Angle-Resolved Photoemission Spectroscopy And Ultrafast Spectroscopy Studies Of Low-Dimensional Materials And Unconventional Superconductors

The novel physical properties exhibited by low-dimensional materials and unconventional superconductors are believed in having great potential in future device applications.Thus,exploring the origin of exotic physical properties in such materials,especially the mechanism of the formation of order parameters（e.g.,charge density waves,antiferromagnetic sequence,etc.）closely related to superconductivity,has been the focus of research in condensed matter physics.Angle-resolved photoelectron spectroscopy（ARPES）and ultrafast spectroscopy are powerful experimental methods for probing the electronic structure of materials in equilibrium and the interactions between degrees of freedom in nonequilibrium,respectively,and play an important role in helping to understand the physical properties of materials.In this thesis,a systematic and thoroughly investigated of low-dimensional materials(quasi-two-dimensional co-linear antiferromagnetic materials Co_1/3Nb S₂ and charge density wave materials Eu Te₄)and unconventional superconductors（heavy fermionic superconductors Ce Pd₅Al₂ and high-temperature superconductors KCa₂Fe₄As₄F₂）using ARPES and ultrafast spectroscopy have been discussed.The main results obtained are as follows:（1）Large-sized,high-quality Co_1/3Nb S₂ single crystals were grown using the chemical vapor transfer method.The details of the growth,characterization,and physical property measurements are described.Afterward,the electronic structure of Co_1/3Nb S₂ was investigated in detail using ARPES.The experimental data indicate that the bands constituting the Fermi surface are quasi-two-dimensional.Otherwise,the d electrons introduced by the magnetic element Co upon its intercalation not only lead to an increase in the chemical potential of the parent compound Nb S₂ but also participate in the composition of the Fermi surface and induce many non-trivial topological properties.（2）The electronic structure of the charge density wave（CDW）material Eu Te₄ has been investigated in detail using ARPES.The experimental results demonstrate that Fermi surface nesting drives the formation of CDW order in Eu Te₄ and leads to the opening of an anisotropic CDW gap on the whole Fermi surface.Quantitative analysis reveals that there is another gap opening at the high binding energy,which is the key to the anisotropy of the CDW gap.The temperature-dependent measurements reveal an anomalous temperature dependence of the CDW gap that can not be explained by conventional theory.（3）The electronic structure of the heavy fermionic superconducting material Ce Pd₅Al₂ has been investigated in detail using ARPES.Combined with the results of density-function theory（DFT）calculations,it is found that the 4f electrons of Ce are involved in the composition of the Fermi surface.This finding indicates that the 4f electrons of Ce have a dual nature,both itinerant and localized character.In addition,band folding and Fermi surface reconstruction are also observed.These results suggest that imperfect Fermi surface nesting may have occurred along the k_z direction in Ce Pd₅Al₂ and induced the formation of antiferromagnetic order.（4）The quasiparticle relaxation dynamics of the high-temperature superconducting material KCa₂Fe₄As₄F₂ have been investigated in detail using an ultrafast spectroscopy system.At lower light intensities,two distinct characteristic temperatures can be observed from the temperature-dependent and light fluence-dependent experimental data,corresponding to the superconducting transition temperature（T_c=33.5 K）and the temperature at which the pseudogap opens（T^*≈50 K）.By using the RT model,the sizes of the superconducting gap(Δ_SC（0）≈4.3±0.1 me V)and the pseudogap(Δ_PG（0）≈2.4±0.1 me V)were extracted.In addition,at higher laser fluence,a significant oscillatory behavior is observed.Quantitative analysis results reveal that coherent oscillation is a superposition of two A_1g phonon modes of different frequencies.The electron-phonon coupling constant for the high-frequency A_1g（2）can be obtained by using a non-thermal model.And the temperature-dependent of this phonon frequency can be reasonably explained by the anharmonic effect of the optical phonon.