Magnetotransport Properties Of Dirac Semimetal Cd₃As₂ Nanoplates

Recently,the investigation of topological semimetals has attracted intense interest in condensed-matter physics.Due to the nontrivial topology of electronic structures near the Dirac/Weyl nodes in momentum space,the three dimensional(3D)topological semimetals exhibit a number of unusual physical properties,such as ultra-high carrier mobility,negative longitudinal magnetoresistivity(NLMR),planar Hall effect(PHE),and Fermi-arc surface states.Compared with other topological semimetals,Dirac semimetal Cd3As2 has an extremely simple electronic structures with only one pair of the symmetry-protected Dirac nodes at the Fermi energy and has a nearly sphere-like Fermi surface.Therefore,Cd3As2 provides an ideal platform to investigate the novel transport properties in 3D Dirac/Weyl semimetals.In this thesis,we systematically investigate the magnetotransport properties of high-quality Cd3As2 nanoplates with the nanoplates grown by chemical vapor deposition method and then patterned into a standard Hall-bar devices by Nano-fabrication techniques.The main results can be concluded as follows:1)The chiral anomaly in Cd3As2 nanoplatesChiral anomaly is one of the most exciting transport properties in topological semimetals,which usually manifests as NLMR.Whereas,the NLMR can actually be induced by other physical mechanisms.Recent theories indicate that the PHE combined with NLMR in nonmagnetic topological semimetals,can provide a key transport signature of the chiral anomaly.In this chapter,we carry out the PHE investigation of the high-quality Cd3As2 nanoplates.For magnetic field B oriented parallel to the electric field E,these nanoplates exhibit large NLMR in low temperature region.Tilting the in-plane magnetic field B,the observed planar longitudinal magnetoresistivity displays a cos2 ? dependence,and after subtracting the magnetoresistivity components originating from the experimental misalignments,the obtained planar transverse magnetoresistivity ?xyPHE displays a?sin0cos?dependence,which are well with the theoretical predication.Further experiments demonstrates that both the behaviors of NLMR and ?xyPHE can be suppressed synchronously with increasing the temperature and can survive at room temperature,indicating the same origin of both effects:Chiral anomaly.The amplitudes of the PHE show B2-dependence and B-dependence in weak magnetic field limit and the quantum limit,respectively.Furthermore,the current jetting effect is excluded by presenting systematically experimental results and carefully theoretical calculations.Our experiments provide key transport signal for the chiral anomaly in Weyl Fermion systems.2)Fermi-arc surface states induced quantum oscillations in Cd3As2 nanoplatesThe most direct way to probe the Fermi-arc surface states is the angle-resolved photoemission spectroscopy(ARPES).However,the Fermi-arcs in Cd3As2 have not been observed by ARPES up to now.Recent theory indicate that the disconnected Fermi-arcs on opposite surfaces can form an unusual closed Weyl magnetic orbits and can contribute to the quantum oscillatiomns.In this chapter,we carry out the quantum oscillations study of the Weyl magnetic orbits in the Dirac semimetal Cd3As2 nanoplates.We find that,the nanoplates exhibit extra weak 2D oscillations superimposed on 3D bulk oscillations at high fields with the magnetic field B perpendicular to the sample surface.Combined the analysis of the angle dependent oscillation frequency and the theoretical calculation,we can conclude that the observed 2D oscillations at high fields can be attributed to the Weyl magnetic orbits connecting the Fermi arcs from opposite surfaces.Further experiment demonstrated that the 2D oscillations of Weyl magnetic orbits can be well detected by nonlocal detection.Our results provides an alternatively effective way to study the quantum transport properties of Fermi-arc surface states.3)Thermal tuning of the carrier density in Cd3As2 nanoplatesConventional gating technologies could gain an effective modulation of carrier density for thin samples.However,the Dirac nodes of Dirac semimetal Cd3As2 would open a gap with the thickness less than 60 nm.Therefore,a feasible route to effectively tune the carrier density in Dirac semimetal Cd3As2 nanoplates is highly desirable.In this chaprer,we demonstrate that the carrier density of Cd3As2 nanoplates can be efficiently tuned by the thermal cycling treatment(TCT).As TCT goes on,both the carrier density and mobility exhibit an anomalous evolution.Moreover,with the upwardly shifted Fermi energy,the oscillations of Fermi-arc surface states and the NLMR induced by chiral anomaly can be suppressed apparently with an anomalous phase shift of the bulk quantum oscillations.Meanwhile,the coupling between the quantum oscillation of the 2D Fermi-arc states and 3D bulk states leads to a shift of peaks of bulk oscillations at high fields.Our results provide a thermal control knob for manipulations of the carrier density in Dirac semimetal Cd3As2.4)High-temperature competition between NLMR and weak antilocalization in Cd3As2 nanoplatesWe systematically investigate the high-temperature competition between the NLMR and weak antilocalization(WAL)by magnetotransport measurements on high-quality samples of Dirac semimetal Cd3As2 nanoplates.We find that,for B I || E,the observed large WAL effect that almost overwhelms the NLMR induced by chiral anomaly in weak fields at low temperatures.With the increase of temperature,the WAL becomes to weaken gradually and finally vanishes at 200 K.However,the NLMR is robust against temperature and persists up to room temperature.This high-temperature competition between these two effects is further corroborated by the well fitted curves using sem iclassical formula with the experimental data.Further experiments demonstrate that the high-temperature competition can be understood in terms of the low carrier density in Cd3As2 nanoplates.In addition,the NLMR reverses it sign at the critical field Bc,which is can be viewed as a signature of the Cd3As2 nanoplates enter the quantum limit,and the observed non-saturating linear LMR can be understood by the linear quantum magnetoresistance.Our results provide a better understanding of the novel magnetotransport properties of Dirac semimetal Cd3As2.