應力對於釹鍶錳氧薄膜電子結構的影響之X光吸收能譜學研究

Abstract

價電子軌域的對稱性與電荷、自旋、以及晶體結構之間的關聯性深深地影響著過渡金屬氧化物其豐富的物理特性，釹鍶錳氧(Nd1-xSrxMnO3)是一種摻雜不同陽離子比例的鈣鈦礦結構錳氧化合物，特別的是Nd1-xSrxMnO3會隨著不同的摻雜比例而呈現出不同的磁性相變，也由於Nd1-xSrxMnO3具有如此豐富的相變特性加上近年來磊晶製程和理論計算的進步也促進了對於Nd1-xSrxMnO3材料的科學研究。 本論文利用X光繞射實驗解析Nd0.35Sr0.65MnO3薄膜沉積於SrTiO3基板的晶體結構，由於Nd0.35Sr0.65MnO3薄膜與SrTiO3基板之間的晶格結構的差異使得薄膜受到基板應力的影響，進而影響錳離子的3d軌域對稱性。除此之外，我們量測Nd0.35Sr0.65MnO3薄膜之錳 L-edge以及氧K-edge的軟X光吸收能譜之線二向性來探究Nd0.35Sr0.65MnO3薄膜其電子結構以及軌域等等的特性。實驗結果顯示膜厚為40 nm之Nd0.35Sr0.65MnO3薄膜其Mn3+的eg電子呈現出與塊材不同的軌域對稱性，我們推測其差異是來自於膜厚為40 nm之Nd0.35Sr0.65MnO3薄膜受到SrTiO3基板應力的影響，因此改變了其Mn3+的eg電子軌域對稱性。

The rich physical properties of transition-metal oxides (TMO) arise from the strong coupling among the orbital, charge, spin and lattice degrees of freedom. Nd1-xSrxMnO3 is a perovskite-type doped manganite with a rich phase diagram and magnetic transitions as a function of doping concentration x. In addition, recent advances in the epitaxial growth and theoretical analysis expedite the research of the Nd1-xSrxMnO3 system. We performed the X-ray diffraction experiments to investigate the crystal structure of Nd0.35Sr0.65MnO3 thin films epitaxially grown on SrTiO3. The symmetry of Mn 3d orbitals is affected by the strain, which originates from the difference in the lattice structure between bulk Nd0.35Sr0.65MnO3 and SrTiO3 substrate. Furthermore, we measured polarization-dependent X-ray absorption spectra to study the electronic structure and orbital symmetry of Nd0.35Sr0.65MnO3 thin films. Our results reveal that the thin film of a thickness 40 nm exhibits a different eg orbital polarization in Mn3+ in comparison with that of a bulk sample. The difference of eg orbital polarization in Mn3+ is caused by the strain of the Nd0.35Sr0.65MnO3 thin film.