多鐵性LiCu2O2與鐵基超導體之晶體成長與物性分析之研究

Abstract

LiCu2O2是一個典型的準一維螺旋磁性材料，同時也是第一個被發現具有多鐵性質的銅氧化物。它在低溫環境下會形成磁有序行為，且自身擁有低維度量子自旋系統的幾何變動特性，這些不尋常的物理性質近來引起了廣泛的關注；有許多實驗皆證實了這個s=1/2的磁系統大約在22-24 K自旋有序溫度下擁有一個不對稱的螺旋磁性基態。在此論文的第一部份，我們成功地生長了一系列的LiδCu2O2 (LCO) 與 LiδTMxCu2-xO2 (LTCO, TM = 鈦、鎳、鋅) (δ = 1 - 1.2 與 x = 0 - 0.1)晶體，典型的LiCu2O2單晶具有ab面的特性，不僅呈現出高純度與高結晶度的X光繞射圖譜，同時也顯示出非雙晶的結構，沒有TM摻雜的晶體尺寸可達9×6 mm2。我們利用軟X射線偏振光的吸收光譜量測法來瞭解銅與TM在不同晶格結構時其價態與軌域的特性，同時隨著不同TM摻雜量成功地摻雜進入銅的位置，可以獲取不同TM摻雜量單晶的物理特性。

另一方面，近年來鐵基超導體是個熱門有趣的議題。自從具有層狀結構的鐵基氮磷族氧化物LaO1-xFxFeAs超導體被發現擁有相對高的超導轉變溫度26 K之後，從那時起，更多的鐵基超導體陸續被提出且轉變溫度甚至可高達56 K。而硒化鐵具有最簡單的晶體結構以及8K的超導轉變溫度，因此吸引了廣泛的注意；在鐵硫族化合物與鐵氮族化合物系統中有許多共同的物理特性可以被提供來瞭解高溫超導的超導機制。在論文的第二部份中，我們利用高壓合成技術成功獲得一系列的硒化鐵晶體並描述其生長條件與物理特性，硒化鐵晶體皆呈現(101)優選方向且擁有高結晶性，晶體的尺寸最大可達2×2 mm2，其中Fe1.03Se晶體藉由適當的熱處理後超導轉變溫度可達到9 K。相較於多晶的硒化鐵樣品，由磁化率的量測證實硒化鐵晶體無論在零場冷或場冷的量測曲線中都呈現出相對輕微的磁背景訊號，這些晶體具有相對高的純淨度將有助於研究超導機制的形成。

LiCu2O2 is a typical quasi-one dimensional helimagnetic material which also is the first member of the cuprate family found to be mutiferroics. Due to its unusual magnetic order at low temperature and low dimensional quantum spin systems with geometric frustration. Therefore, it has recently attracted a great deal of attention. More experiments clearly demonstrated that this s=1/2 magnetic system possess an incommensurate helimagnetic ground state during the spin ordering temperature about 22-24 K. In first part of this thesis, a systematic investigation of LiδCu2O2 (LCO) and LiδTMxCu2-xO2 (LTCO, TM = Ti, Ni and Zn) (δ = 1 - 1.2 and x = 0 - 0.1) were undertaken to determine growth parameters and crystal properties. Typical LiCu2O2 crystals with ab-plane habit not only show a clean X-ray diffraction pattern without impurities and high crystalinity but also exhibit untwined structure. A typical size of large single crystals LiCu2O2 was 9×6 mm2. The orbital character of the valence states of Cu and TM ions on different sites were investigated by polarization-dependent soft X-ray absorption (XAS) spectroscopy measurement with in- and out-of-plane X-ray E vectors. These crystals exhibit varied physical properties owing to successful incorporation of TM dopants.

On the other hand, in recent years the iron-based superconductors are well received by everyone interested in the issues. With layer structure iron-based quaternary oxypnictides, LaO1-xFxFeAs superconductors were found with its superconductivity transition TC ~ 26 K which is a relatively high transition temperature. Since then, more iron-based superconductors have been discovered and the transition temperature TC up to 56 K. The FexSe arouse the majority of interest because of its simplest crystal structure and with superconducting transition around 8 K without any carrier doping. In the iron-chacogenides system, excluding arsenic but with iron-pnictides have many common physical characteristics which can provide important information to understand the superconducting mechanism of high-temperature superconductors. In the second part we reported the growth and characterization of FexSe single crystals. High-pressure cubic-anvil technique successfully obtained FexSe single crystals of a size up to 2 × 2 mm2 at an optimum growth pressure of 2.8 GPa. As-grown crystals showed a characteristic of (101) crystal orientation with good out-of-plane and in-plane crystallinity. The maximum TC = 9 K in Fe1.03Se crystal was reached after appropriate heat treatment. The crystals exhibit more slight magnetic background both in ZFC and FZ curves in the normal state corroborate by susceptibility measurement. Such crystals have significantly purer quality compared to the polycrystalline samples and are promising for future investigation of superconductivity.