The Study Of Transport Properties Of Topological Semimetals

The study of topological properties of materials is a major topic in condensed mat-ter physics.Topological semimetals,as an important family of topological materials besides topological insulators,have drawn great intention in these years.People have found varieties of exotic transport properties in topological semimetals,such as non-saturated large magnetoresistance,nontrivial Berry phase,chiral anomaly,large intrin-sic anomalous Hall effect and so on.It is of great significance in condensed matter physics to further investigate and study the abundant transport properties of topological semimetals.In this dissertation,we studied transport properties of some topological semimetal systems at low temperature and high magnetic field.We observed a magnetic-field-induced electronic phase transition in the Dirac semimetal state of black phosphorus under hydrostatic pressure.We demonstrated the anomalous Hall effect in ZrTe5 orig-inates from the combination of large Zeeman splitting and strong spin-charge coupling by analyzing the quantum oscillation and the change of the anomalous Hall effect un-der pressure.We found the anomalous Hall conductivity in the half-Heusler compound GdPtBi can be suppressed by the hydrostatic pressure and it vanishes above 1.5 GPa.Further analysis excludes the disappearance of Weyl nodes under pressure as the origin of the vanishment of the anomalous Hall conductivity.We observed a large topological Hall effect in antiferromagnet EuCd2As2 and analyzed its possible origin.In antiferro-magnet EuMnSb2,we observed an extremely large anisotropic magnetoresistance and attributed it to the anisotropic band structure under magnetic field.This dissertation is divided into six chapters as follows:1.IntroductionIn this chapter,we briefly introduce the conception of Berry curvature and topol-ogy in condensed matter physics,and some basic properties of topological insulators and topological semimetals.Besides,we also review the properties and the recent in-vestigations of black phosphorus.2.Magnetic-field-induced electronic phase transition in the Dirac semimetal state of black phosphorus under hydrostatic pressureIn this chapter,we studied the magneto-transport properties of black phosphorus under hydrostatic pressure.We found that the Dirac semimetal state of black phosphorus under 1.23 GPa has tiny Fermi pocket and very low quantum limit.We observed a magnetic-field-induced electronic phase transition above the quantum limit field.Both the out-of-plane magnetoresistance and Hall resistivity show anomalous behavior at the transition field.The exponentially decayed transition temperature behavior indicates the special form of the electronic phase transition is density wave ordering,which is also supported by the theoretical calculations.Our results show that the topological semimetal is a good platform to observe the instability of the 3D pseudo-relativistic electron gas in the quantum limit regime.3.The anomalous Hall effect originated from the combination of large Zee-man splitting and strong spin-orbital coupling in ZrTe5In this chapter,we investigated the origin of the anomalous Hall effect in non-magnetic material ZrTe5.We observed a large anomalous Hall effect in ZrTe5.No magnetic order occurred in ZrTe5 and the weak temperature dependence of anomalous Hall conductivity indicate that the anomalous Hall effect in ZrTe5 comes from the in-trinsic Berry curvature contribution.By the analysis of the quantum oscillation,we demonstrated that Weyl nodes do not exist in ZrTe5.Further analysis indicates that the AHE in ZrTe5 originates from the combination of large Zeeman splitting and strong spin-orbital coupling.The anomalous Hall resistivity changes its sign from negative to positive at 1.3 GPa,which further supports the intrinsic contribution of anomalous Hall effect in ZrTe5.4.The study of the pressure-controlled anomalous Hall conductivity in half-Heusler antiferromagnet GdPtBiIn this chapter,we systematically studied the magneto-transport properties of half-Heusler antiferromagnet GdPtBi under hydrostatic pressure.We found that despite the minor change of the band structure and magnetic structure below 2 GPa,the anomalous Hall conductivity of GdPtBi can be suppressed by the pressure continuously and van-ishes at 1.5 GPa.The chiral anomaly and electronic band structure calculation demon-strate the existent of Weyl nodes in GdPtBi under 2 GPa,which indicates the suppres-sion of the anomalous Hall conductivity does not arise from the disappearance of Weyl nodes under pressure.The magneto-transport measurement and the analysis of the band structure indicate that the disappearance of anomalous Hall conductivity in GdPtBi orig-inates from the shift of the Fermi level relative to the Weyl nodes or the minor change of the canted spin structure of Cd3+ ions in finite magnetic field under pressure.5.The study of topological Hall effect in EuCd2As2In this chapter,we mainly studied the magneto-transport properties of antifer-romagnet EuCd2As2.The antiferromagnetic transition temperature TN is 9.1 K in EuCd2As2.The magnetic susceptibility and resistivity show that there is a strong spin fluctuation below 50 K.We found that EuCd2As2 shows large topological Hall effect below 40 K.The topological Hall resistivity shows its maximum 511 ?? cm at TN and it decreases sharply with the temperature deviating from TN.We came to the conclu-sion that the large topological Hall effect in EuCd2As2 arises from the Berry curvature in real space from the special spin texture,or the non-zero Berry curvature around the Weyl nodes in the momentum space.6.Field-induced insulator-to-metal transition and colossal anisotropic mag-netoresistance in a Dirac material EuMnSb2In this chapter,we observed a magnetic field-induced insulator-to-metal transi-tion and an extremely large anisotropic magnetoresistance in antiferromagnet EuMnSb2.The anisotropic magnetoresistance reaches its maximum?1.84×106%at 2 K,which is 4 orders of magnitude larger than previously reported AMR values in conventional antiferromagnet.Band structure calculations show that EuMnSb2 is a massive Dirac system.The Dirac-like linear band dispersion at Y point is strongly affected by the spin-orbital coupling effect and magnetism.Consequently,the band structure of EuMnSb2 can be significantly modulated by the direction of Eu2+ moments under magnetic field,thus resulting in an anisotropic band gap.The combined anisotropic band gap and the corresponding Fermi level position generate the extremely large anisotropic magnetore-sistance in EuMnSb2.