硫摻雜對 BiOCuSe 晶格熱膨脹與磁阻特性之影響;Effects of S-doping on the lattice thermal expansion and magnetoresistive characteristics of BiOCuSe

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题名:	硫摻雜對 BiOCuSe 晶格熱膨脹與磁阻特性之影響;Effects of S-doping on the lattice thermal expansion and magnetoresistive characteristics of BiOCuSe
作者:	劉仕彬;Liu,Shih-Bin
贡献者:	物理學系
关键词:	熱膨脹;磁阻;硫摻雜;氧硫化合物;thermal expansion;magnetoresistance;Sulfur doping;oxychalcogenide
日期:	2014-07-25
上传时间:	2014-10-15 14:48:45 (UTC+8)
出版者:	國立中央大學
摘要:	本論文主要是透過X光繞射、電傳輸量測、巨觀磁性量測等實驗方法，來觀測硫摻雜對層狀晶體的BiOCuSe 氧硫化合物粉末，其晶格熱膨脹、磁阻與磁特性等物理特徵之影響。在這一系列的粉末化合物中，晶體結構仍維持P4/nmm 四方對稱，但晶格常數會隨著硫摻雜的增加而明顯遞減。在樣品中發現可藉由硫含量調控的熱膨脹行為，並在硫比例為10 %時，其熱膨脹係數最小。這個特殊的物理表現，來自晶體中(Bi2O2)2+ 和(Cu2Se2)2- 兩層間的電荷轉移現象。除了硫比例為30%的樣品，實驗量測所得之電阻，均能以三維VRH傳導行為加以描述。而磁阻隨外加磁場的變化量會因溫度和硫摻雜之不同而不同，並非常見的平方增加之關係。R(T)與MR(T)圖，皆能被30%的硫劃分成兩個區域，且與銅氧離子上電子密度分佈之可變性相關。摻雜程度較分界點(x=0.3)低時，電子密度分佈會隨硫增加變化，電阻和磁阻亦隨硫增加遞減。摻雜程度高於分界點時，電子密度分佈不隨硫增加變化，電阻會再次增高，磁阻則有從正到負的變遷現象。上述之傳輸行為的改變，推測可能來自硫摻雜對傳導帶與價電帶之影響，被改變後的電子能隙是造成電阻與磁阻變化的主要原因。而在這一系列的BiOCuSe1-xSx摻雜樣品中，沒有量測到明顯的磁性變化。;This article reports on the observations of lattice thermal expansion, magnetoresistive characteristics, and magnetic properties in layered oxychalcogenide BiOCuSe1-xSx powder, through the x-ray diffraction, electric transport, ac magnetic susceptibility, and magnetization measurements. In those powder compounds, crystallizes still into a tetragonal P4/nmm symmetry, but the lattice constants will be suppressed by substituting the sulfur for selenium in BiOCuSe. An alterable lattice thermal expansion is achieved mainly by tuning the sulfur doping level, and this exceptional small thermal expansion will be detected when x = 0.1. Partially electronic charge transfer between (Bi2O2)2+ and (Cu2Se2)2- layers will produce the minimal thermal expansion at this special sulfur composition. The temperature dependences of resistivity R can be described by the 3D variable range hopping conduction, except the data of x = 0.3 sample. A vicissitude of magnetoresistance (MR) can be observed from positive to negative by altering the sulfur composition, and there is significant relation with the variability of electron distribution near Cu and O ions. R(T) and MR(T) curves both can be classified into two types with the sulfur composition. This borderline between the two regions is at 30 %. We suppose this phenomenon can conjunction with the S-doping effect of conduction band and valence band. The changed energy gap leads to the variation of transport and magnetoresistive characteristics. No apparent magnetism is observed of BiOCuSe1-xSx samples.
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