Explorations Of New Quantum Anomalous Hall And Quantum Spin Hall Materials

Topological insulators?TIs?and topological crystalline insulators?TCIs?are characterized by gapless surface states that are protected by the time-reversal symmetry and the crystalline symmetry,respectively.Many exotic quantum effects,such as quantum anomalous Hall effect?QAHE?and quantum spin Hall effect?QSHE?,can be realized in TI-and TCI-based materials.The QAHE and QSHE not only make possible the applications of dissipationless quantum Hall edge states in energy-saving electronics but lays the foundation for many other topological quantum effects such as topological magnetoelectric effect and chiral topological superconductivity.However,in the existing materials for the QAHE and QSHE are usually complex and hard to prepare and requires a very low temperature to show the effects.Finding better materials for the QAHE and QSHE is still a major task for the field of topological states of matter.This dissertation presents our explorations and studies on new QAHE and QSHE materials by combining molecular beam epitaxy?MBE?,scanning tunneling microscope?STM?,angle-resolved photoemission spectroscopy?ARPES?,superconducting quantum interference device?SQUID?and transport measurements.The main results we obtained are as follows:?1?We discovered a stoichiometric magnetic TI compound MnBi₂Te₄ with orderly arranged magnetic atoms which can be an excellent QAHE system in a thin film.By alternately growing Bi₂Te₃ quintuple-layer and MnTe bilayer,we fabricated single-crystalline thin films of metastable MnBi₂Te₄ with MBE and realized an accurate control of the film thickness.Combining APRES,magnetic measurements and first-principles calculations,we found that a MnBi₂Te₄ film thicker than 1 septuple-layer?SL?is a 3D TI above magnetic ordering temperature.Each MnBi₂Te₄ SL is ferromagnetic with out-of-plane easy magnetic axis,and adjacent SLs are antiferromagnetically coupled.MnBi ₂Te₄films of even number SLs have non-zero Chern number and magnetically induced gap up to 60 meV,which suggests that they can show the QAHE at higher temperature than the existing materials.?2?In TCI SnTe?111?films,we found the evidences for the electrically switchable QSHE phase.We carried out a systematic ARPES study on the bandstructure of SnTe?111?films depending on film thickness and substrate conditions.An oscillation in the gap size of the SnTe films with increasing thickness was observed,which indicates a topological phase transiton between the normal insulator and QSHE phases.By modifying the electric field across the films via the charge transfer between PbTe and its substrate,we realized the control of the topological phase transition.The result lays the foundation for developing field effect devices of dissipationless QSHE edge states.