錳與鈷氧化物之軌域極化與電子關聯

Abstract

過渡金屬氧化物的物理性質決定於其價電子的電子結構，而電子的基本屬性：電荷、自旋，以及晶體結構，皆與價電子的軌域對稱性緊密相互關聯，它們之間的相互作用與價電子間的強關聯效應產生豐富的物理特性，例如：高溫超導、金屬絕緣相變及龐磁阻效應等。因此，探討價電子軌域對稱性是研究電子強關聯系統的重要途徑之一。本論文係結合電子能譜技術及脈衝式雷射薄膜成長技術，來探討錳氧化物與鈷氧化物的軌域極化與電子關聯。 在錳氧化物方面，我們利用軟X光吸收能譜之線二向性實驗方法，來研究La1-xSr1+xMnO4 材料的軌域。另一方面，應用脈衝式雷射薄膜磊晶技術成長高品質的La0.5Sr0.5MnO3薄膜樣品，來進行軌域對稱隨應力改變之物理性質研究。在鈷氧化物方面，利用極化相關的軟X光吸收能譜實驗方法來探測NaxCoO2材料中CoO2二維三角形晶體層的軌域對稱性及電子強關聯效應。

Transition-metal oxides exhibit a variety of physical properties determined by the electronic structure of valence electrons. The electron attributes: charge, spin, and lattice structure strongly couple with orbital symmetry of the valence electrons. The strong interplay among these attributes gives rise to fascinating phenomena, such as high-temperature superconductivity, metal-insulator transition, and colossal magnetoresistance. The study of orbital symmetry is therefore an important subject in strongly correlated electron systems. In this dissertation, we present soft x-ray spectroscopic measurements and epitaxial growth of manganese and cobalt oxides to investigate their orbital ordering and electron correlations. For manganese oxides, we used linear dichroism in soft x-ray absorption to unravel the orbital character of the valence electrons of La1-xSr1+xMnO4. To study the strain induced orbital ordering, we employed the pulsed laser deposition technique for the growth of high-quality epitaxial thin films of La0.5Sr0.5MnO3. We also used polarization-dependent soft x-ray absorption technique to determine the orbital symmetry of electrons responsible for the low-energy excitations in layered NaxCoO2. In addition, the measurements provide spectral fingerprint of electron correlation of NaxCoO2.