氧含量對六角晶格銣鎢青銅礦Rb0.23WOy物理性質的影響

Effect of Oxygen Content on Physical Properties of Hexagonal Rubidium Tungsten Bronze Rb0.23WOy

邵禹成(Yu-Cheng Shao)

指導教授 ： 林大欽

淡江大學/理學院/物理學系碩士班/碩士(2010年)

https://doi.org/10.6846/TKU.2010.00823

摘要

在多晶銣鎢氧化物Rb0.23WOy(2.9＜y＜3.05)系統中，經由先前的研究得知氧含量對其正常態電阻及超導轉變溫度都有影響。由正常態的電阻率溫度對溫度關係圖中發現在氧含量3.0附近存在金屬-非金屬轉變，氧含量較少的樣品(y≦3.0)傳輸性質為金屬性，氧含量較多的樣品( y＞3.0)則為非金屬性，而且此轉變也會讓樣品電阻率大幅改變。藉由霍爾效應的量測可以發現在金屬-非金屬轉變時會伴隨傳導載子性質的改變(電子-電洞)，另外也說明氧含量較多的樣品其電阻率的增加是由於傳輸載子減少的關係。藉由X-ray的GSAS精算以及第一原理計算的結果，我們認為在氧含量較多的樣品中，多餘的氧是佔據銣的空缺位置，致使在傳輸性質中扮演重要角色的鎢氧八面體產生些微的扭曲。經由W L3-edge X光吸收光譜的測量，這個扭曲的現象會造成W-O鍵結變弱，使可移動的載子數量減少，這結果與霍爾量測相吻合。此外，鎢氧八面體的些微扭曲也會影響樣品中電子-聲子的交互作用，使超導性質出現改變。藉由氧含量的改變會使樣品有三個區域出現，分別為超導抑制區(Tc＜2 K)、較低區(Tc∼3 K)以及較高區(Tc＞3 K)。在氧含量對樣品超導態的影響這一部份，我們作了低溫比熱的量測，藉由比熱的數據可以發現當溫度低於超導轉變溫度時，樣品似乎並不是全部都變成超導態，可能存在著非超導性的電荷密度波。

關鍵字

銣鎢青銅礦 ； 金屬-非金屬轉變 ； 超導抑制區

並列摘要

Superconducting transition temperature Tc and normal-state resistivity as a function of oxygen content for hexagonal tungsten bronze Rb0.23WOy with 2.90 < y < 3.05 were obtained from transport measurements. It is remarkably interesting that Tc enhances about 50% and room-temperature resistivity increases about three orders of magnitude as oxygen content varies from 2.90 to 3.05. The low-temperature specific heat data indicate that the Einstein-like mode associated with Rb vibration has a dimensionality crossover from 3D to quasi-2D as oxygen content increases from 2.90 to 3.04﹐and show that the superconducting state is not condensed in some part of the sample.The Hall effect measurement indicate that the change of the transport charge occur in the vicinity of y = 3.00(electron-hole transition ) and the room-temperature resistivity increasing induced by the decreasing in transport charge. W L3-edge x-ray absorption spectra further show that W-O bond intensity gradually weakens as oxygen content increases, indicative of more oxygen disorder present in the oxygen-rich samples. The observed results strongly suggest that the local lattice distortion induced by oxygen disorder not only modulates Rb vibration, possibly coupled to electron-phonon interaction responsible for superconductivity, and also reduces the charge transfer between O 2p and W 5d orbital in the vicinity of y =3.00. This scenario can possibly account for significant increases of Tc and normal-state resistivity of Rb0.23WOy as
oxygen content slightly changes from 2.90 to 3.05.

並列關鍵字

rubidium tungsten bronze ； metal-nonmetal transition ； superconductivity-suppressed zone

參考文獻

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