Sr(Pd1-xNix)2Ge2的超導性和Sr(Co1-xNix)2Ge2的磁性

Abstract

在本文中，主要討論鍶鈀鎳鍺化合物隨鎳含量的增加，晶格常數和超導轉變溫度有著系統性的變化。這個系統和最近發現的鐵基超導中的鋇鉀鐵砷(122系統)有著相同的結構。Ｘ光繞射分析表示樣本帶有少量雜質但大部分的繞射峰可以被有著空間群I4/mmm (No. 139)的四方晶格所標定代表說鎳真的進入結構裡面。而隨著鎳含量的增加，四方晶格的晶格常數a變短，c變長，且伴隨體積減少。在鍶鈀鎳鍺化合物中，當鎳含量佔百分之10 %左右，超導轉變溫度增加從3 K緩慢低增加到3.2 K，對於鎳含量大於70%超導轉變溫度慢慢低減少到1.8K之下。我們對鍶鈀鎳鍺化合物中鎳含量達到百分之百的樣本做進一步的低溫電阻量測確認他是超導且有超導轉變溫度大約0.87 K，所以鍶鈀鍺化合物中鎳含量從0~100 %都是超導。此外也觀察到對於x< 0.1超導轉變溫度Tc隨著外加壓力的微微增加時有增加的趨勢，但對x≥ 0.1隨著外加壓力的增加Tc總是減少的。 做為一個鎳取代系統的延伸，我們調查了鍶鈀鉑鍺系統。結構分析指出這個系統有溶解度限制在x大約為0.15左右。在鍶鈀鉑鍺系統中磁化率測量表示不論鉑參雜了多少在T ≥2 K都沒有觀察到塊材的超導。 此外，鋇鐵砷系統經由鈷的取代有一個反鐵磁-超導轉變，做為一個類比我們也調查了鍶鈷鎳鍺系統的磁性變化。在2 K以上磁化率測量指出鍶鈷鎳鍺系統有著複雜的磁性行為但都沒有觀察到超導。

The systematic variations of lattice structure and superconducting transition temperature in Sr(Pd1-xNix)2Ge2 are reported, which is iso-structural with the iron based superconductors (Ba1-xKx)Fe2As2 (122-system). The x-ray diffraction analysis shows most peaks can be indexed by the tetragonal lattice with space group I4/mmm (no. 139) with minor impurities indicating Ni really entered the structure. The tetragonal lattice parameter a decreases and c increases monotonically, while the unit cell volume decreases with Ni concentration x. The superconducting transition temperature Tc increases slightly from 3 K to 3.2 K for light Ni doping of x ~ 0.1 then slowly decreases to Tc< 1.8 K for x > 0.7. Further investigation for SrNi2Ge2 by low temperature resistivity measurement indicates a Tc(mid point) of 0.87 K suggesting that whole system were superconducting. Furthermore, we observed the pressure effects on the superconducting transition temperature by magnetic susceptibility. A crossover of Tc dependence on pressure was observed from which Tc increases at low pressure for x < 0.1 to Tc always decreases for any applied pressure for x ≥ 0.1. As an extension of the Ni substitution system, we investigated the Sr(Pd1-xPtx)2Ge2 system. The structure analysis indicates that this system has a solubility limit around x ~ 0.15. The magnetic susceptibility shows that no bulk superconductivity at T ≥ 2 K was observed in the Sr(Pd1-xPtx)2Ge2 system whenever Pt was doped. Besides, we also observed the variation of magnetism in Sr(Co1-xNix)2Ge2 system as an analogy to the Ba(Fe1-xCox)2As2 system which undergoes antiferromagnetism- superconductivity transition through Co substitution. Magnetic susceptibility measurement indicated that Sr(Co1-xNix)2Ge2 system had a complex magnetic behavior with no superconductivity observed down to 2 K.