Preparation And Properties Of Quantum Anomalous Hall Bilayers And Multilayers

Magnetic topological insulators?TIs?showing the quantum anomalous Hall?QAH?effect can be used to construct various topological quantum states of matter.This dissertation presents the studies on the preparation and physical properties of QAH bilayers and multilayers by using molecular beam epitaxy?MBE?,atomic force microscope?AFM?,angle-resolved photoemission spectroscopy?ARPES?,and low-temperature transport measurement.The main results and conclusions are as follows:?1?We systematically studied the properties of chromium?Cr?and vanadium?V?co-doped?Bi,Sb?₂Te₃?CVBST?TI films MBE-grown on strontium titanate?SrTiO₃?and indium phosphide?InP?and their dependences on substrate surface structure,position of magnetic TI layer,film thickness and so on.This enables us to engineer the magnetic properties and QAH behaviors of the magnetic TI films and to develop a comprehensive understanding on the mechanisms of the magnetism and the QAH effect.Based on these results,we realized a synthetic quantum spin Hall?QSH?system by combining two CVBST QAH films with different coercive fields.The transport measurements of various devices made from the bilayer system show the similar results to those of the QSH effect observed in two-dimensional TIs.?2?We prepared QAH bilayers and multilayers with MBE techniques.We found a normal insulator material that can be used as the spacing layers in QAH bilayers and multilayers:Cadmium selenide?CdSe?which has a bandgap of 1.74 eV and a small lattice mismatch?<2%?with CVBST in the?001?surface.We realized the epitaxial growth of CVBST/CdSe heterostructures with atomically sharp interface and found that the CdSe layer has little influence on the electric and magnetic properties of the CVBST film.We grown superlattices of CVBST and CdSe films and observed the high plateau QAH effect in them,which represents the realization of a high Chern number QAH system.The successful preparation of QAH multilayers not only paves the road for the applications of the chiral QAH edge states in low-energy-consumption electronics,but can also be engineered into magnetic Weyl semimetal phase with only one pair of Weyl points.Besides,we also realized MBE growth of QAH bilayers with different coercive fields in the two layers with CdSe as the spacing layer,which can be used to simulate the helical edge states of the QSH effect.Such a QAH bilayer provides us an excellent platform to study the physics related to the QSH effect.