Infrared Spectroscopic Studies Of The Topological Material AMnSb₂ System

Recent years,the observations and investigations of topological materials have attracted much attentions in the development of modern condensed matter physics.For topological materials,albeit the bulk states are those of a insulator or semiconductor,the surface should have gapless nontrivial edge states,showing the two-dimensional Dirac semimetal behaviors.Owing to the high carrier density and low energy depletion,these materials are highly applied in various devices in the future.In recent years,topological materials investigations include topological insu-lators,Dirac semimetals,Dirac nodal-line semiemtals,and Weyl semimetals.For the enormous interesting topological nontrivial phenomena,they are meaningful for the research of particle physics.To my best knowledge,most of the observed topological materials are formed by Carbon,Nitrogen,or Chalcogen elements,among which the electronic dispersions near the Fermi level are highly related.Recently,a new kind of topological materials AMnSb₂and AMnBi₂com-pounds have attracted much attention.In 2011,Park et al.identified the anisotrop-ic Dirac band dispersions in MnBi and MnSb layers compounds.For the antifer-romagnetic phase below 250K,the research of the proposed type II Weyl semimet-als YbMnBi₂and the magnetic Weyl fermions have aroused amount of interests.While YbMnBi₂is proved not as the type II Weyl semimetal by Chaudhuri et al.,the optical spectroscopic contributes to the studies of topological materials.Different topological materials can have different electronic response in the optical studies.Together with the first-principles calculations,we can offer de-tailed investigations of the topological materials.In this dissertation,we have investigated the optical response of the AMnSb₂compounds,which are synthe-sized by self-flux method.As the Sb square net is staggered type in CaMnSb₂,two linear components have been observed in the low-temperature optical con-ductivity,but none of them extrapolate to the origin,at odds with the optical response expected for three-dimensional Dirac fermions.We have extracted the temperature evolution of the Dirac gap through the absorption edge,that shows an inverted-bands behavior.A noteworthy phenomenon is the Van-Hove Singu-larity like behavior in the optical spectrum at low temperature.With the help of theoretical calculations,we ascribe the Van-Hove Singularity to the Mexican-hat dispersion which is caused by band inversion.In SrMnSb₂case whose square net resembles CaMnSb₂,we detect the obvious shift of plasma edge at different tem-perature,distinct from the CaMnSb₂case.Finally,in the research of YbMnSb₂,we find a range of constant optical conductivity which is not changed at various temperatures.This behavior is unusual in three dimensional topological mate-rials.Compared with the band structure calculated by first-principles method,we prove YbMnSb₂is a new kind of robust topological nodal-line semimetal with antiferromagnetic order.Even though the electronic states near the Fermi level are predominantly derived from the p_xand p_yorbitals of Sb atom in AMnSb₂family,different cation A can affect the topological properties a lot.These research offer a great amount of important informations to the topological investigations of AMnSb₂compounds.