| Summary: | 博士 === 國立中正大學 === 物理系 === 92 === Physical properties of condensed matter are mainly determined by valence electrons. Fascinating phenomena related to electron-electron correlations, such as high temperature superconductivity, colossal magnetoresistance, and metal-to-insulator transition, are important topics in modern condensed
physics. The advances in experimental techniques, such as synchrotron radiation, electron spectroscopies, and epitaxial growth of thin films, provide us with great opportunities to unravel the underling physics of these novel phenomena. In this thesis, we performed advanced electron spectroscopic measurements to study magnetic materials in which electron-electron correlations are important. Particularly, we present a viewpoint different from the conventional band theory to describe the half-metallic behavior of
Fe$_3$O$_4$ and CrO$_2$.
First we studied the half-metallic feature and the electronic structure of Fe$_3$O$_4$ using spin-polarized photoemission. The measured spin polarization in the vicinity of the Fermi level shows that Fe$_3$O$_4$ is not half-metallic as predicted by band theory. We can successfully interpret the experimental
results using a cluster model. With the measurements of magnetic circular dichroism in Fe $2p$ resonant photoemission and cluster model calculations, we obtained the parameters of the electronic structure of Fe$_3$O$_4$. These results indicate that Fe$_3$O$_4$ is a system with strong electron-electron
interactions. Furthermore, we developed a spin-resolved soft x-ray absorption technique to explore the unoccupied electronic structure of CrO$_2$. The spin-polarization in the vicinity of the Fermi level is close to 100\%, providing direct evidence of half-metallicity. The measurements also show the
existence of an atomic-like Cr $3d$ state not far away from the Fermi level with a spin polarization of only 50\%, establishing its Mott-Hubbard character. We conclude that CrO$_2$ has a dualistic electronic nature.
In a strongly correlated system, electron correlations play an important role in its magnetic properties. With advanced electron spectroscopic measurements, we studied the electronic structure of important magnetic materials for spintronics. However none of the present theory can fully describe our new
findings; the results presented in this thesis might shed some light on the physics of half-metallic oxides.
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