利用μ子自旋旋转解纠缠NaFe_（1-x）Ni_xAs中的超导序和磁序
Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotation
摘要：
对铁基超导体NaFe_（1-x）Ni_xAs的μ子自旋旋转和弛豫进行了研究。静态磁序的特征在于获得的局部有序的磁矩大小和磁有序区域的体积分数的温度和掺杂的依赖性。对于x = 0和0.4%，观察到向接近均匀的长程磁有序态的转变，而对于高于0.4%的x，磁有序变得更加无序，并且对于x = 1.5%被完全抑制。磁体积分数随着x的增加而不断减小。磁性和超导体积的组合意味着磁性和超导性的空间重叠共存跨越NaFe_（1-x）Ni_xAs的T-x相图的大区域。当x = 0.6，1.0和1.3%时，在低于超导T_C时，有序矩尺寸和体积分数都有明显的减小，而其它铁磷属元素化物的有序矩尺寸和体积分数只有一个在低于T_C时减小，而不是两个都减小。随着Ni掺杂量的增加，磁有序的抑制进一步增强，当x = 1.3%时，在低于T_C时出现了重入非磁态。重入行为表明反铁磁性和超导性之间的相互作用，涉及对相同电子的竞争。这些观测结果与符号变化的s波超导态一致，这种超导态预计将出现在微观共存和与磁性相分离的边缘。我们还提出了一个普遍的线性关系之间的局部有序矩大小和反铁磁有序温度TN在各种铁基超导体。我们认为，这种线性关系是一致的巡回电子的方法，其中费米表面嵌套驱动反铁磁有序。
英文摘要：
Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering.
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