Experimental Realization And Study Of The Quantum Anomalous Hall Effect In Magnetically Doped Topological Insulator Thin Films

Quantum anomalous Hall effect(QAHE) is a quantum Hall effect induced by spontaneous magnetization and it occurs without any need of an external magnetic field. Although anomalous Hall effect was discovered more than 130 years ago, its quantization has not been realized yet. Because of the very peculiar band structure of topological insulator, according to theory, QAHE possibly exist in ferromagnetic topological insulator thin films. The dissertation is devoted to experimental realization of QAHE. In our previous study, we have obtained topological insulator thin films with ferromagnetic insulator phase and could finely tune their chemical potential, as required by the theoretical proposals for QAHE. However, the measured anomalous Hall resistance is not high and far below the quantized value of 25.8 k?. This dissertation presents a thorough investigation on the electronic, magnetic and transport properties of Cr-doped(Bi,Sb)2Te3 topological insulator thin films grown by molecular beam epitaxy(MBE). We have paid special attention to the material details that cannot be accessed easily by theory and eventually observed the quantized anomalous Hall effect. The main results and conclusions are as follows:(1) Combining angle-resolved photoemission spectroscopy(ARPES), atomic force microscope(AFM) and transport measurement, we have systematically studied the morphology and atomic structure of the SrTiO3(111) substrates under various treatment conditions, the electronic energy band structure of Cr-doped(Bi,Sb)2Te3 thin films at different doping levels, the capping protection layers and their influences on the anomalous Hall resistance. By optimizing the SrTiO3(111) substrates preparation and by finely tuning the band structure, we observed that the Hall resistance of 5 QL Cr0.15(Bi0.1Sb0.9)1.85Te3 thin films without any capping layer reaches the quantized value of 25.8 k?, accompanied by a significant drop in the longitudinal resistance at a temperature of 30 m K. The longitudinal resistance quickly disappears once a strong magnetic field is applied. During the disappearance process, we do not see any trace of Shubnikov-de Haas oscillation or jumps, the Hall resistance remains at the quantized value, which implies that no quantum phase transition takes place. The results mark the experimental realization of QAHE.(2) We have systematically studied the thickness dependence of QAHE in Cr-doped(Bi,Sb)2Te3 thin films. It is found that a transition from QAH insulator phase to ordinary insulator phase occurs as the thickness reduces to 3 QL. The phase transition results from the gap induced by the hybridization between top and bottom surface states of the film when it is larger than exchange field induced energy gap at this thickness. In addition, we also observed the deviation of anomalous Hall resistance from the quantized value in thicker films due to delocalization of dissipative electrons.(3) We have investigated the growth and electronic band structure of Sb2Te3 thin films on graphene terminated 6H-SiC substrate. Because of ?3 × ?3 30° epitaxial relation between the lattice basis vectors of graphene and Sb2Te3, the Dirac cone of graphene at K point of graphene's Brillouin zone is found to fold to ? point, overlapping with the surface states of Sb2Te3. As a result, the electronic states of graphene and Sb2Te3 will couple together. It implies that if ferromagnetism is introduced into Sb2Te3 by magnetic doping, the coupling may break the time reversal symmetry of graphene and lead to QAHE in this heterostructure. The work may provide a new and simple system for realizing QAHE.