The Energy Band Calculation And Electronic Properties Of Topological Semimetals

Graphene and three dimensional topological semimetals have received tremendous attention in condensed matter physics due to its intriguing physical properties and potential application in electronics.Firstly,the low-energy quasiparticles of graphene and three-dimensional Dirac and Weyl semimetals have special linear dispersions,which obey the Dirac or Weyl equations in high energy physics and are different from the traditional quadratic dispersion Schr?dinger fermions.Theoretical studies have shown that graphene and three-dimensional Dirac and Weyl semimetals exhibit novel physical properties due to their linear dispersion.With the deepening of research on topological semimetals,it has been found that some basic symmetries in high-energy physics,such as Lorentz symmetry and particle hole pair symmetry,are not necessary in condensed matter physics.The reduction of symmetry constraints means that,compared with the classification of high-energy particles,the types of quasiparticles in condensed matter physics are more abundant.Specifically,the Weyl?Dirac?cone can be tilted,the degeneracy of the band crossing can be 3,6,and 8,and the dimension of the band crossing can be one-dimensional?Nodal line?and two-dimensional?Nodal Surface?.New fermions that beyond Weyl and Dirac paradigm has injected new vitality into the research of topological semimetals,and this also puts forward new requirements for the current first-principles calculations research.To find out the potential ideal new fermions material candidates and provide theoretical guidance for the experimental observation.Experimentally,the prerequisite for observing these intriguing properties is to prepare high quality topological semimetals.For the experimental research of graphene,high-quality graphene samples have long been synthesized,but the fabrication of smooth edged graphene nanostructures with specific edge orientation still has some challenges.This leads to the current transport measurement of specific edged graphene nanostructures research is still lacking.In this dissertation,the research work on topological semimetals can be categorized into two parts:theoretical and experimental.Theoretically,we have predicted relatively ideal novel topological semimetals using first principle calculations and its related physical properties are also discussed.Experimentally,we studied the transport properties of the carriers in graphene under the periodic superlattice potential modulation by patterning high quality zigzag edged graphene antidot lattices.The research results in this thesis are as follows:1.Based on the first-principles calculation,we predict the new fermions in ZrO family with space group 225.Without considering spin-orbit coupling,Zr O is a type?triple degenerate point and three-band Nodal Rings semimetals.The appearance of the triple degeneracy point usually indicates the existence of the three band Nodal Rings.Zr O turned into type?Dirac semimetal when considering spin-orbit coupling.2.Based on the first-principles calculations,we predict that crossing Nodal line semimetals can be realized in the nonsymmorphic space group 194 in BaTiS₃ family.The crossing Nodal line in BaTiS₃ differs from the previous studied ones in that it is composed of six rings.The six nodal rings are protected by the mirror symmetry.In addition,the size of the nodal ring can be modulated via stress.3.In the experimental research on two-dimensional semimetal graphene,based on the hydrogen plasma anisotropic etching technique and Van der Waals heterojunction transfer method and microfabrication methods,we have prepared high-quality zigzag edged graphene antidot lattice.Electrical transport and magnetic transport measurements indicate that the graphene antidot lattices fabricated via this method is of high quality.At low magnetic fields,we observed the commensurate oscillation resistance peak around one antidot.Crossover from sdH oscillation to quantum Hall effect can be observed by gradually increasing the magnetic field.