The Chiral Transport Of Weyl Semimetals

Gapless topological phases,especially the Weyl semimetal(WSM),have attracted much interest.Weyl semimetal with broken either inversion or time reversal symmetries,is a new topological state of metals,characterized by the nontrivial band-touching nodes,shifted to different energy or separated in momentum space and analogous to ’3D graphene’.The topological aspects of Weyl semimetals include chiral anomaly,anomalous Hall effect,chiral magnetic effect,and Fermi arcs are reviewed in many excellent papers.Especially,the chiral anomaly and the nonzero Berry curvature around the energy band touching nodes drive the Weyl semimetals to chiral electromagnetic response and exotic thermoelectric properties.Recently,TaAs-family materials have been proposed and confirmed as possible candidates for realizing the WSM.TaAs is a kind of nonmagnetic material with preserved time reversal symmetry(TR)and broken inversion symmetry(Ⅰ),and unlike previously proposed pyrochlore materials,which have been studies widely on experiments.The main works in this thesis are listed as follows:In chapter one,we give a brief introduction to topological semimetals,includes Dirac semimetals and Weyl semimetals,its possible applications on optics,electronics etc.,and connection with hot topics,such as graphene,topological insulators,two-dimensional layered transition-metal dichalcogenides.We conclude some important developments on material realizations and novel transport on Weyl semimetals,both theoretically and experimentally.Weyl semimetal(WSM)is a novel topological gapless state with promises exotic transport due to chiral anomaly.Therefore,in chapter two,we focus on the physical origin and its derivation of chiral anomaly.We derive the chiral anomaly equations using several methods,which include Fujikawa technique,Boltzmann kinetic equation,Landau quantization and topological field.We find that chiral anomaly could emerge in different systems,and could help us to apply its exotic properties on condensed matter physics.In chapter three,we discuss two experimental fingerprints of Weyl semimetal,that is,Fermi arc and negative magnetoresistance(MR).Fermi arc is a discontinuous topological surface state,which could be measured by ARPES and STM.One the other hand,negative MR results from chiral anomaly when external electric field parallel to magnetic field.In experimental data of TaAs,there are three mechanisms contributed to MR,which are weak anti-localization(WAL),chiral anomaly and anomalous MR.In chapter four,we investigate the unsaturated positive MR on Weyl semimetals TaAs family,TMD WTe2,Dirac semimetals Na3Bi/Cd3As2 etc.The explanation of unsaturated MR is still open question and under debate.We summary and compare that several theoretical models on LMR are proposed,such as Abrikosov LMR,charge resonance,effective medium theory,and guiding-center diffusion.In chapter five,we try to classify topological semimetals with Dirac masses.The mass classification of Dirac semimetals helps us to realize Weyl semimetals and nodal-line semimetals,and even double-Weyl phase.Finally,we discuss the classification of short-range interaction and superconductivity in WSM.In the last chapter six,we make a summary about this thesis and give some further supplementary topics on Weyl semimetals,especially about optical field,such as topological magnetoelectric effect and electric-magnetic duality with chiral anomaly.