| In the past two decades,topological theory has played an increasingly important role in condensed matter physics.Topological excitation can be found in various condensed matter systems,and these novel topological quantum states greatly enrich condensed physics.Novel physical phenomena and new materials such as anomalous quantum Hall effect,topological insulator and Weyl semimetal have been discovered.In addition,there is a new kind of topological state,which exhibit non-abelian anyonic statistical properties under exchange operations,leading to the possibility of topological quantum computation.In conclusion,it is highly possible that new materials and phenomena associated with topological order and topological state play an important role in the next generation of electronic devices applications and quantum computing.There are two main aspects in this thesis.The first part focuses on a novel topological material.Three-dimensional topological Dirac semimetal Cd3As2 has an extremely simple electronic band structures with only one pair of the symmetry protected Dirac cones at the Fermi energy.The special band structure makes Cd3As2 an excellent platform for realizing new energy-free electronic devices,therefore,has received widespread attention.It has been shown that the interplay between topological states and electronic correlation in magnetic topological semimetals may potentially lead to new topological phases.In addition,magnetic Weyl semimetal show the anomalous Hall effect,which has potential application value in the energy-free transport without external magnetic field.In this thesis,we report the tunable magneto-transport properties by adjusting Mn-doping composition in Cd3As2.The polycrystalline(Cd1-xMnx)3As2(x=0-0.20)was synthesized by controlling the ratio of the raw materials Mn3As2 to Cd3As2.The magnetic measurement results indicated that Mn-element doping transforms diamagnetic Cd3As2 to antiferromagnetic(Cd,Mn)3As2,providing an approach to control topological protected Dirac materials by manipulating antiferromagnetic order parameters.In addition,strong quantum Shubnikov-de Hass(SdH)oscillations were observed in the(Cd,Mn)3As2.By analyzing the oscillation of longitudinal resistivity in magnetic field,we found that Mn-element doping has a strong influence on the Fermi level positions,and the Fermi energy approaches to Dirac point with higher doping concentration.The Hall resistance measurement results suggested that the SdH oscillations originate from the surface state,and the Landau fan diagram yields a nontrivial Berry phase,indicating the existence of massless Dirac fermions in the(Cd1-xMnx)3As2 compounds.These results show that the antiferromagnetic(Cd1-xMnx)3As2 compounds retains original topological properties and does not transform to trivial band insulators.Our present work may pave a way for further investigating antiferromagnetic topological Dirac semimetal and expand the potential applications in optoelectronics and spintronics.The topological states in topological semimetals/insulators are called'symmetry protected topological phases',and the topological orders originate from the local symmetry of neighbor particles.However,the quantum spin liquid is considered as a 'true' topological state by some researchers since its topological order related to the long-range mode of quantum entanglement.Quantum spin liquids may be considered 'quantum disordered' ground states of spin systems,in which zero-point fluctuations are so strong that they prevent conventional magnetic long-range order.This novel state host significant value in theory,which cannot be described by the Landau symmetry breaking theory.What is more,the novel high-temperature superconductivity and quantum computing are also closely connected to quantum spin liquid.NaYbO2 has been reported as a possible host for quantum spin liquid state.In the second part of this thesis,the composition dependent polycrystalline Na1-xYbO2(x=0,0.03,and 0.07)have been investigated by combing high-field magnetizations and inelastic neutron scattering techniques.For the x=0 sample,no signature of a magnetic order is observed down to 0.3 K.Inelastic neutron scattering measurement suggests a continuous low-energy excitation spectrum centered at momentum transfer(Q)?1.25 A-1 and extending up to energy transfer(E)?2.0 meV.In contrast,x=0.03 and 0.07 samples exhibit magnetic transitions at 1.1 K and 2.3 K,respectively.High-field magnetization measurements indicate similar behaviors for x=0 and 0.03 samples including plateau-like features at the 1/3 saturated magnetization,which implies that the spin disorder in the x=0 sample might be suppressed preceding the emergence of the up-up-down phase.This composition and field dependent study allows to construct complete phase diagrams that indicate NaYbO2 is a promising candidate for quantum spin liquid state in proximity to the antiferromagnetic instability tuned by the application of magnetic fields as well as controlling the concentration of Na+ ions'vacancies.Finally,we analyze the magnetization data in details,and obtain the possible critical transition points of quantum spin liquid state and 1200 antiferromagnetic state,which further improve the temperature-field phase diagram of NaYbO2.In addition,Er and Ho elements were selected to synthesized NaEro2 and NaHoO2 and studied the magnetic and Magnetocaloric properties.Our results show that the NaEro2 and NaHoO2 exhibit excellent magnetic refrigeration capacities.Under the external magnetic field of 0?5 T,the magnetic-entropy change ?Sm reach 18.2 and 18.5 J/kg·K at 3 K and 6 K,respectively.Additionally,NaHoO2 performs great magnetic refrigeration capacities under relevantly weak magnetic fields.Under the external magnetic field of 0?1 T and 0?2 T,the magnetic-entropy change ?Sm reach 9.0 and 12.8 J/kg·K at 3 K,respectively. |
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