Study Of The Transport And Magnetic Properties Of Novel Topological Semimetals And Frustrated Iridium Oxides

In recent years,quantum communication technology has received extensive interest due to its high efficiency and safety,and has become one of the most hot spots of quantum physics and information science research in the world.Searching for new-type and practical materials is primary importance in condensed matter physics.At present,topological materials and quantum-spin liquid materials have been widely studied.Topological materials have many excellent electrical and magnetic properties are very valuable in the future quantum computing and quantum communication.From the three-dimensional topological insulator,to the quantum anomalous hall effect,to the Weyl semimetal,to the topological quantum materials,design,preparation,characterization and application research of topological material have gathered great efforts of a large number of researchers at home and abroad.In addition,the quantum spin liquid material caused by geometrical frustration is also a hot topic.The spins in the quantum spin liquid are disordered,but entangled with each other over long distances,and lead to a potential application in quantum communication in the future.In addition,the study of quantum spin liquid also contributes to the understanding of the mechanism of high temperature superconduction.Topological materials and quantum spin-liquid materials have rich physical properties and broad application prospects,but the research in this field has just started,and more and deeper studies are urgent.In the first chapter,we introduced the research progree of new topological materials,and enumerated the classification of the Hall effect by the corresponding formation mechanisms and electromagnetic transport characteristics.Then,the theory and experimental research progress of topological insulator and topological semimetal was introduced in detail.Finally,we introduced the measurement methods of Fermi surface,resistance and magnetism.In the second chapter,the energy band structure,transport and magnetic properties of single crystal DySb were studied theoretically and experimentally.In experiments,DySb single crystal was prepared with Sn as the cosolvent at high temperature.The structure and basic electrical and magnetic properties were measurand with EDX,XRD,PPMS at low temperature.The magnetoresistance shows obvious quantum oscillation and giant magnetoresistance effect under high field up to 37 T.The energy band structure of materials was studied by quantum oscillation and first-principles calculation.Four different frequencies are obtained by cryogenic quantum oscillation analysis,indicating the multiple energy bands at Fermi surface.The theoretical calculation in the case of ferromagnetic configuration by considering spin-orbit coupling shows that energy band inversion exists below the Fermi surface,and DySb may be a topological semimetallic.The giant magnetoresistance effect in materials is associated with high electron mobility due to topologically linear band structure.In the third chapter,we studied the formation mechanism of planar hall effect in,Td-MoTe2,the type-II Weyl semimetal High quality monocrystalline MoTe2 samples were prepared experimentally by self-solvent method.The magnetic field rotates in a-b plane,and the angle between the magnetic field direction and the current direction is 0.The variation rule of hall resistance and transverse resistance with the angle 0 under different temperatures and magnetic fields were measured by six-electrode method.The change of planar hall resistance and transverse resistance with magnetic field angle can be well described by the theoretical formula.We attributed the change of anisotropic resistance ??chiral mainly to ?? caused by orbit magnetoresistance rather than ?||,caused by the chiral anomaly(?? is the resistance when the magnetic field perpendicular to the current direction,?|| is the resistance when the magnetic field parallel to the current direction).Comparing to the chiral anomaly,the orbital magnetoresistance plays a leading role and to the planar hall resistance.Meanwhile,it is because of the existence of large orbital magnetoresistance that it is difficult to observe the negative magnetoresistance effect caused by chiral anomaly in this Weyl semimetallic material.One conclusion can be inferred that only when the negative magnetoresistance effect is observed at the same time,the measured planar hall effect may be related to the chiral anomaly.In the fourth chapter,we systematically studied the electrical and magnetic properties of phyrochlore iridium oxide Bi2Ir2-xMnxO7.Experimentally,five type of polycrystalline samples with different Mn components,with x=0,0.05,0.1,0.15 and 0.2,were prepared by high-temperature melting method,and the actual component proportion and crystallization were measured by EDX and XRD.According to the magnetic study,it was found that the anti-ferromagnetism of the material first increased with the incorporation of Mn elements,and the anti-ferromagnetism tended to become weaker from stronger with the continuous increase of Mn.This change of magnetism is mainly attributed to the intense competition among the anti-ferromagnetic interaction between Ir-Ir,the anti-ferromagnetic interaction between Ir-Mn and the ferromagnetic interaction between Mn-Mn.The results of electrical transport analysis showed that with the introduction of Mn4+ ion,the material changes from metal state to insulation state.And this phenomenon is related to the enhancement of electron localization induced by the 3d element doping.At low temperature,Bi2Ir2O7 shows positive magnetoresistance,while the doped samples all exihibites negative magnetoresistance.The change of magnetoresistance corresponds to the metal-insulator transformation caused by doping,indicating that the substitution of 3d-Mn has a great influence on the magnetic and electric transport properties of this material.In the fifth chapter,the electrical,magnetic and thermodynamic properties of the polycrystalline iridium oxide Ba3MgIr2O9 were studied under to 0.4 K.GGA,GGA+U(2 eV)and GGA+U(2 eV)+SOC were used to calculate the energy state density of the ground states.The calculation results under the GGA+U+SOC model considering spin-orbit coupling and coulomb interaction show that the first type of anti-ferromagnetic structure AFM1,with anti-ferromagnetic intra and inter Ir2O9 dimer,has the lowest energy state and exists energy gap.The analyses of magnetic susceptibility and specific heat under low temperature showed that there was no magnetic order until 0.4 K.The results of Curie-Weiss fitting of susceptibility show the existence of antiferromagnetic state,consistent with the theoretical calculation of AFM1 magnetic configuration.In addition,there are very few Ir4+ ions(Mess than 1‰)in the polycrystalline Ba3MgIr2O9 sample,resulting in the tiny ferromagnetic moment of 1.4×10-3 ?B/Ir.Due to the existence of strong triangular geometrical frustration in the a-b plane,the system is still quantum spin-orbital liquid at extremely low temperature.