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논문 기본 정보
- 자료유형
- 학위논문
- 저자정보
- 지도교수
- 정윤희
- 발행연도
- 2017
- 저작권
- 포항공과대학교 논문은 저작권에 의해 보호받습니다.
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We synthesize and investigate strongly correlated materials by using the fundamental characterization tools such as magnetic property and electric transport measurement system. To perform the comprehensive investigation of exotic magnetoelectric materials, we found the microscopic origin and information about the cupric divanadate Cu2V2O7 and topological non trivial metal Bi1−xSbx (x ∼ 0.04).
Single crystals of orthorhombic polar oxide α-Cu2V2O7 with space group Fdd2 are synthesized and their physical properties are measured. Neutron powder diffraction is also performed on a poly-crystal sample to extract the magnetic structure. The ground state is shown to be weakly ferromagnetic, that is, collinearly antiferromagnetic in the a-direction with a small remanent magnetization in the c-direction. When an external magnetic field is applied in the c-direction, further spin canting, accompanied by the induced electric polarization, occurs. It is demonstrated that the magnetoelectric effect in α-Cu2V2O7 is adequately described if spin-dependent p-d hybridization due to spin-orbit coupling as well as magnetic domain effects are simultaneously taken into account. We discuss the implication of the present result in the search for materials with multiferroicity and/or magnetoelectricity.
The Weyl semimetal is a hopeful system to apply on the future spintronic devices. Antimony doped bismuth alloy,BixSb1−x, allows us to reach the critical point
which is a boundary between the topological non trivial insulator and trivial band insulator. About 3-4% Sb doped case, electron and hole bands are touched at a point, becomes a Dirac metal. When external magnetic fields are turned on, degenerated Dirac bands are split to two Dirac bands along the applied field direction.
These Dirac nodes have different chirallity and generate anormalous current when electric and magnetic fields are applied parallel direction. Thus, the negative value
of longitudinal magneto resistance appears and this has been regarded as a clue of the existence of Weyl phase. However, we found more concrete measurement method to justify Weyl state by using measurement of non-linear dependence on the longitudinal magnetoresistance. Comparing with transverse magnetiresistance, other
possibilities are ruled out which originate from extrinsic effects such as Joule heating and Schottky junction problem. Semi classical Boltzmann transport theory with topological aspect is introduced to explain the nonlinearlity. We find a quadratic dependence of conductivity with respect to E and B, reveals only in the presence of the Weyl phase.
Single crystals of orthorhombic polar oxide α-Cu2V2O7 with space group Fdd2 are synthesized and their physical properties are measured. Neutron powder diffraction is also performed on a poly-crystal sample to extract the magnetic structure. The ground state is shown to be weakly ferromagnetic, that is, collinearly antiferromagnetic in the a-direction with a small remanent magnetization in the c-direction. When an external magnetic field is applied in the c-direction, further spin canting, accompanied by the induced electric polarization, occurs. It is demonstrated that the magnetoelectric effect in α-Cu2V2O7 is adequately described if spin-dependent p-d hybridization due to spin-orbit coupling as well as magnetic domain effects are simultaneously taken into account. We discuss the implication of the present result in the search for materials with multiferroicity and/or magnetoelectricity.
The Weyl semimetal is a hopeful system to apply on the future spintronic devices. Antimony doped bismuth alloy,BixSb1−x, allows us to reach the critical point
which is a boundary between the topological non trivial insulator and trivial band insulator. About 3-4% Sb doped case, electron and hole bands are touched at a point, becomes a Dirac metal. When external magnetic fields are turned on, degenerated Dirac bands are split to two Dirac bands along the applied field direction.
These Dirac nodes have different chirallity and generate anormalous current when electric and magnetic fields are applied parallel direction. Thus, the negative value
of longitudinal magneto resistance appears and this has been regarded as a clue of the existence of Weyl phase. However, we found more concrete measurement method to justify Weyl state by using measurement of non-linear dependence on the longitudinal magnetoresistance. Comparing with transverse magnetiresistance, other
possibilities are ruled out which originate from extrinsic effects such as Joule heating and Schottky junction problem. Semi classical Boltzmann transport theory with topological aspect is introduced to explain the nonlinearlity. We find a quadratic dependence of conductivity with respect to E and B, reveals only in the presence of the Weyl phase.
목차
1 Introduction 11.1 General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Outlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Overview of the General Backgrounds 52.1 Multiferroicity: Magnetoelectric Effects . . . . . . . . . . . . . . . . . 52.1.1 Ferroelectricity in non-collinear magnetic system . . . . . . . . 62.1.2 Ferroelectricity in collinear magnetic system . . . . . . . . . . 62.1.3 Spin-dependent Hybridization mechanism . . . . . . . . . . . 82.2 Magnetoelectric Tensor Analysis . . . . . . . . . . . . . . . . . . . . . 102.3 Brief review of Cu2V2O7 . . . . . . . . . . . . . . . . . . . . . . . . . 122.4 Electromagnetic properties of Topological non-trivial metal:Weyl metal 172.4.1 Weyl metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.5 Transport Theory for Weyl metal . . . . . . . . . . . . . . . . . . . . 222.5.1 Boltzmann transport equation . . . . . . . . . . . . . . . . . . 222.5.2 Application to Weyl metal . . . . . . . . . . . . . . . . . . . . 233 Experimental Techniques 293.1 Sample Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1.1 Solid State Reaction Method . . . . . . . . . . . . . . . . . . . 293.1.2 Flux Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.1.3 Bridgman and Stockbarger Method . . . . . . . . . . . . . . . 313.2 Structural Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2.1 X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . 313.3 Physical Properties Measurements . . . . . . . . . . . . . . . . . . . . 333.3.1 Magnetic Properties Measurements . . . . . . . . . . . . . . . 333.3.2 Pyro-, Magneto-electric Current Measurement . . . . . . . . . 334 Cupric Vanadate α-Cu2V2O7 394.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394.2 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 424.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534.5 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544.5.1 Neutron Powder Diffraction . . . . . . . . . . . . . . . . . . . 544.5.2 Polarization calculation . . . . . . . . . . . . . . . . . . . . . . 574.5.3 Domain Effects . . . . . . . . . . . . . . . . . . . . . . . . . . 605 Weyl metal: Time Reversal Symmetry Broken Bi1−xSbx (x=0.04) 655.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655.2 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675.3 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675.4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 705.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746 Summary 79
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