First-Principles Study Of Topological Magnetism In Graphene Nanoribbons

Based on first-principles research methods and density functional theory,this thesis studies the band structure and magnetic properties of topological graphene nanoribbons.On this basis,the electronic structure and magnetic properties of nanoribbons can be regulated by introducing vacancy defects,atomic substitution doping,and changing their edge structure.The main findings are as follows:First,we investigated the electronic structure and magnetic properties of two configurations of 7/9 armchair graphene nanoribbons(A-7/9AGNR and B-7/9AGNR).7/9AGNR is constructed by alternately connecting topologically mediocre 7AGNR and topologically non-trivial 9AGNR.Introducing different types of vacancy defects(single-atom vacancy defect,di-atom vacancy defect,tri-atom vacancy defect,and four-atom vacancy defect)at the 7AGNR subband,9AGNR subband,and the junction of the two subbands,respectively,thereby changing the physics of the 7/9AGNR.Through a systematic study,we found that the introduction of single-atom vacancy defects at different positions produces magnetism,while di-atom vacancy defects are always non-magnetic.Therefore,the electronic structure and magnetic properties of 7/9AGNRs can be effectively regulated by introducing different types of vacancy defects at different positions.In this thesis,the electronic structure and magnetic properties of 7/9AGNRs doped by atomic substitution are investigated.The radii of B atoms,N atoms,and C atoms are similar in size,and it is easy to carry out substitution doping.Therefore,the effects of ordered B atom doping and ordered N atom doping at different positions on graphene nanoribbons were studied,respectively.We found that the substitution doping along the length direction of the entire 7/9AGNR can effectively change its energy band structure and adjust its bandgap,but it does not induce magnetism.Only the substitutional doping on the topologically non-trivial 9AGNR subbands can induce magnetism with a magnetic moment magnitude of 0.78?_B 0.99?_B.When the doping site is located at the edge of the9AGNR subband,the magnetic moment is the largest.Finally,the magnetic properties of 7/9 AGNRs with zigzag edge extensions were investigated.It is found that arranging zigzag edge extensions in the same direction produces stable magnetism,and each zigzag edge extension provides a magnetic moment close to 1.0?_B.This is because changing the edge structure of graphene nanoribbons will produce sublattice imbalance,resulting in a localized zero-energy state,which ultimately results in stable magnetic properties of the entire system.However,aligning the zigzag edge extensions in opposite directions did not make the graphene nanoribbons magnetic.The study shows that the generation and disappearance of magnetism of 7/9AGNR depend on the relative arrangement of zigzag edge extensions.In conclusion,this research has discovered a control method that can make topological graphene nanoribbons generate stable magnetism and fixed magnetic moment.This work provides a direction for exploring graphene nanoribbons with complex magnetic ground states and has particular research significance.