Exploring And Investigation On The Physical Properties Of Low-Dimensional Topological Materials

In recent years,accompanying with the rapid development of topological theory in condensed matter physics,many topological quantum materials(such as topological insulators and topological semimetals)with exotic band structure have been discovered successively.A lot of peculiar physical properties were observed in these materials,such as topology protected boundary states,linearly dispersing bands,spin momentum locking,and low-energy electrons robust against impurity and disorder in topological semimetals,which might highly support the development of stable low-energy consumption electrical devices in the future.Compared with the widely investigated three-dimensional system,the two-dimensional layered topological materials possess rich tunable properties based on quantum confinement and heterojunction stacking and readily integrate on the devices,due to the inherent van der Waals interaction and easy reduction of the dimension.In the past few years,hundreds of magnetic or non-magnetic two-dimensional topological materials have been theoretically predicted.However,only a few of them have been experimentally proved to have non-trivial topological properties.Conversely,searching for new layered topological materials and tuning their physical properties will facilitate the development of topological theory.In this thesis,the subject was performed based on new two-dimensional layered topological materials TaNiTes,Ta2PdSe6,EuCd2As2.The crystal growth methods and concepts for improving the quality of single crystals are introduced in detail,and the magnetic properties,magnetic transport and spectral properties are systematically studied in our work.The details are as follows:(1)The high-quality TaNiTe5 single crystals were synthesized by self-flux method.The quantum transport of TaNiTe5 single crystals was then systematically studied.The single crystal in this work exhibits diamagnetism and metallic behavior,which differs from previous report of paramagnetic metal.Besides,strong quantum oscillation can be observed in both magnetization measurements and high magnetic transport measurement at low temperatures.By analyzing the magnetoresistance oscillation at high magnetic field,three fundamental oscillation frequencies were obtained.In order to comprehensively research the quantum oscillation,we further carried out the magnetic torque experiment at low temperature.By analyzing the oscillation of the magnetic torque,four basic oscillation frequencies were obtained,which is consistent with the magnetoresistance oscillation.In addition,the small effective mass and non-trivial Berry phase are obtained by fitting two of the frequencies,which proves that the non-trivial topological semimetal band structure in the system.Through the magnetic torque measurement of different magnetic field angles,three-dimensional Dirac fermions existing in the two-dimensional TaNiTe5 system was revealed.(2)In the second work,we systematically studied the Ta2PdSe6 single crystals whose topological nature is controversial in theoretical predictions.The high-quality single crystals were grown by chemical vapor transport,with residual resistance rate as high as 1244 and diamagnetism with typical metallic behavior.In addition,the hallmark features of the topological semimetals were found in this system with colossal magnetoresistance at 14 T as high as 1.7 ×104%,low carrier concentration nH=6.5×1020 cm-3,and high mobility μH=1.3 × 105 cm2·V-1·s-1 at T=2 K.The magnetotransport at low temperatures also shows SdH oscillation and a hole-dominated transport behavior.In order to analyze the quantum oscillation of the system,we carried out high-field transport measurements,in which the magnetoresistance at 31 T is as high as 1.8 × 105%.After analyzing the quantum oscillations,six fundamental oscillation frequencies were obtained.Among them,small effective mass and nontrivial Berry phase of the two bands were obtained by fitting the SdH oscillations.Combined with the transport measurements with magnetic field rotating from plane normal to the current direction,we conclude that the layered Ta2PdSe6 compound possesses two-dimensional topological electronic structure.(3)In the third work,we synthesized high-quality A-type antiferromagnetic topological insulator layered EuCd2As2 single crystal.The antiferromagnetic order transition induced resistance variation is over 5 orders of magnitude higher than previously report.Besides,a small magnetic field-induced metal-insulator transformation was observed in magnetotransport measurement.The two-dimensional weak antilocalization effect at low temperature indicates that the crystalline(001)plane holds a gapless topological surface state with π Berry phase.Combining with the spectral measurement data,we believe that this system has a large bulk energy gap and an intrinsic topological insulator ground state.Furthermore,EuCd2(As0.85P0.15)2 single crystals were synthesized by substitutional doping the system for tuning investigation.The structural and magnetic properties of EuCd2(As0.85P0.15)2 single crystal are similar with those in EuCd2As2 single crystal.However,strong SdH oscillations were observed in the doped system at high magnetic field.By careful analyzing the quantum oscillation.a single oscillation frequency of 34 T with small effective mass and nontrivial Weyl fermion behavior was obtained.Through the angular dependance of magnetotransport measurement,we confirmed that an ellipsoid Fermi surface exists in EuCd2(As0.85P0.15)2 system.Combining with two parts of work,we believe that the topological insulator ground state in EuCd2As2 single crystal and elemental doping can effectively tune the Fermi level and successfully realize the phase transition from topological insulator state to Weyl semimetal state.