[[alternative]]Study of the Phase Transition and Modulation in Transition Metal Oxide and Sulfide Using X-Ray Scattering

計畫編號：NSC99-2112-M032-005-MY3 研究期間：20110801~20120731 研究經費：1,595,000[[abstract]]過渡金屬氧化物與硫化物的相變化與調制結構的X-光散射研究 此計畫主要是計畫使用X-光散射來探討過度金屬氧化物及硫化物中由電荷或電子自旋所產生的調制結構(modulated structure)和傳輸行為的關聯性。除了研究單晶樣品外，我們也計畫使用X光掠角繞射(glancing incident x-ray diffraction)及X-光散射來探討氧化物多層薄膜樣品由於相變化而在界面和表面所產生之改變。 在過渡金屬氧化物中由於電荷、電子自旋、電子軌道、及晶格之間的作用力可產生一豐富且複雜的相圖，也導致了很多異常的物理傳輸行為的發現；如高溫超導、或龎磁阻效應。為了要進一步了解此類化合物的異常物理行為，我們計畫研究以鐵為基底的氧化物SrFeO3。在SrFeO3中Fe4+具有跟錳氧化物中的Mn3+等電子態(isoelectronic state)，且SrFeO3具有一簡單的正立方體結構，也沒有Jahn-Teller效應的產生，所以此系統可用以探討由氧缺陷所造成的原子結構、電子結構、磁結構與傳輸行為之間的關聯性。此計畫所需的單晶樣品將在台大凝態中心周方正教授的實驗室成長製做，及量測電性與磁性，並利用共振X-光散射來探討由氧缺陷所造成的晶格形變、電荷及電子自旋所形成的有序結構，傳輸行為間的關聯性。 本計畫想要研究的第二個材料為層狀的金屬硫化物TaS2。TaS2在78K時會有一個電荷密度波(charge density wave, CDW)的相變化，並在0.8K時轉變為超導體，而形成一個超導與電荷密度波共存的態。這與一般所認知的超導態與CDW不可供存是相違背的。最近的研究甚至發現在此層狀的硫化物中參雜少許的過度金屬(Ni, Cu, Fe)，可提升超導的轉換溫度至4K，而且電荷密度波依然存在。這些的異常現象正吸引著研究學者在探討電荷密度波與超導態的交互作用。本計畫是要使用X-光的共振散射及吸收光譜來研究這些參雜對電荷密度波的影響，進而探討CDW跟超導態的關聯性。 除了以上所提的單晶樣品的研究外，本計畫也將研究多層膜樣品的表面與介面結構。一般來說，新穎材料(如高溫超導及龐磁阻材料)的塊材(bulk)的物理特性及傳輸行為已被廣為研究，也被了解的比較多些。但當這些材料被製作成薄膜時，往往由於存在於膜與基底之間的應力，而導致膜的傳輸行為與塊材有所差異。尤其當不同特性的塊材(如龐磁阻材料+超導體、或多鐵性材料+超導體)被製作成多層膜時，往往也可導致與塊材完全不同的物理特性及傳輸行為。已知存在於膜之間的介面結構、與膜與膜之間的交互作用，對多層膜的傳輸行為扮演很重要的角色。由於同步輻射光源的高解析度與能量可調性，本計畫也將使用X-光的掠角繞射、及X-光散射來研究此類多層膜樣品在低溫下由膜本身的相變化所導致的多層膜之間的應力效應與介面結構。[[abstract]]Study of the phase transition and modulation in transition metal oxide and sulfide using x-ray scattering This proposal aims to investigate the modulated structures caused by the charges and spins in SrFeO3-δ and RxTaS2 (R= Fe, Cu, Ni, Na) using mainly resonant and nonresonant x-ray scattering, and further study their correlation with the unusual transport behavior. In addition, we will also study the structural information of surface and interface of multilayered films using x-ray reflectivity, glancing incident x-ray diffraction. It has been known that interactions, in transition metal oxides, between the different degrees of freedom of charge, spin, orbital and lattice produce a complicated phase diagram and result in many unusual physical properties, such as the high-Tc superconductivity and colossal magnetoresistivity (CMR). For instance, the interplay between the charge disproportionation and spins results in the phase separation between charged-ordered insulating state and charge-disordered metallic state, and play an important role in driving the CMR effect as observed in manganites. Lots of experiments have revealed many unusual physical properties closely relating to interactions between the different degrees of freedom, but puzzles remain. In order to improve our understanding of these unusual physical properties, it is interesting to consider the iron (IV) based oxide, SrFeO3, because the Fe4+ ion is isoelectronic with the Mn3+ ion, and exhibits a cubic perovskite structure at all temperatures without any evidence of a Jahn-Teller effect. By controlling the oxygen deficiencies, SrFeO3-δ produces a rich phase diagram including the structural transition, charge ordering, and magnetic phase transition. Therefore, we plan to study the correlations between the unusual transport behavior caused by the formation of charge ordering, spin ordering and lattice distortion using resonant/nonresonant x-ray scattering, and conductivity and magnetization measurements. The second objective of this proposal is to study the correlation between the superconductivity and charge-density wave (CDW) in the layered transition metal dichalcogenides, such as TiSe2 or TaS2 using resonant x-ray scattering. 2H-TaS2 undergoes a CDW transition at ~78 K and superconducting transition at 0.8 K. It has been observed that the intercalation of 3d transition metal can increase the superconducting transition temperature upto ~4 K. As synchrotron x-rays possess the merits of energy turnability and high spatial resolution and with the availability of high quality single crystals of RxTaS2 (R= Fe, Ni, Cu), we intend to study the doping effect in the electronic structure of CDWs using resonant x-ray scattering and absorption spectroscopy. In addition to the study of single crystals proposed above, we also intend to study the surface and interface structures of multilayered films. It has been known that the strain or size effects play an important role in the transport behavior of films. For instance, the multilayered films A/B/C (A=YBa2Cu3O7, B=Nd2-xSrxMnO4, C=SrTiO3) have been observed to show the very different transport behavior, and this is caused by the coupling between the layers and the strains existing in the films. As synchrotron x-rays have the merits in high resolution and energy turnability, we plan to investigate the strain effect and the interface structure of the multilayered films using x-ray reflectivity and high resolution x-ray scattering.[[sponsorship]]行政院國家科學委員