.2 H Structure Of The Transition Metal Disulfide Is The Electronic Structure Of Compounds High-resolution Angle-resolved Photoelectron Spectroscopy Study

The transition metal dichalcogenides(TMD's),which are one class of the first discovered 2D charge density wave(CDW) compounds,have received continuous and considerable interest because of their relatively simple crystalline structures but complex electronic ground states.Their particular features,such as the layered crystal structures and the competition and/or coexistence between different electronic ground states,are especially intriguing in view of the anomalous properties of another class of layered materials,the high-Tc cuprate superconductors. The angle resolved photoemission spectroscopy(ARPES),which is the sole tool to simultaneously detect the electron's energy,moving direction and scattering property near Fermi energy in solids,is appropriate for investigating the novel electronic structures,phase transitions,various orderings etc in such layered materials.In this dissertation,we report some progresses on studying the electronic structures of the TMD's by means of ARPES.We would like to reveal the microscopic mechanism for the formation of their different electronic ground states.The corresponding results are listed as follows.1.We report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide 2H-Na_xTaS₂,where the two essential ingredients of the CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure.Namely,the CDW gap is only partially open,and charge density wave vector match is fulfilled through participation of states of the large Fermi patches.In this scheme,some long-standing difficulties in understanding the CDW of 2H-Na_xTaS₂,such as no gap opening near the Fermi surface and no match of the CDW wavevector with the Fermi surfaces,are sovled successfully. Unlike other CDW mechanisms based on band structure effects,it is rooted in the strongly coupling nature of its electronic structure,which provides phase space needed for CDW fluctuations.2.2H-NbSe₂ is a prototype CDW material.However,the nesting required by the conventional CDW mechanism,i.e.separation of certain Fermi surface sections should match the ordering wavevector,is mysteriously missing here.With ARPES, we show that the long-lost nesting condition is fullfiled not just between the six saddle points,but the major contributions come from a large honeycomb region away from the Fermi surface.Although the spectral suppression in these regions is weak,it clearly tracks CDW formation,and the summed suppression overweights those around the saddle points.The estimated charge susceptibility further confirms that the CDW instability in 2H-NbSe₂ are mostly caused by electrons in these regions.3.For the first time,we report the detailed evolution of the electronic structures of 2H-Cu_xNbS₂.The extracted self-energy information on these compounds implies rather weak and anisotropic electron-phonon couplings therein,which naturally answer for the anomalous properties of their superconductivity.On the other hand,we compared the estimated charge susceptibility for two different TMD's with and without CDW,2H-Na_0.05TaS₂ and 2H-Cu_0.09NbS₂ respectively.While there is a clear maximum at the CDW wavevector for 2H-Na_0.05TaS₂,the result for 2H-Cu_0.09NbS₂ does not show such behavior,which is consistent with the absence of charge density wave in the system.