Angle-Resolved Photoemission Spectroscopy on Itinerant Electrons in a 2D Electride and 3D Perovskite Oxides

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자연과학대학 물리·천문학부(물리학전공)
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서울대학교 대학원
학위논문 (박사)-- 서울대학교 대학원 : 자연과학대학 물리·천문학부(물리학전공), 2018. 8. 노태원.
Itinerant electrons in solids play an important role since they determined physical properties such as conduction or magnetism. In some simple systems such as alkali or alkaline-earth metals in which interactions between electrons are negligible, it is relatively easy to study relation between electronic structures and physical properties by both experiments and theory. However, if the interactions or correlations become important, studying electronic structure of such systems is complicated since conventional band theory fails to describe correct electronic structures. In this dissertation, we studied three different itinerant electron systems by photoemission spectroscopy to overcome the limitations. Each work had experimental and theoretical difficulties at the moment starting the experiments. We tried to resolve the difficulties by adopting user facilities and state-of-the-art experimental techniques.

For the first part, we presented the first study of electronic structures for an electride Ca2N by angle-resolved photoemission spectroscopy (ARPES). An electride is a system in which electrons themselves behave as anions. Adopting synchrotron radiation, we could perform ARPES using various photon energies. It determines kz dispersions of the electronic structure of Ca2N, and we found Fermi momenta kF was almost constant with respect to kz values. It directly revealed that low-energy excitation governed by the electronic structure should follow quasi-two-dimensional behavior, which was originally suggested by density functional theory (DFT) calculations. Such good agreement also proved that physical properties related with electronic structures would be consistent with those predicted from DFT calculations. Hence, we provided first experimental evidence for the existence of quasi-two-dimensional anionic excess electrons in Ca2N.

In the second part, we investigated possible surface metallic states predicted by DFT calculations on BiO2-terminated BaBiO3 thin films. We used an up-to-date in-situ core-level photoemission and ARPES systems to study electronic structures of BaBiO3 thin films. By analyzing angle-dependent Bi 4f and Ba 3d core-level spectra, we found that our BBO films had BiO2 termination, which suited the prerequisite for realizing surface metallic states. We also measured momentum-resolved valence band spectra and found our results were consistent with the previous report. Contrary to the theoretical prediction, we did not find any signature of surface metallic states. We studied possible surface states in detail using in-situ K evaporation on our BaBiO3 films, and revealed surface states located –3.8 eV below the Fermi level. We proposed that electronic structure studies for BaBiO3 is delicate and need sophisticated methods.

As the last part of this dissertation, we studied three-dimensional electronic structure of SrRuO3 (SRO) single crystals. We determined the inner potential of SRO single crystals for the first time. We also measured the band dispersions on high symmetry planes and opened a way to cross-check theoretical electronic structures of SRO, which has not been accomplished during last 20 years. From temperature dependence of a band dispersion, we found that the ferromagnetic exchange splitting of SRO decreased when temperature rose up. However, it did not become zero at the TC, and remained as finite values. We suggested that this finite exchange splitting above TC indicates existence of short-range magnetic order above TC and supported dual nature of itinerant/localized magnetism in SRO.
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College of Natural Sciences (자연과학대학)Dept. of Physics and Astronomy (물리·천문학부)Physics (물리학전공)Theses (Ph.D. / Sc.D._물리학전공)
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