Photoelectron Spectroscopy Studies Of Novel Physical Properties Of Complex Transition Metal Compounds

The strong electronic correlation in the transitional metal compounds has attracted more and more attention due to the abundant novel phenomena in these systems. Varies materials with peculiar properties has been synthesized, triggering intensive research wave in condensed matter physics and promoting the development of the physics, chemistry and material science as well as their interdiscipline. These novel properties suggest that the strongly correlated systems might be a very good candidate for the fabrication of the next generation of electronic devices after Si. The potential application value of them has received attention from the industry. Moreover, the state-of-art technologies such as ultra-high vacuum, ultra-low temperature and ultra-short LASER system lead us to a world which is more complex, more microcosmic and ultra fast. As we known, the physical and chemical properties of the materials, especially the single crystals, are determined by their electronic structure to a great extent. Thus the research on the electronic structure is always the key problem both theoretically and experimentally. Angel Resolved Photoemission Spectroscopy (ARPES), as the unique instrument that could depict the electronic structure directly in the momentum space, is comprehensively used in the physics, chemistry and material science. It plays a key role in the research on the novel phenomena such as phase transitions, ordering and the competition between different degrees of freedom and reveals the microscopic nature of them. On the other hand, Time Resolved Two Photon Photoemission (TR2PPE) is a powerful compensation for the conventional ARPES. It could detect the electronic structure as well as the ultra-fast dynamical processes such as electron excitation, relaxation directly in the time domain and momentum space, while the transient behavior of the excited charge carriers and the non-equilibrium sates are crucial in the surface (interface) electronics, charge (thermal) transport and chemical reaction dynamics. The understanding of these dynamical processes provides us necessary information on the basic interaction in the strongly correlated system.In this dissertation, we report some interesting results in the electronic structure of the "122" and "1111" series of newly discovered iron based superconductors. Besides, the TR2PPE system in our group and some preliminary results are introduced. The corresponding results are as follows:1. The parent compound of "122" series of iron based compounds is studied by high resolution ARPES. The band splitting in the spin density wave (SDW) state is revealed. The system saves total energy by band splitting, which favors the SDW transition. On the other hand, we observed possible gap in the Fermi surface of certain band, suggesting the role of Fermi surface nesting mechanism in the phase transition. Our results set the foundation for the further study of the SDW transition in the iron based compounds. It would be helpful for the understanding of the relationship between superconductivity and SDW.2. We studied the electronic structure of the representative parent compounds of "1111" series in the iron based compounds. We identified the bulk states and the surface states induced by surface charge redistribution. We found that the system saves total energy by band reconstruction instead of gap opening. This is consistent with the results in the other series of iron based compounds. On the other hand, the sharp quasi-particle peaks and weak three dimensional nature of "1111" series might relate to the myth why it holds the record of the superconducting transition temperature of iron based superconductors. Our results would help to construct a global picture of the physics in the iron based compounds.3. We studied the electronic structure of half doped La1-xSr1+xMnO4. The spectra are quite broad and incoherent, which is consistent With the strongly correlated property of the system. The charge ordering in such a strongly correlated insulator should be described by a local picture. However, we observed the nesting of the remnant Fermi surface. Moreover, according to the analysis of auto-correlation, the nesting of the remnant Fermi surface play a key role in the charge ordering transition. Our results suggest that there is certain similarity between itinerant charge density wave in the metallic system and the charge ordering in such strongly correlated insulators. The pure local picture might not be enough to describe such charge ordering transition.4. Based on the ultra-short LASER system, we construct a TR2PPE system for the research on the unoccupied states, carrier lifetime and ultra fast phase transition. At present, the system has been completed. It has different excitation mode, which could be switched over easily. We have got some preliminary results on single crystalline Cu.