The Experimental Study On Transport Properties Of Type ? Topological Semimetals WTe₂ And PtTe₂

The Weyl Semimetal(WSM)is a new type of topological materials that has attracted much attention after the study of topological insulator.It is typically characterized by pairs of Weyl points with opposite chiral charges in the body of Brillouin zone(BZ)and open Fermi arcs on the surface of BZ.The massless,chiral Weyl fermion emerges as the low-energy excitations near the Weyl points.The research of the topological materials has been rapid developed and the author has witnessed important achievements as a graduate student.The first-principles calculations and angle-resolved photoemission spectroscopy(ARPES)play important roles because they can predict and identify the topological nature of various materials,respectively.However the transport research is also important because it lays the foundation for the future application of WSM in electronic devices.Shortly after the TaAs family was confirmed as WSM,a new group of type ? WSM began to draw widespread attention.Type ? WSMs have many unique properties that differ from the type ? WSMs because their Lorentz invariance is broken.These properties include momentum space Klein tunneling,field-selective chiral anomaly and intrinsic anomalous Hall Effect.My work mainly focused on tuning the transport properties of type-? WSM WTe2 nanodevices.In addition,I conducted the study of the quantum oscillations of the type II Dirac Semimetal(DSM)PtTe2.The main research results are listed as following:(1)In the first work,the author tuned the transport properties of type ? WSM WTe2 nanodevice using scanning electron microscope equipped with a Ga+ ion beam.Ga+implantation introduces Frenkel defects and substitution defects in WTe2 nanodevices and the lattice defects further lead to the change of carrier properties and Fermi surface.These changes affect the electrical transport of WTe2 devices.Comparing the electrical transport data before and after ion implantation,I show that the introduction of lattice defects reduces the carrier concentration and mobility,which in turn leads to the decrease of the magnetoresistance.Raman spectroscopy quantitative analyses combined with first-principles calculations determine that the Frenker defect of Te is the dominant defect type introduced by Ga+ ion implantation in the WTe2 devices.By first-principles calculations,the author also studied the effect of various lattice defects on the band structure and the position of the Fermi level.This work explores a new way of tuning device over a depth much longer than the electrostatic screening length,which makes the thicker devices tunable.(2)In the second work,the author systematically studied the effect of Mo doping on the electrical transport properties of WTe2 nanodevices.Both WTe2 and MoxW1-xTe2 have been theoretically predicted to be type ? WSM and have been confirmed by ARPES or transport experiments however transport study on nanodevices of this system has been rarely reported.Through the analysis of the two-band model,I found that Mo doping suppress the mobility of electrons and holes.The higher the Mo concentration is,the lower the mobility will be.The low mobility is the main reason for the decrease of magnetoresistance induced by Mo doping.Interestingly,the electron-hole carriers' balance in WTe2 is not broken by the doping of Mo.Shubinikov-de Haas(SdH)oscillation analysis shows that Mo doping suppresses the oscillation amplitude.The higher the Mo doping concentration is,the less the observed oscillation frequency will be.But it seems that these frequencies still correspond to the four frequencies observed in WTe2.At the same time,I found that the effective mass of each frequency increases with increasing Mo concentration,while the quantum mobility decreases.In this work,the author completed a comparative study of type ? WSM WTe2 and MoxW1-xTe2,and understood the effect of Mo doping on the transport of WTe2 nanodevices.(3)In the third work,we systematically studied the de Haas-van Alphen(dHvA)effect of type ? Dirac semimetal PtTe2,and conducted the first-principles calculations to deepen our understanding of its electronic structure.After the discovery of type ?WSM,people have been considering whether there is a DSM that breaks the Lorentz invariance.The discovery of the PtSe2 family confirms the existence of type ? Dirac semimetals.Although the topological properties of PtTe2 have been well confirmed,studies on its transport properties have rarely been reported.The author measured the isothermal magnetization of PtTe2 crystals,and observed a strong dHvA effect in the in-plane and out-of-plane magnetization.Quantum oscillation analysis shows that the effective mass of PtTe2 is comparable to that of PdTe2,and the quantum mobility is significantly higher than that of the latter.It can be seen that PtTe2 has superior transport properties than PdTe2.Based on the first-principles calculations,we further confirmed the topological nature of PtTe2 and determined the origin of different oscillation modes.It is worth mentioning that in the dHvA oscillation we also observed the electronic pockets that participate in the formation of the type ? Dirac cone.