Singular Electronic Structure Of Metal Oxide Functional Materials, High Resolution Angle-resolved Photoelectron Spectroscopy Study

Strong correlated electronic materials, especially the metal oxidefunctional materials, which possess the characteristic of competition betweendifferent degrees of freedom(such as the charge, spin and orbital) and the lowdimentionality, present a variety of physical properties(e.g., high-temperaturesuperconductivity, giant thermoelectric potential effect) and enormous potentialvalue. Thus, they have not only become the hottest research topic incondensed matter physics and material science but also aroused extensiveconcern in the industrial community. In recent years, with the further advanceof research and the emergence of novel materials(such as the unconventionalsuperconductors and the materials with giant thermoelectric Seebackcoefficient), the metal oxide functional materials have attracted tremendousinterests, and also promoted close cooperation among scientists in the field ofphysics, chemistry and material science. At present, with the progress of thesynchrotron radiation techniques and instruments, it offers a direct andeffective experimental probe to understand the fundamental problems in thesenovel materials. Among them, 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, has beenwidely used in investigating the novel electronic structures, phase transitionsand various orderings in such strong correlated materials. In this dissertation,we report some progress in studying the electronic structures of themisfit-layered oxides, triangular-lattice cobalt oxides and Iron-basedsuperconductors by means of ARPES. Besides, we would like to reveal themicroscopic electronic structure of the formation of their different electronicground states. The corresponding results are listed as follows.1. The misfit-layered oxide with giant thermoelectric Seebeckcoefficient, Bi₂Ba_1.3K_0.6Co_2.1O_7.94, is studied by high resolution ARPES,which revealing the electronic structure of a highly strained oxide interface.We find that low-energy states are confined within alternatingrocksalt-structured[BiO/BaO] layers and hexagonal[CoO₂] layers on bothsides of the interface respectively, but still affected by the incommensurate crystal field scattering from the other side. Furthermore, the high strain onthe rocksalt layer induces large charge transfer to the[CoO₂] layer.Besides, a novel effect, the interfacial enhancement of electron-phononinteractions, is discovered. Our findings provide an electronic structurefoundation for understanding oxide interfaces and have significantguidance in designing oxide devices.2. The electronic structure of hexagonal structure cobalt oxidesA_xCoO₂(A=Na, K) is studied by means of high resolution ARPES. Thesetypes of materials not only possess a relatively high thermoelectric powerin the thermoelectric materials but are the parent compounds of theunconventional superconductors A_xCoO₂·H₂O as well. Therefore,revealing the general electronic structure and the Fermi surface topologyplays an auxiliary role in understanding the mechanism ofsuperconductivity and high thermoelectric power. The results of differentdopings of A_xCoO₂ are compared and the detailed electronic structure andthe information of Fermi surface are obtained, which help tocomprehensively understand the superconductivity and highthermoelectric power.3. The temperature dependence of the density-of-states in theiron-based superconductor SmO_1-xF_xFeAs(x=0, 0.12, 0.15, 0.2) isinvestigated by high resolution angle-integrated photoemissionspectroscopy. The density-of-states suppression is observed with thedecrease of temperature in all samples, revealing two characteristicenergy scales(10 meV and 80 meV). However, no obvious dopingdependence is observed. We argue that the 10 meV suppression is due toan anomalously doping-independent normal state pseudogap, whichbecomes the superconducting gap once in the superconducting state; andalert the possibility that the 80 meV-scale suppression might be an artifactof the polycrystalline samples.4. The electronic structure of the Iron-based superconductorsBaFe_2-xNi_xAs₂(x=0.1, 0.16, 0.2) single crystals is studied systematically. Our results demonstrate that, with the partially substitution of Fe with Ni inthe parent compounds BaFe₂As₂, there is no SDW transition exiting inthese three dopings but only superconducting transition. With theincreasing doping of Ni, the electronic structure only shows the characterof rigid band model: there is no band splitting and folding, but Fermi levelraise and bands shift towards to higher binding energy. Our results providea simple and clear band picture, which is useful for the study of otheriron-based superconductors.