Synthesis And Physical Properties Of LnM₄Pn₂ （Ln=Eu,Sr;M=Cu,Ag;Pn=P,As,Sb） Single Crystals

In condensed matter physics,the interaction of charge,spin,orbital and lattice can lead to a variety of fantastic phenomena,such as superconductivity,colossal magnetoresistance,charge density wave(CDW)and so on.For the past few years,with the development of this field,various topological materials such as topological insulators and topological semimetals emerged,exhibiting many novel physical properties such as Fermi arcs,quantum oscillations,and ultrahigh mobility.This dissertation focuses on the material with CaCu4P2 structure family,Earlier studies have shown that many of the compounds with the CaCu4P2 crystal structure exhibit various physical properties,such as magnetic phase transitions,strange transitions,and charge density waves.Some of the materials also exhibit topological properties.But in general,many materials in this system are only at the synthetic stage,and most of them have not been systematically studied.Single crystals of LnCu4-δP2,LnCu4As2,and LnAg4Sb2(Ln=Eu,Sr)were systematically synthesized and characterized by means of electrical transport,magnetization,specific heat,thermal expansion and so on.The following results were obtained:(1)Anomaly physical properties related to structural distortion were observed in EuCu4P2 and SrCu4P2 single crystal.High-quality single crystals of LnCu4P2 were grown and systematically characterized.Magnetization measurements reveal EuCu4P2 and SrCu4P2 exhibit antiferromagnetic and paramagnetic behavior,respectively.A significant anomaly irrelated to magnetism in resistivity and heat capacity was observed in both compounds,i.e.,at～120 K/160 K in EuCu4P2 and 140 K in SrCu4P2.Thermal expansion reveals a subtle lattice parameters variation near anomaly,i.e.ΔLc/Lc～0.013%and 0.062%for EuCu4P2 and SrCu4P2,respectively.The results of temperaturedependent SAED prove that there is no crystal structure transition below and above anomaly.The anomaly transition in cooling resistivity in EuCu4P2 and SrCu4P2 should be associated with structural distortion.A large and nonsaturating linear magnetoresistance was observed in SrCu4P2 single crystals.It could originate from mobility fluctuation caused by defect by analysis of different models.(2)Large positive magnetoresistance was observed on EuCu4As2 and anomaly physical properties associated with structural distortion also were observed in LnCu4As2.High-quality single crystals of EuCu4As2 and SrCu4As2 were grown and systematically characterized.Magnetization measurements reveal EuCu4As2 and SrCu4As2 exhibit antiferromagnetic and paramagnetic behavior,respectively.EuCu4As2 shows large positive magnetoresistance up to 750%at 2 K on electric measurements.A significant anomaly irrelated to magnetism in resistivity and heat capacity was observed in both compounds.Thermal expansion reveals a subtle lattice parameters variation near anomaly,i.e.(ΔLab/Lab)～0.196‰ and 0.007‰,(ΔLc/Lc)～0.8‰ and 1.037‰ for EuCu4As2 and SrCu4As2,respectively.The results of temperature-dependent SAED prove that there is no crystal structure transition below and above anomaly for SrCu4As2,the anomaly should be associated with structural distortion before and after transition.(3)Magnetic phase diagram of EuAg4Sb2 was obtained and EuAg4Sb2 was proved to be a topological semimetal candidate.High-quality single crystals of EuAg4Sb2 were grown and systematically characterized.Two magnetic phase transitions were observed in the low-temperature region at～8 K and～11 K in electrical transport and magnetization measurements.The results of specific heat measurements indicate that the transition at 8 K is a first-order phase transition related to magnetism and this transition can be tuned by magnetic field.Large magnetoresistance with SdH oscillation was observed up to 2.0×103%at 1.8 K and 28 T in electrical transport under high magnetic field in the single crystal.Multi-period SdH oscillation analysis indicates that it has three band with non-trivial Berry phases,this prove that EuAg4Sb2 may be a topological semimetal.(4)SrAg4Sb2 was proved to be a topological semimetal candidate.High-quality single crystals of SrAg4Sb2 were grown and systematically characterized for their physical properties.Our experiments verified that there is a transition at 15 K and confirmed that the transition has no relation to magnetism.Giant magnetoresistance with Shubnikovde Haas(SdH)oscillation was observed up to 2×104%at 1.8 K and 28 T in the high field electrical transport.The analysis of SdH oscillations indicate that there are two energy bands with three-dimensional Fermi surface.The dHvA oscillations on the magnetic torque measurements of the single crystal show that the results of fitting of four energy bands suggest β band has non-trivial Berry phase.The angle-rotating measurements show three bands have 3D Fermi surface structure.Combined with DFT,SrAg4Sb2 was proved as aweak topological semimetal.