Neutron scattering investigation of iron pnictides superconductors

在所谓的奇异超导体中，库珀对超导凝聚体的形成是由磁涨落所调节的。已知有两类转变温度处于相反极端的材料。在d−电子铜酸盐材料中，超导电性出现在T_c高达160K的电荷掺杂反铁磁绝缘体中。在标度的另一端，在f−电子超导体中，基态很容易在巡回自旋密度波磁序和T_c~1K的超导电性之间进行调谐。最近发现的FeAs超导体弥补了这一差距，T_c高达55K，母相已被明确地证明是自旋密度波金属。对这些化合物的早期研究发现，母体化合物中的SDW不稳定性是理解这些材料行为的关键特征。这一巡回观点揭示了母体化合物的耦合结构和磁相变，并为超导序参数的对称性提供了一个稳健的模型。尽管取得了这些成功，但强局域化物理的作用在诸如磁性和超导或光学光谱共存的观测中是显而易见的。黑潮的一个显著特征是，它们可以在压力下调谐到超导状态，这与它们的铜酸盐形成了鲜明的对比。这一行为为我们提供了实现超导电性的两个调谐参数，以及探索这些材料潜在物理的新手段。我们认为，通过仔细研究磁性的发展作为压力和化学掺杂的函数，可以在理解铁多肽中超导的途径方面取得相当大的进展。这两个调整超导电性的参数有可能访问截然不同的物理。超导性的开始或其在相对中等压力下的巨大增强提供了一种在没有化学无序的情况下将系统调整为超导电性的显着不同的方法。我们相信，这两个调谐参数之间的对比可以揭示巡回物理和强关联物理之间的二分法。我们将集中研究最简单的铁磷酸盐超导体，FeTe1−xSex系统，因为它们化学简单，超导T_c相对较高，而且有相对大的高质量单晶可用。

In the so-called exotic superconductors, formation of a superconducting condensate of cooper pairs is mediated by magnetic fluctuations. Two classes of materials with transition temperatures at opposite extremes are known. In the d−electron cuprate materials, superconductivity emerges from a charge doped antiferromagnetic insulator with Tc as high as 160 K. At the other end of the scale, in the f−electron superconductors, the groundstate is easily tuned between itinerant spin density wave (SDW) magnetic order and superconductivity with Tc~1 K. The recently discovered FeAs superconductors bridge this gap with Tcs of up to 55 K and a parent phase which has unambigously been shown to be a SDW metal. Early investigation of the pnictides have identified the SDW instability in the parent compounds as a critical feature in understanding the behavior of these materials. This itinerant view has shed light into the coupled structure and magnetic transition of the parent compounds and provided a robust model for the symmetry of the superconducting order parameter. Despite these successes the role of strongly localized physics is evident in the such observations as the co-existance of magnetism and superconductivity or optical spectroscopy. One remarkable feature of the pnictides is that they can be tuned into the superconducting state using pressure, in sharp contrast to their cuprates counterparts. This behavior provides us with two tuning parameters to achieve superconductivity as well as a new means probe the underlying physics of these materials. We propose that considerable progress can be made in understanding the route to superconductivity in the iron pnictides by closely examining the development of magnetism as both a function of pressure and chemical doping. These two parameters that tune superconductivity have the possibility of accessing decisively different physics. The onset of superconductivity or its enormous enhancement under relative modest pressure provides a markedly different means in tuning the system to superconductivity in the absence of chemical disorder. We believe that the contrast between these two tuning parameters can shed light into this dichotomy between the relevance of itinerant and strongly correlated physics. We shall focus our investigation to the simplest of the iron pnictide superconductors, the FeTe1−x Sex system as they are chemically simple, superconducting Tc’s are relatively high and relatively large high quality single crystals are available.