Study On Size Effect Of Low-dimensional Topological Materials MBi₂（M=Pt,Pd）

The discovery of the quantum Hall effect has led to a new field of research and applications in quantum material devices such as topological insulators and topological semimetals.As a new quantum state,topological semimetal materials have unique magnetic transport properties（such as chiral negative longitudinal magnetoresistance and giant magnetoresistance）and extremely high carrier mobility,which are rich in unique electronic properties and potential application in future spinner devices.The discovery of graphene initiated the study of two-dimensional materials,and various layered material systems such as transition metal dichalcogenides,two-dimensional oxide superconductors,and two-dimensional magnetic materials were subsequently derived.The excellent scalability of two-dimensional materials not only facilitates the accurate study of material properties,but also facilitates the coupling of magnetic,topological,superconducting and other properties,providing a reference to explore novel physical phenomena.In this paper,the two-dimensional layered topological semimetalβ-MBi₂（M=Pt,Pd）is studied systematically and comprehensively from the device preparation,physical characterization,magnetic transport measurement and so on.The specific research content and innovative results are summarized as follows:（1）We have performed a systematic study of the thickness-dependent magnetotransport properties of the trigonal layered PtBi₂.As PtBi₂ decreases in thickness,the metallicity is progressively suppressed,and ultimately displays semiconductor-like behavior when the thickness is reduced to 22 nm.Interestingly,the trigonal layered PtBi₂ magnetoresistance gradually decreases with decreasing thickness,but increases abruptly at 22 nm nanoflakes.The Hall resistance demonstrates the distinct evolution of electronic state on thickness,revealing that the carrier compensation mechanism may play a role on the large magnetoresistance in 22nm PtBi₂ nanoflake.High-resolution scanning transmission electron microscopy images clearly reveal the surface reconstruction of trigonal PtBi₂ and provide a qualitative explanation for understanding the thickness-dependent transport properties.Our results provide insight into the electronic properties in the low-dimensional limit of topological semimetal trigonal PtBi₂.（2）The chiral anomaly is one of the important physical properties of 3D topological semimetals.We have systematically measured the planar Hall effect and anisotropic longitudinal resistivity of bulk trigonal layered PtBi₂.The negative longitudinal resistivity and the planar Hall effect are consistent with the transport properties of chiral fermions.Further heating experiments show that both the planar Hall effect and the negative longitudinal resistivity are suppressed when the temperature is raised to 25 K,suggesting that the origin of both phenomena is due to the chiral anomaly.However,further analysis suggests that the origin of the Hall effect may not be a pure chiral anomaly and may be accompanied by contributions from anisotropic orbital resistance.Theρ_yx andρ_xx parameter plots of the bulk PtBi₂ indicate that the Hall effect is caused by the combination of chiral anomaly and orbital magnetoresistance.Our experiments provide critical transport evidence for the chiral anomaly of PtBi₂.（3）We have successfully fabricated a micro-nano device for trigonal layered PtBi₂ by mechanical exfoliation method.The planar Hall effect in PtBi₂ micro-nano devices has been detected by systematic transport experiments.The results suggest that the planar Hall effect in PtBi₂ nanoflakes is caused by anisotropic orbital magnetoresistance,which in turn is caused by the anisotropic Fermi surface properties of PtBi₂ rather than the chiral anomaly.Further analysis of the anisotropic"shock wave"parameter plots diagram of the PtBi₂ micro-nano device indicates that the planar Hall effect is not due to the chiral anomaly,possibly caused by the anisotropic orbital resistance.Combined with the thickness-dependent transport properties of PtBi₂,the reconstruction of atoms on the surface of PtBi₂ nanoflake may be the reason for obscuring the transport properties of chiral fermions.With its underlying topological properties and natural layered crystal structure,the trigonal layered PtBi₂ provides a good platform for understanding the underlying physical transport mechanisms in PtBi₂ micro-nano device applications.（4）We have successfully prepared layered topological superconductorβ-PdBi₂ single crystal by fusion method.Superconductivity and non-trivial topological electronic states are two important properties of topological superconductors.We focus on the transport signals of topological surface state in topological superconductor candidateβ-PdBi₂.When reduced the thickness to 21 nm,theβ-PdBi₂ nanoflake undergoes a semiconductor-metal transition followed by an upturned resistance when the temperature decreases.A large unsaturated longitudinal magnetoresistance is accompanied by the distinct Shubnikov-de Hass oscillation.The analysis of Hall resistance reveals that the carriers present the features of relativistic fermions with small effective mass and extremely high mobility.The angle-dependent quantum oscillations demonstrate a two-dimensional Fermi pocket.A giant anisotropic magnetoresistance as large as 98%is detected,which is comparable to the typical nonmagnetic topological semimetals.The results provide distinct transport signals of nontrivial topological electronic state,establishing further understanding on the topological properties in low dimensional topological superconductor candidateβ-PdBi₂.