鐵硒母系化合物之無序化鐵空缺位之超導性質之研究

The Evolution of Superconductivity in beta-Fe(1-x)Se parent phase with Fe vacancy disorder

葉耕余(Keng-Yu Yeh)

指導教授 ： 吳茂昆 ； 張廖貴術

國立清華大學/原子科學院/工程與系統科學系/博士(2021年)

摘要

摘要

本研究通過納米元件製程技術針對具有√5×√5鐵有序空缺位的Fe4 +δSe5單晶奈米線進行電性傳輸的研究。在未外加磁場下觀察到溫度28 K的一階金屬-絕緣體（MI）相轉變，並且發現與磁場相關的正巨磁阻(Colossal positive magnetoresistance)現象，其中該正巨磁阻表現出c軸優選方向(Prefer orientation)的異向性(anisotropic)的磁場依賴性。在磁場強度B = 9 T以及T <〜17 K時，Fe4 +δSe5單晶納米線保持與磁場無關的電阻狀態，推斷該磁場在零磁場下發生了自旋極化，可能與自旋-軌道耦合(spin-orbital coupling)的破壞有關。同時，該鐵有序空缺位的Fe4 +δSe5的金屬-絕緣體的相轉變效應具有頻率的依賴性，可以阿瑞尼斯定律（Arrhenius-law）的關係表示出其能量與頻率的關係，使這種行為與四氧化三鐵的Verwey轉變具有類似的電子電荷有序效應，也是第一個發現的非氧化物中具有類似Verwey轉變的實驗結果。數據表示自旋軌道耦合效應主要在低溫下對√5×√5 鐵空位有序的Fe4 +δSe5可以被觀測，此提供了解FeSe的電子軌道性質與超導電性的出現可能關聯。
另一方面，本論文亦進行了對FeSe超導體的絕緣母相(Parent phase)的詳細研究。通過化學共沉澱法合成Fe1-xSe時，以FeCl2的不同比例進行製備具有不同Fe空缺有序的Fe1-xSe。從電性的量測中發現，Fe空位有序的絕緣Fe1-xSe基本上表現出3D-Mott變程跳躍行為(3D-Mott variable range hopping)，在37〜45 K左右具有類Verwey轉變特性(Verwey-like transition)。通過調控Fe空缺有序地排列，Fe1-xSe具備不同傳輸特性如單能帶(single-band)和雙能帶(two-band)行為。再者，本研究利用快速退火處理(Rapid thermal annealing, RTA)進行對鐵空缺有序性的破壞，同時增加Fe3 +在Fe1-xSe中的含量。在不改變Fe1-xSe分子組成劑量的條件下，產生出更多的電子載流子，導致在 8 K時會出現完全的超導現象。更在675 oC的RTA處理下達到起始超導相轉變溫度14.5 K，為第一次在常壓下出現FeSe系統高於超導相轉變溫度高於8 K的研究結果。最後，本研究論證通過激子耦合機制(Exciton coupling mechanism)可以誘導和增益FeSe的超導性，證明具有鐵控缺有序的Fe1-xSe為FeSe超導系統的母相。

關鍵字

鐵硒超導體 ； 費耳威相變 ； 鐵空缺有序性 ； 莫特絕緣體 ； 混合價態 ； 巨磁阻效應

並列摘要

Abstract

The electrical transport of Fe4+δSe5 single-crystal nanowires exhibiting √5 × √5 Fe-vacancy order and mixed valence of Fe were investigated in this study by a single nanowire measurement. A first-order metal-insulator (MI) transition of transition temperature TMI ≈ 28 K is observed under zero magnetic fields (B). Colossal positive magnetoresistance emerges, resulting from the magnetic-field dependent MI transition. TMI demonstrates anisotropic magnetic-field dependence with the preferred orientation along the c-axis. At T < ~17 K, the state of near magnetic-field independent resistance is preserved under magnetic fields up to B = 9 T, deduced to be spin-polarized at zero fields. The c-axis preference and the loss of magnetic-field dependence below T ≈ 17 K to the spin-orbital coupling in the tetragonal system have been claimed. The Arrhenius-law shift of the transition on the source-drain frequency dependence reveals that it is the first non-oxide compound with the Verwey-like electronic correlation. The data indicate that the spin-orbital coupling is crucial in √5 × √5 Fe-vacancy-ordered Fe4+δSe5 at low temperatures. The findings can provide valuable information to better understand the orbital nature and the emergence of superconductivity in FeSe-based materials.
On the other hand, a detailed study to investigate the existence of an insulating parent phase of FeSe superconductor has been carried out. Fe1-xSe with different Fe-vacancy orders were prepared by a chemical co-precipitation method under the nominal molar ratio of FeCl2 precursor during the synthesis. The insulating Fe1-xSe with Fe-vacancy order basically shows a 3D-Mott variable range hopping behavior with a Verwey-like electronic correlation at around 37~45 K, single- and two-band behaviors of the transport properties can be observed in different types of Fe1-x¬Se by controlling the vacancy order. The application of the rapid thermal annealing (RTA) process results in the destruction of Fe-vacancy order and induces more electron carriers by increasing the Fe3+ valence state. Superconductivity emerges with Tc ~ 8K without changing the chemical stoichiometry of the sample after the RTA process by resulting in the addition of extra carriers in favor of superconductivity and reach to 14.5 K with the appropriate thermal annealing temperature of 675 oC is firstly performed under the ambient condition. In addition, the results show that the tetragonal Fe(1-x)Se with the specific Fe-vacancy order is the parent phase of FeSe superconductors, and the superconductivity can be induced and correlated by the exciton coupling mechanism, which implies the parent phase of FeSe superconductors.

並列關鍵字

FeSe superconductors ； Verwey transition ； Fe-vacancy order ； Mott-insulator ； Mix-valence state ； Colossal magnetoresistance

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