Experimental Studies Of Electronic Structure On Novel Topological Materials

The unveiling of topological concept in condensed matter physics provides a further understanding on the intrinsic properties of the quantum materials,which inspires a great enthusiasm to explore novel topological materials.In recent years,topological quantum materials,such as topological insulators,Dirac semimetal,Weyl semimetal,triple degenerate points metal,topological superconductor,linear giant magnetoresistance material and so on have been attracting great interests and lots of studies.In this thesis,we studied the electronic properties of several novel topological materials using molecular beam epitaxy（MBE）,angle-resolved photoemission spectroscopy（ARPES）,scanning tunneling microscope（STM）and low temperature transport measurements.We investigated the triple degenerate points metal γ-PtBi2,topological superconductor parent films Sr_xBi₂Se₃,and ruthenates Ca₃Ru₂O₇.Main results are shown below:1.Combining experiment and ab-initio calculation,our ARPES results support the existence of triple degenerate fermion in γ-PtBi2.And we observe a canted tubular fermi surface for the first time,which provides a reasonable explanation for odd SdH quantum oscillation founded in this material.We systematically investigated the electronic structure and surface morphology using ARPES and STM.We observed two kinds of surface termination,which results in two different surface states and confirmed its non-centrosymmetric crystal structure.Through the photon-energy dependence measurements,we determined its bulk electronic structure and found a nice consistency between the experiment data and calculations.Our results provided the band structure evidences on the existence of triple degenerate point fermion.Furthermore,we observed a canted fermi surface,which gave a good explanation to the strong SdH signals in tilted applied field configurations.2.We succeeded in growing high quality Sr_xBi₂Se₃ films by MBE and systematically studied the effect of Sr concentration and growth condition on the transport properties.For the first time,we obersve linear magnetoresistance in thick Sr_xBi₂Se₃ films.We also discussed the possible reason why films are not superconducting.We studied the transpot properties of Sr_xBi₂Se₃ films with different doping concentrations,annealing conditions and thickness.We found that Sr doping can enhance the contribution of surface states to the magnetoresistance.We observed the weak antilocalization and linear magnetoresistance related to quasi-two-dimensional states.In addition,we investigated the Sr doping sites and the change of lattice parameter in both bulk and thin film samples.There is no obvious difference in Sr sites between bulk and thin film Sr_xBi₂Se₃ samples.However,the lattice parameter shows a clear difference between bulk and thin films sample.Lattice parameter of neighboring Bilayers in the quintuple layers（QL）compresses after Sr doping in thin films sample,while expanding in bulk samples.Such behavior could be a possible origin why there is no superconducting behavior in Sr_xBi₂Se₃ films.3.We studied Ca₃Ru₂O₇ single crystals by high-resolution ARPES and found a strong anistropy in band structure.Dirac-like linear band disperion exists around the fermi level,implying a possible topological non-trivial properties in low temperature.Ca₃Ru₂O₇ has rectangular-type crystal structure and is easy to have twin domain.We studied the band structures of twinned and detwinned samples to reveal the intrinsic electronic structure.We found that fermi surfaces are consisting of two types of tiny pockets.Electron pocket locates at M point along the kadirection and hole pocket locates near the M′point along the kbdirection.At 48K of first-order transition,bands change drastically,which is the result of multi-order in this system and sample is sensitive to the change of c axis lattice constant.High-resolution ARPES measurements found that electron pocket around the fermi level formed by a linear dispersive band,which implies that Ca₃Ru₂O₇ could be a candidate of topolgical material.