Study On Electrical Transport Properties Of Topological Semimetals

Following the emergence of topological insulators,the development of topological band theory in condensed matter physics has promoted the discovery of topological semimetals.Topological semimetals contain diverse new types of topological fermions,which are of crucial importance in the study of topological electronics.In addition,various material systems and distinct band structures in topological materials bring new opportunities for the study of many novel physical phenomena.In this thesis,we mainly studied the electrical transport properties of topological semimetals.The major results are outlined as follows:1.Introduction of the basic physical properties of topological semimetals,two dimensional materials and layered topological semimetals.The device fabrication and measurement techniques used in our studies are also introduced.2.Experimental studies on the transport properties of p-type Cd3As2.We observed the magnetic field induced insulator-metal-insulator transition on the temperature dependent resistivity curves.Theoretical analysis indicates that these two phase transitions are related to the Dirac cone.The insulating phase at low temperature originates from the Landau localization under magnetic field,and insulating phase at high temperature arises from the thermal excitation of defect states near Dirac cone.3.Experimental studies on the anisotropic chiral anomaly effect in type-II Weyl semimetal WTe2.The anisotropic chiral anomaly effect is the distinct transport property of the type-II Weyl fermion,we verified this propertie in WTe2 by fabricating thin film devices with different crystal orientations.We chose thin films with specific thickness to suppress the magnetoresistance(MR)contribution of normal bulk state,and fabricated high quality devices using shadow mask method.In our experiments,we observed obvious negative longitudinal MR induced by chiral anomaly effect with current along b axis,whereas it disappears with current along a axis,which provides the transport evidence of type-II Weyl fermions in WTe2.In addition,by applying the gate voltage,we can in-situ tune the Fermi energy accross the Weyl point.4.In addition to the property of type-II Weyl fermions,WTe2 possesses large non-saturating MR but its physical origin is still under debate.We fabricated high quality h-BN protected WTe2 thin film devices using dry-transfer method,and studied the mechanism of large MR through electrostatic doping approach.We found that the MR is closely related to the charge compensation,which reaches maximum at the charge compensation point.This result demonstrates that the large MR is induced by charge compensation mechanism.In addition,with charge compensation retained in a wide temperature region,we revealed the charge scattering mechanisms for the temperature dependent magneto-transport.5.Based on the spin polarized band structure of WTe2,we explored its intrinsic spin transport using nonlocal method.We found that the spin mediated nonlocal transport in WTe2 depended on the crystal orientation.When the current is applied along b axis,the spin mediated nonlocal transport is dominated by the large intrinsic spin Hall effect.But it is dominated by the current-induced spin polarization(CISP)with current along a axis.Moreover,we also achieved the long spin diffusion length(-1 μm)by studying the length dependence of nonlocal resistance.These results reveal promising spintronic applications of WTe2.The main innovations of this thesis are:1.Observatied the anisotropic chiral anomaly in type-Ⅱ Weyl semimetal,and realized the in-situ tuning of the Fermi energy across the Weyl point for the first time.2.Realized the in-situ tuning of the Fermi energy in WTe2 across the charge compensation point for the first time,and revealed the physical origin of large MR in WTe2.3.Observed crystal orientation dependent spin transports for the first time,and discovered diverse spin transport mechanisms in WTe2 which can be distinguished by the crystal orientation.