【作者】 刘旻昊；

【导师】 王亚愚；

【作者基本信息】 清华大学 ， 物理学， 2012， 博士

【摘要】 拓扑绝缘体是一种具有受时间反演对称性保护的拓扑表面态的新型半导体。拓扑表面态具有线性色散关系，并且其自旋和晶格动量满足一定的手性关系。拓扑绝缘体在自旋电子学和量子计算领域具有潜在的应用背景，已经引起极大关注。Bi2Se3、Bi2Te3及Sb2Te3这一类拓扑绝缘体体系具有较简单的能带结构，即对于某一表面，其体能带带隙中只存在一个表面能带简并点，有利于实验研究。但该体系的载流子浓度较高，为能够将体能带在输运性质测量中的贡献尽量降低，选择对分子束外延方法生长的薄膜体系进行研究。理论预言表面态的输运性质将表现出弱反局域化行为。实验表明未掺杂的Bi2Se3薄膜在磁场垂直于表面情况下的磁电导表现为弱反局域化行为，而低温下的电阻-温度曲线则表现出与预期相反的绝缘体行为。通过对磁场平行于薄膜平面情况下的磁电导行为的研究，确认了电子-电子相互作用对输运性质的影响，从而解释了电阻-温度曲线的反常行为。对不同浓度Cr元素掺杂Bi2Se3薄膜的磁电导测量表明，随着掺杂浓度的提高，低温磁电导表现出从弱反局域化到弱局域化的转变。原位的角分辨光电子谱测量显示，表面态能带的带隙随着掺杂浓度升高逐渐增大。这与理论计算得到的结论一致，即表面态能带带隙的增大会导致局域化行为的转变。更有趣的是，重掺杂样品的磁电导随温度升高表现出从弱局域化行为到弱反局域化行为的转变，与稀磁半导体类似。角分辨光电子谱测量也显示了重掺杂样品表面态能带的消失。因而上述局域化行为的转变应与磁性掺杂导致的能带拓扑相变具有紧密联系，其具体机制需要进一步的理论和实验研究。通过对Sb2Te3进行Bi元素掺杂，获得了具有低载流子浓度的(Bi,Sb)2Te3薄膜。对其在超强磁场下的输运性质测量观察到表面态的朗道能级导致的量子振荡现象。分析表明，由于其线性色散关系和较大的塞曼分裂，表面态具有非常规的朗道能级能谱。本论文通过对分子束外延方法生长的拓扑绝缘体薄膜的输运性质的系统研究揭示了表面态的特殊的局域化行为和朗道能级能谱，为理解拓扑绝缘体这一体系的奇特性质提供了有价值的线索。更多还原

【Abstract】 Topological insulator is a new kind of semiconductors with topological surfacestates protected by time-reversal symmetry. The topological surface states showspin-polarized linear dispersion. Topological insulator has attracted intense studies forits potential applications in spintronics and quantum computation.The topological insulator family of Bi2Se3、Bi2Te3and Sb2Te3possess relativelysimple band structure with only one degenerate point of surface bands in the bulk bandgap, which is ideal for experimental studies. However, the carrier densities of thesebinary compounds are so high that both the surface states and the bulk states contributeto the transport properties. Thin films grown by molecular beam epitaxy are chosen inorder to limit the contribution from the bulk states.Weak antilocalization behavior of the topological surface states is expectedtheoretically. Magnetoconductance measurements on pure Bi2Se3thin films withmagnetic field perpendicular to the films are consistent with weak antilocalizationbehavior. The resistance-temperature curve shows insulating behavior at lowtemperatures, in contradiction with the expected metallic behavior for weakantilocalization. Electron-electron interaction is revealed in magnetoconductancemeasurement with magnetic field parallel to the films, explaining the emergence of theinsulating behavior.Magnetoconductance measurements on Cr-doped Bi2Se3thin films exhibit acrossover from weak antilocalization behavior to weak localization behavior withincreasing doping level. In situ ARPES measurements demonstrate that the band gap ofthe surface states increases with the doping level. These results are qualitativelyconsistent with related theoretical calculations, which show that a crossover oflocalization behavior takes place due to enlarged band gap of surface states band bymagnetic doping. More interestingly, the magnetoconductance of heavily-doped Bi2Se3films changes from weak localization to weak antilocalization with increasingtemperature. Similar behavior was observed in diluted magnetic semiconductorswithout topological surface states. The disappearance of the surface state band in theheavily-doped samples is also observed by ARPES. Thus the crossovers of localization behavior are intimately related to the topological phase transition of the band structureof topological insulator thin films. The accurate description, however, needs furthertheoretical and experimental investigations.Lower carrier density is achieved in Sb2Te3films doped with Bi. Quantumoscillations due to Landau levels of surface states are observed in transportmeasurements under ultrahigh magnetic field. Quantitative analysis shows that thecombination of linear dispersion and large Zeeman splitting leads to an unconventionalspectrum of Landau quantization of surface states.The systemic transport studies on topological insulator thin films uncover thespecial localization behavior and Landau levels of topological surface states, providinginsights into the exotic properties of topological insulator.更多还原