Aup-(PrBa2Cu3O7)1-p之熱電勢的展透行為研究

Abstract

對於金屬-絕緣體複合材料的導電率σ所具有的percolation行為，是早已眾所皆知。其值在p>pc時，將遵循一個簡單的power law：σ~(p-pc)t，其中pc為臨界體積比，t為臨界指數。但對於熱電勢所具有的percolation行為卻僅有理論上的探討，而缺乏有效且直接的實驗證據來支持。本實驗的目的便是為了驗證理論上認為：熱電勢在percolation threshold 附近的行為與σI/σM，γI/γM及(p-pc)有關，其中γ為熱導率，下標I及M分別代表絕緣體及金屬。而本實驗的結果符合此一理論，並得到在σI/σM << γI/γM時，S~(p-pc)-q且pc =pc’。 本篇論文中，我們研究一系列不同成份的三維粒狀結構金屬-半導體複合物：Aup -(PrBa2Cu3O7)1-p，其中ρPBCO/ρAu =100000，是良好的percolation系統。在實驗中採用4He低溫系統以及自製的樣品座來量測樣品從室溫到5 K的熱電勢以及導電率。

The percolation behavior of electrical conductivity,σ, in metal-insulator composites has been well established. For p > pc, a simple power law governs the electrical conductivity:σ ~ (p-pc)t, where t is a critical exponent and pc is the critical volume fraction. On the other hand, the percolation behavior of thermoelectric power has been only discussed theoretically, while there are few experimental results to test the theoretical predictions. In this work, we show that the behavior of thermoelectric power near the percolation threshold depends on three dimensionless parameters, namely, the electrical and thermal conductivity ratios of the two constituent components σI/σM, γI/γM , and the distance away from the percolation threshold Δp=(p-pc), where I and M stand for the insulator and metal, respectively. Our experimental result agrees with the theoretical prediction. In particular, we observe that, for σI/σM << γI/γM (which is pertinent to our case), S~(p-pc)-q. Our result also confirms that the percolations thresholds for electrical conductivity and thermoelectric power are the same, i.e., pc = pc’ to within our experimental uncertainty. Our samples are a series of three-dimensional granular metal-insulator composites: Aup-(PrBa2Cu3O7)1-p, which is an ideal percolation system with a resistivity ratio ρPBCO/ρAu =100000. Our resistivity and thermoelectric power measurements between room temperature and 5 K were performed on a 4He cryostat equipped with a homemade sample holder.