Exploration And Physical Properties Of Topological Semimetal Materials

In recent years,topological materials have become a research hotspot in the field of condensed matter physics and material physics.According to the band structure characteristics,topological materials can be divided into topological insulators,topological semimetals,etc.The existence of protected conducting states on the edge or surface in topological insulators can help us to distinguish topological insulators from ordinary insulators.And in topological semimetals,there are bands with linear dispersion relations,forming Dirac cones or Weyl cones.Topological semimetals can be further divided into Dirac semimetals,Weyl semimetals,nodal-line semimetals,etc.There are many unique transport properties in these topological semimetals:negative longitudinal magnetoresistance induced by chiral anomaly,anomalous Hall effect that may be caused by topologically nontrivial Berry curvature,etc.,showing broad application prospects.Dirac semimetals that have been discovered early include Na₃Bi,Cd₃As₂,etc.,and Weyl semimetal state can be realized by breaking the time or space inversion symmetry in Dirac semimetals.Ta As with non-centrosymmetric structure is the first Weyl semimetal to be verified experimentally.In addition,it is possible to break the time-reversal symmetry to obtain Weyl semimetals in some magnetic systems,such as Hg Cr₂Se₄,Co₃Sn₂S₂,etc.RAl X（R=rare earth;X=Si,Ge）series compounds recently are theoretically predicted and experimentally confirmed as a new class of Weyl semimetals for the simultaneous breaking of time and space inversion symmetry.In this thesis,by adjusting the chemical composition,we tried to find new topological semimetals in the similar rare earth intermetallic compound systems and explore their physical properties.We focused on three different Nd-Ga based compounds,among which NdGaSi has a similar crystal structure as RAl X,and has obvious anomalous Hall effect at low temperature.In addition,two new Nd-Ga-Ge ternary phase compounds,Nd₄Ga₅Ge₆ and Nd₃Ga₇Ge₃,were synthesized,both of which exhibited abnormal resistivity behaviors at low temperature.The thesis is divided into the following four chapters:In the first chapter,we briefly introduce several topological semimetals according to their energy band structures,then introduce the magnetic anisotropy in the materials and the common electromagnetic transport properties at low temperature.Finally,we briefly introduce various commonly used single crystal growth methods.In the second chapter,we introduce the crystal structures and first-principles calculation of the band structures for the RAl X（R=rare earth;X=Si,Ge）Weyl semimetal family.Then we find that the similar system,NdGaSi,has almost the same band structure,and is a potential Weyl semimetal candidate.NdGaSi single crystal is grown by the self-flux method.It is found that the material has obvious magnetic anisotropy and may possess a more complicated non-linear spin structure.At T_m1～12.5K,it is ferromagnetically order along c axis,while at T_m2～8 K,there is an antiferromagnetic order in the ab plane.The resistivity decreases with decreasing temperature,increases slightly below T_m1,and then decreases rapidly after reaching a peak at T_m2.The increase in resistivity may be related to enhanced scattering between ferromagnetic domain walls,and the subsequent rapid decrease in resistivity is due to weakened carrier scattering come from the formation of new magnetic order.At low temperature region,it is found that there is an obvious anomalous Hall effect in NdGaSi.Further analysis showed that the intrinsic deflection mechanism played a dominant role.In the third chapter,we present the single-crystal growth of two newly synthesized Nd-Ga-Ge ternary rare earth intermetallic compounds and the research on their magnetic and electromagnetic transport properties.Nd₄Ga₅Ge₆ crystallizes in the monoclinic structure（space group:C2/m）.It is antiferromagnetic below 4 K,with c axis as the easy magnetization axis.There is a metamagnetic-like behavior induced by the magnetic field.The resistivity decreases with decreasing temperature,the abnormal increase of resistivity below the antiferromagnetic phase transition temperature may come from the formation of a new magnetic Brillouin zone boundary and the opening of the energy gap（or the formation of spin-density-wave state）.The physical origin remains to be further studied.Another ternary phase,Nd₃Ga₇Ge₃ also crystallizes in the monoclinic structure（space group:C2/c）.It is antiferromagnetic below 5 K,with b axis as the easy magnetization axis.It shows spin-flop transitions at high fields.The resistivity decreases as the temperature decreases,and the resistivity increases abnormally at the magnetic phase transition temperature and then decreases,the mechanism of the increase of resistivity may be similar to that of Nd₄Ga₅Ge₆,and the subsequent resistance decreasing behavior is related to the weakening of carrier scattering due to the formation of new magnetic order.When the magnetic field is perpendicular to the current,the sample exhibits positive magnetoresistance,and the magnetoresistance curve appears bulging at low temperatures,corresponding to the magnetic phase transition induced by the magnetic field.Negative magnetoresistance occurs when the applied magnetic field is parallel to the current and persists over a wide range of angles and temperatures,which needs further study to figure out the physical origin.In addition,the Hall resistivity of the sample shows that electrons contribute to the transport properties,and the concentration of electrons is not greatly affected by temperature changes.The fourth chapter is the summary and outlook,which briefly introduces the main research results and significance of this paper,as well as the prospect of future work.