Magnetotransport Properties Of Topological Semimetal PtBi₂

In recent years the rising of topological semimetals extended topological classification from topological insulators to metals.The low-energy state electrons excitation in topological semimetals satisfies the Dirac equation,which induces many novel physical properties such as the ultrahigh carriers mobility,extremely large magnetoresistance(MR)and chiral abnormal in applied field et al,so the research of topological semimetals is not only of great significance in science but also provides a new material platform to develop electronic devices with lower energy consumption,higher integration and more powerful.Further research classifies the topological semimetals into Dirac semimetals,Weyl semimetals,nodal-line semimetals and triply degenerate semimetals.Searching for new topological semimetals system and taking magnetotransport research on them are also hot research topic in the field of condensed-matter physics.In this thesis,the cubic phase PtBi2 and trigonal phase PtBi2 were studied systematically from the synthesis of single crystal,physical characterization,magnetic transport measurement aspects.The main results are presented as follows:(1)We synthesized the high quality cubic PtBi2 single crystal by self-flux method took the quantum transport measurement under high magnetic field.We observed extreme large unsaturated magnetoresistance(XMR)up to 11.2 million percent in a magnetic field of 33 T,which surpasses the previously reported Dirac materials.Analyzing of the significant Shubnikov-de Hass(SdH)quantum oscillations reveals ultrahigh mobility(?105 cm2V-1S-1),light effective masses and nontrivial Berry's phase in cubic PtBi2,all are the fingerprint features of topological semimetals.Further band structure calculation based on first principles reveals nontrivial electronic structure and Dirac point with linear dispersion in cubic PtBi2,besides,there are three kinds of carriers with different effective masses near the fermi level.Furthermore,the magnetization performed remarkable de Haas-van Alphen(dHvA)oscillations,analyzing of the oscillations reveals light effective masses,nontrivial Berry's phase and three-dimensional Fermi surface,all were consistent with the transport results,so we suggested that pyrite PtBi2 is a new topological semimetal.(2)Based on the ab initio calculations,we have performed electronic structure calculation and analysis on the trigonal PtBi2,which is another structure phase of PtBi2 system.We found that there are as many as six fermi pockets near the fermi level and two of them hold nontrivial topological structure,more importantly,a pair of triply degenerate Dirac point appear along the C3z rotation axis H-K and(-H)-(-K)in its Brillouin zone,both of them are topological protected.Then we synthesized the high quality trigonal PtBi2 single crystal by self-flux method and took the quantum transport and magnetization measurement under high magnetic field.Similarly,the trigonal PtBi2 performed extremely large unsaturated MR,but the angle-dependent MR exhibits strong anisotropic character with a "butterfly" shape.Analyzing of both the SdH and dHvA oscillations through Fast Fourier transform reveals six principal frequencies,the fermions in two low frequencies pockets hold nontrivial Berry's phase and light effective masses,which is consistent with theoretical calculation.Thus,we made a conclusion that the triply degenerate point fermions are highly likely exist in trigonal PtBi2 and it is a new triply degenerate topological semimetal.(3)The trigonal PtBi2 is a two-dimensional layered material combine by Van der Waals force,so we exfoliated the trigonal PtBi2 bulk single crystal into nanosheets and fabricated nanodevices using electron beam lithography(EBL)technology,then we studied their magnetic transport properties.We found that the PtBi2 nanosheets perform different carriers property compared with the bulk single crystal,which may because of the fermi level was tuned through size effect.Meanwhile,the apparent weak anti-localization(WAL)effect was observed,suggesting strong spin-orbit coupling was exist in trigonal PtBi2.Further analyzing reveals the WAL effect is mainly originates from the three-dimensional bulk state contribution.Moreover,compare to the bulk single crystal,the MR of PtBi2 nanosheets shown less anisotropy,the "butterfly" shape MR and the SdH quantum oscillations were vanished as well.