Study On Magnetoresistance And Electronic Structure Of Nonmagnetic Metal Oxide And Superconductor Nb₃Sb

Since the discovery of quantum Hall effect in two-dimensional electron gas in the 1980s,sci-entists have gradually realized the importance of topological properties of electronic band structure of solid materials to the classification of materials and the understanding of physical properties.Many new concepts such as topological insulator(TI),Dirac semimetal,Weyl semimetal,nodal-line semimetal and topological superconductor have been proposed and exotic properties as the extremely large magnetoresistance(XMR),chiral anomaly,quantum spin Hall effect and non-zero berry have been found.Since XMR effect has a great potential in applications,it once again s-timulates people to explore the physical mechanism.Experimentally,it is hard to identify which mechanism is responsible for MR in a particular sample.It is necessary to make a clear connection between experimental observations and theoretical models.Based on the previous studies,this thesis chooses nodal-line semimetal MoO2,topological trivial metal ReO3 and high symmetry point semimetal Nb3Sb and systematically studied their large MR effect.Many interesting results were obtained as follows:(1)We have calculated the band structure and Fermi surface of the the monoclinic structure(P21/c,No.14)MoO2 and found it is a nodal-line semimetal.We have also grown high quality single crystal of MoO2 and systematically measured the longitudinal resistivity,Hall resistivity and quantum oscillation of magnetization.Similar to most topological trivial/nontrivial semimetals,MoO2 also exhibit large quadratic MR(MR?5.03 × 104%,2 K and 9 T),and up-turn behavior at low temperature on pxx(T)curves under magnetic field.The Hall resistivity and Fermi surface calculations indicate that those behaviors originate from the compensation of hole and electron carriers.Interestingly,The observation of significant negative MR when the magnetic field is applied along the current and non-zero Berry phase obtained from dHvA oscillation imply that Weyl nodes may exist due to the crystal symmetry breaking caused by the oxygen vacancies.(2)The ordinary nonmagnetic oxide metal ReO3 has simple cubic structure(Pm3m,No.221).Previous research has studied its negative expansion coefficient,temperature dependence of resistivity and dHvA oscillation.We have successfully grown high quality ReO3 single crystal by transport method,then calculated the band structure and Fermi surface.We systematically mea-sured the longitudinal resistivity,Hall resistivity and magnetization and found that ReO3 exhibits unsaturated large MR(9.43 × 103%,10 K and 9 T)with strong anisotropy.The magnetic field dependence of MR shows different power exponent(H1.63?1.90)when the field is along different crystal axis.According to the Fermi surface calculation,we believe that the Fermi surface pro-jected onto the plane which is perpendicular to the magnetic field varies when the field is along different crystal axis,thus both hole-electron carriers compensation and open orbits mechanisms lead to various power exponent of MR.(3)A15 structure compound,Nb3Sb is a conventional superconductor with a superconduct-ing transition temperature Tc?0.2 K and its electronic structure and phonon spectrum had been extensively studied decades ago.Recent band structure calculations predict that Nb3Sb is a high symmetry point semimetal.We have successfully grown the single crystals of Nb3Sb and sys-tematically measured the longitudinal resistivity,Hall resistivity and magnetization with quantum oscillation.Similar to other topological trivial/nontrivial semimetals,Nb3Sb exhibits large MR at low temperature(717%,2 K and 9 T),parabolic MR curves and up-turn behavior on ?xx(T)curves under magnetic field.The Hall resistivity data shows that Nb3Sb is a perfect hole-electron compensate material while the non-zero Berry phase obtained from the dHvA oscillation proves that Nb3Sb is topologically nontrivial.The above results indicate that topological surface state and bulk s-wave superconductivity are both exist in Nb3Sb.Our research demonstrate that the details of the Fermi surface plays an important role in the large MR in topological trivial/nontrivial materials and Nb3Sb system provides a new platform for exploring Majorana fermions.