surface band. We have systematically studied the evolution of Dirac Valley on SnTe (111) surface under strain. It is found that compression strain makes the Dirac cones of Gamma and M valleys move in varying degrees, even opposite, and that tensile strain can enhance the coupling between the upper and lower surfaces. Even the energy gaps of different sizes are produced in the Gamma and Mian valleys. A strain heterostructure is designed on the surface of SnTe (111) and it is found that the strong Dirac fermion can be filtered by dynamic local pressure. These results show that the functional application of strain and the application of Dirac Valley electronics can be realized in the topological crystal insulator. The Dirac fermion of the thin film has helical degrees of freedom. Taking the (111) thin film of the topological crystal insulator SnTe as an example, it is found that the Dirac fermion of the thin film can be caused by a large helical splitting with a proper electric field. Based on these results, we have calculated the transport of Dirac fermions through double gate nanostructures and found some helicity related characteristics, including selective transmission of Dirac fermions helicity, helicity switching. Helical negative refraction and double negative refraction. Our results provide the possibility for the realization of helical electronic applications.
【學位授予單位】：清華大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：O469

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