壓力效應下鈰鈀矽與鍶鈀鎳鍺化合物之超導與磁性研究

Pressure effects on superconductivity and magnetism in CePd2Si2 and Sr(Pd1-xNix)2Ge2

曾文彥(Tseng, Wen Yen)

指導教授 ： 徐永源 ； 陳洋元

國立臺灣師範大學/理學院/物理學系/碩士(2011年)

摘要

本論文主要探討重費米化合物CePd2Si2在高壓環境下的反鐵磁與磁的不穩定行為，在量子臨界點（quantum critical point）的附近，將可觀察到其超導性與非費米液體的現象；塊材CePd2Si2反鐵磁相轉變點大約存在於絕對溫度10 K，施予3 GPa 的外在壓力將使得反鐵磁相轉變點被抑制至0 K，並且當外在壓力達到2.4 GPa時，其超導轉變溫度約為0.3 K。一旦外在壓力超過7 GPa，其超導表現將會消失。研究目標為探討CePd2Si2 奈米微粒在壓力效應下是否也擁有與塊材相同的超導性與磁性行為。 本實驗中CePd2Si2奈米微粒是以準分子雷射濺鍍法製備而成，雷射能量約為200 mJ以及工作壓力約為1 torr，靶材至收集盤的的距離為4 cm。分別透過 XRD、TEM以及EDS來做晶體結構分析與成份分析，奈米微粒粒徑大小約為2 nm。奈米微粒的超導性與磁性量測，則是利用鑽石高壓鉆Mcell Ultra以及SQUID來完成。 ThCr2Si2類型的三元金屬化合物，其超導性與磁性已經被廣泛研究；此類晶格結構是找尋較高超導轉變溫度的金屬化合物超導體的關鍵之一。 SrPd2Ge2塊材是以弧光放電法製備而成，結構與成份分析則是透過X光粉末繞射儀，超導性是以自製的壓力鉆進行量測。SrPd2Ge2 常壓下的超導轉變溫度約為3.04 K。研究重點在於系統性的實驗不同摻雜比例的Sr(Pd1-xNix)2Ge2 與其晶格結構和超導轉變溫度的對應關係，也顯示了外在壓力的增加與鎳的摻雜比例造成的化學壓力增加，對於超導溫度的提升有著相同的趨勢。

關鍵字

壓力 ； 超導 ； 奈米 ； 低溫 ； 磁性

並列摘要

The heavy fermion compound CePd2Si2 provides an ideal system to study the antiferromagnetism (AFM), and instability under hydrostatic pressure. Around its quantum critical point (QCP), remarkable features such as superconductivity and non-Fermi liquid behavior can be observed. Bulk CePd2Si2 exhibits a AFM phase transition at ~10 K. In presence of external pressure, the AFM transition temperature TN was suppressed to 0 K as applied pressure was increased to 3 GPa, in the meantime, a superconducting transition with Tc ~0.3 K appears at P = 2.4 GPa. As long as external pressure exceeds 7 GPa, the superconducting transition is no longer observed. The goal of this research is to determine whether the nanoparticles of CePd2Si2 have the similar pressure effects on Tc and TN as those of the bulk. In our work, CePd2Si2 nanoparticles were fabricated by pulse laser deposition (PLD) method with laser energy ~200 mJ and work pressure ~1 torr. The distance between target and collect plate is near 4 cm. The crystal structures, diameters and compositions were examined by XRD, TEM, and EDS respectively. The diameter of nanoparticles is around 2 nm. In this work the superconductivity and magnetism of CePd2Si2 nanoparticles under hydrostatic pressure were studied by Mcell Ultra in SQUID.
Among ternary intermetallic compounds, ThCr2Si2-type intermetallics have been extensively studied, especially for the interest of superconducting and magnetic properties. This structure is one of the keys to search new intermetallic superconductors with higher Tc. SrPd2Ge2 bulk was formed by arc melting and characterized by XRD for its crystal structure and compositions. SrPd2Ge2 is the ThCr2Si2-type superconductors with Tc ~3.04 K. The systematic variations of lattice structure and superconducting transition temperature Tc in Sr(NixPd1-x)2Ge2 were studied, both pressure effects on Tc enhancement through chemical pressure of Ni doping and hydrostatic pressure showed similar tendency. Home-made high pressure cell were employed to determine superconductivity

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