Investigations On One-Dimensional Electronic Structure And Topological Properties In NbSi_XTe₂ And Y_1-XGd_XCr₆Ge₆

The electronic band structure of solid materials directly affects their physical properties.Recently,quasiparticle excitations represented by massless Dirac fermions in the electronic band structure of materials have received extensive attention.The realization of one-dimensional massless Dirac fermions and one-dimensional electronic states is uncompleted.In this paper,we systematically study the electronic structure and quasiparticle excitation of Nb Si_xTe₂ material,and the electronic kagome lattice in Y_1-xGd_xCr₆Ge₆material by using angle-resolved photoemission spectroscopy technology（ARPES）.We experimentally observed one-dimensional massless Dirac fermions and one-dimensional flat band,and we tune two material system.The main contents are as follows:（1）We have experimentally observed one-dimensional massless Dirac fermions in NbSi_0.45Te₂.One-dimensional Dirac-type band dispersion is observed by angle-resolved photoemission spectroscopy:in the in-plane electronic structure we observed Dirac dispersion only along theΓ-X direction.By varying the photon energy,we measure its out-of-plane electronic structure and find that the Dirac dispersion does not disperse alongΓ-Z direction.The one-dimensional Dirac-type energy band dispersion is experimentally confirmed.The one-dimensional long-range ordered stripe structure on the surface of the material is observed by scanning tunneling microscopy:there is a stripe structure along the direction of the atomic chain of the building blocks.The long-range order preserves along the stripe direction,whereas there is no long-range order symmetry perpendicular to the stripe direction.The reason is that the stoichiometric fluctuations of the doped Si atoms will form different（ab×n+c）structures in tiny spatial scales.The one-dimensional massless Dirac fermions in NbSi_0.45Te₂,together with the two-dimensional massless Dirac fermions in graphene and the three-dimensional massless Dirac fermions in Dirac semimetals,constitute massless Dirac fermions family of different dimensions.（2）Combined with theoretical calculations and symmetry analysis,we reveal the formation mechanism of one-dimensional massless Dirac fermions in NbSi_0.45Te₂.First,due to the modulation of the one-dimensional long-range order of the structure,the material exhibits one-dimensional electronic band structures with dispersion only along theΓ-X direction and no dispersion in other directions.Secondly,the non-symmorphic symmetry brings 4πsymmetry into band structure along theΓ-X direction;under the combined effect of the time-reversal symmetry in the system and the mirror symmetry along the z direction,a symmetry-enforced quadruple degeneracy point appears at the boundary of the BZ,and the one-dimensional Dirac band dispersion is realized.（3）The electronic structure and quasiparticle evolution of NbSi_xTe₂（x=1/2,1/3）material system are experimentally studied.In the single crystal with x=1/2,the system consists of the（a-b）structural units without c-structural unit,thus the three-dimensional translational symmetry of the system is restored.It is experimentally observed that the three-dimensional electronic band structure in NbSi_1/2Te₂ is highly anisotropic,showing the in-plane effective mass ratio of holes in two perpendicular directions of 10,which is consistent with the first-principles calculations;the semiconductor energy gap evolution is investigated through the in situ surface alkali metal doping.In the single crystal with x=1/3,the system consists of（a-b-c）structural unit with the three-dimensional translational symmetry restored.Experiments and theoretical calculations demonstrate that the system evolves into a symmetry-enforced weak topological insulator state,coexisting with hourglass fermions in the bulk state.We characterize the physical properties of Nb Si_xTe₂ thin films prepared by mechanical exfoliation is performed.Optical and electrical transport properties that are highly dependent on crystal orientation were observed in thin-layer samples with x=0.45 and 1/2 composition.（4）We experimentally observe evidence of electronic Kagome lattice in YCr₆Ge₆.The electronic structures are investigated by angle-resolved photoelectron spectroscopy,such as flat bands,Dirac dispersions and saddle points.Flat band in YCr₆Ge₆ is widly obseverd in the k_x-k_y plane of momentum space.Flat band shows no dispersion in k_x-k_y plane while it is dispersive along k_z direction.Therefore the flat band exsits as one-dimensionl elecreonic state.Combining experiments and theoretical calculations,we discover that the planar flat band arises from the d_z²electrons with intra-kagome-plane hopping forbidden by destructive interference.We explicitly demonstrate that orbital character plays an essential role in the realization of electronic kagome lattice in bulk materials with transition metal kagome layers.Anomalous transport results due to the flat band are also observed,such as anisotropic resistivity and relatively large Sommerfeld coefficient.At the same time,the magnetic element Gd is introduced in Y_1-xGd_xCr₆Ge₆material to regulate its transport performance.We obseve ferromagnetism in Y_1-xGd_xCr₆Ge₆.In conclusion,the electronic structure and topological properties of the NbSi_xTe₂ material system and Y_1-xGd_xCr₆Ge₆material system are investigated by angle-resolved photoemission spectroscopy.We tune the discovered electronic stuctures of two systems.The results show that there are symmetry-protected one-dimensional massless Dirac fermions in the x=0.45 samples;three-dimensional translational symmetry of the system is restored in the x=1/2 and 1/3 samples;for x=1/2 and 1/3 samples,their electronic band structure evolves into narrow-gap semiconductors and symmetry-enforced weak topological insulator states,respectively.We experimentally observe evidence of electronic kagome lattice in YCr₆Ge₆.Flat band exsits as one-dimensionl elecreonic state.The evidence inludes the spectral signatures of flat bands,Dirac dispersions and saddle points,transport features such as anisotropic resistivity and relatively large Sommerfeld coefficient.We obseve ferromagnetism in Y_1-xGd_xCr₆Ge₆.