含鐵超導材料FeSe1-δ與銅摻雜之(Fe1-xCux)Se1-δ的核磁共振研究

Abstract

我們利用固態核磁共振的方法研究最近發現的鐵基超導體FeSe1-δ及摻雜銅元素取代部分鐵的(Fe1-xCux)Se1-δ。這些樣品具有微量的Se缺陷(δ≈0.12,0.15)。我們主要是探討Se缺陷和鐵元素在FeSe1-δ中超導性所扮演的角色。由Spin-lattice relaxation rate(1/T1)的實驗我們發現樣品中spin fluctuation的強度隨著Se缺陷的增加而變小，spin fluctuation一般被懷疑與非傳統超導體有關，也確實出現在FeSe1-δ的樣品中；另外分析1/T1數據發現並不是整體FeSe1-δ樣品內部都具有超導性。藉由AC susceptibility實驗的量測，我們亦發現FeSe1-δ在Se缺陷較多樣品中的superconducting volume fraction也較少。這些結果暗示著超導性只出現在Se缺陷較少甚至無Se缺陷的相中。。至於摻銅的(Fe1-xCux)Se1-δ，我們的實驗發現摻銅效應導致超導臨界溫度下降且在極微量的銅摻雜(x≈0.03)時FeSe的超導性消失，由我們的1/T1數據發現樣品的Spin-lattice relaxation rate在摻雜銅元素後並無明顯改變，表示摻雜銅元素對(Fe1-xCux)Se1-δ內的spin dynamic影響不大 。對照Williams等人對摻雜銅(Fe1-xCux)Se1-δ的電阻量測實驗發現有metal-insulator phase transition，摻銅的效應對FeSe電子組態比spin dynamic的影響大，因此我們認為電子組態的變化可能是超導性消失的主因。

The roles of Se deficiency and Fe atoms, for the superconductivity of FeSe, have been respectively investigated in FeSe1-δ and (Fe1-xCux)Se1-δ, by our NMR experiments. The data, for nuclear spin-lattice relaxation rate (1/T1), show that the spin fluctuations are weakened at a larger δ, and are correlated with the superconductivity in FeSe. The superconducting volume fraction, estimated by our ac susceptibility experiments, is found to vary inversely with d. Our findings suggest that the Se-deficient FeSe has an inhomogeneous phase, where the superconductivity is associated with the regions having few or no Se vacancies. As for (Fe1-xCux)Se1-δ, Tc is rapidly suppressed by Cu doping and vanishes around x=0.03. The 77Se and 63Cu NMR linewidths suggest that a local moment is induced at the Fe sites, and not at the Cu sites. However, 1/T1 shows no obvious change with the Cu doping. We suspect that other effects, such as disorder or change in the density of states, have more influence on Tc suppression, since a metal-insulator transition, induced by Cu substitution, occurs in the resistivity measurements.