Bi2Sr2CoO6+δ單晶之角解析光電子能譜研究

Angle-resolved Photoemission Studies in Bi2Sr2CoO6+δ

許翔淇(Hsiang-Chi Hsu)

指導教授 ： 崔古鼎

國立清華大學/理學院/物理學系/碩士(2013年)

摘要

層狀鈷氧化物Bi2Sr2CoO6+δ與高溫超導體材料Bi2Sr2CuO6+δ具備極為相似的結構。然而，將Co替換Cu原子後，Bi2Sr2CoO6+δ不再具備超導特性並成為一絕緣體，此一現象引起科學家廣泛注意。隨著氧含量的不同，Bi2Sr2CoO6+δ展現出豐富的磁變化。由於上述兩項原因，且價帶頂部(近費米能階處)的行為與上述所言之電磁特性息息相關，我們以角解析光電子能譜針對Tpeak~280 K和~150K的Bi2Sr2CoO6+δ的電子能帶結構進行研究，期望能觀察到Bi2Sr2CoO6+δ光電子能譜中價帶頂部的能帶色散現象。
近費米能階處的能態特徵主要來自Co-O 面的貢獻，根據角解析光電子能譜結果顯示，近費米能階之能態特徵無顯著色散行為，顯示出局限特性，此種現象與具備明顯色散行為的Bi2Sr2CuO6+δ極為不同。藉由探討不同入射光能量下的光電子能譜結果，我們觀察到靠近布里淵區中心在束縛能約6.0 eV處由Bi-O面貢獻的電子價帶的有效質量會隨入射光能量改變而產生變化，並利用此有效質量變化估算出Tpeak~280 K 的Bi2Sr2CoO6+δ之內位能V0約為13.5 eV。此外，我們所觀察到的有效質量變化週期為以倒晶格常數2π/c預期的結果的2倍，顯示出Bi2Sr2CoO6+δ沿著kz方向上的電子能帶將受到其雙層層狀結構的影響。在kx-ky平面上的色散關係研究中，我們觀察到沿著Γ→M→Γ"方向，EB~6.0 eV能帶上的有效質量小於沿著Γ→X→Γ'的結果。考量Bi2Sr2CoO6+δ材料內原子的鍵結方向，此現象顯示Bi-O的鍵結方向雖然排列不甚規則，但整體而言其在沿著Γ→M→Γ"方向形成了較連續規律的排列，致使電子較容易在此方向上移動。

關鍵字

角解析光電子能譜 ； Bi2Sr2CoO6+δ ； 電子結構

並列摘要

Layered cobalt oxide Bi2Sr2CoO6+δ has attracted attention because of its isostructure to the cuprate high Tc superconductor Bi2Sr2CuO6+δ (Bi2201). However, the replacement of Cu by Co completely suppresses superconductivity and the crystal becomes an insulator. In addition, with various oxygen contents, Bi2Sr2CoO6+δ exhibits a series of surprisingly rich magnetic properties of ferromagnetism and anti ferromagnetism. Because of these reasons, we have carried out an angle-resolved photoemission (ARPES) measurement to study the electronic band structure of Bi2Sr2CoO6+δ with Tpeak~280 K and ~150 K. We are mostly interested in the valence band maximum (bands nearest to EF) in Bi2Sr2CoO6+δ which is closely related to the electronic and magnetic properties in Bi2Sr2CoO6+δ and expect to measure a "k" _"∥" -dependent dispersion.
The ARPES results reveal that the bands nearest to EF, which are originated from the Co-O planes, are nondispersive showing localized characters, in contrast to bands dispersing across EF in Bi2Sr2CuO6+δ. By investigating the photon energy dependence, we observe that near the Brillouin zone center the effective mass of the peak at EB~6.0 eV, which is from the BiO planes, varies with photon energy, and estimate the inner potential V0 at about 13.5 eV in Bi2Sr2CoO6+δ with the Tpeak~280 K sample. Furthermore, the periodicity of the variation is twice as large as we anticipated by the reciprocal lattice constant "2π" /"c" . It indicates that the band structure along the kz direction is strongly influenced by the bilayer structure in Bi2Sr2CoO6+δ. In the in-plane measurement, we discover that the effective mass of the state at EB~6.0 eV in the direction along Γ→M→Γ" is smaller than along Γ→X→Γ'. By concerning the bonding directions, it indicates that although the whole Bi-O bonding is disordered due to large O vacancies, it forms a more continuous, regular order along Γ→M→Γ" and make electrons move more easily.

並列關鍵字

ARPES ； Bi2Sr2CoO6+δ ； electronic structure

參考文獻

Chapter 1

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