Magnetic Correlations in Superconducting Iron Arsenides

自2008年2月以来，基于一系列含有铁和砷原子层或铁和硒原子层的化合物，开发了一种新的超导体家族。这些化合物在相对较高的温度下(截至2008年11月高达56K)表现出超导性(即它们失去了所有的电阻)。目前的想法是，超导的机制与铅和二硼化镁等简单元素和化合物中的机制不同，在这些元素和化合物中，晶格的振动导致电子结合成对，从而降低电子的能量，这些对可以在晶体中无阻力地移动。相反，新的砷化铁和硒超导体存在于靠近磁性相的地方，这表明磁相互作用可能是超导性的原因。拟议的工作的目的是研究砷化铁化合物家族中选定的成员的超导电性和磁性之间的关系。到目前为止，所有发现的具有特殊结构排列的铁和砷原子的化合物都已经通过控制它们的电子计数成功地转变为超导体，尽管晶体结构有相当大的变化。这是通过取代使化学成分发生微小变化来实现的，例如通过用钴取代一些铁金属。这种化学灵活性可能使以许多不同的方式调整材料的物理行为成为可能，这可能被用来进行系统的实验，以研究磁性和超导之间的相互作用。申请者最近为这一领域贡献了新的组合物，如LiLiAs和NaFeAs，并处于有利地位，可以开始对这些材料的磁性和超导性质进行系统的研究。在这个项目中，一名研究生将使用一系列的中子和X射线散射技术进行一系列研究，以获得原子尺度上的基本磁性行为。特别的重点将是使用中子光谱学来探测成分与超导体接近的非超导化合物中的磁动力学，并探索超导成员中超导能隙的性质。在ISIS、ILL和钻石设施的拟议研究将建立在申请者在这一领域的最新工作的基础上，并将为研究生提供一个连贯的研究方案，该研究生将获得化学合成、物性测量和广泛使用中央设施的经验。

Since February 2008 a new family of superconductors has been developed based on a series of compounds containing layers of iron and arsenic atoms or alternatively iron and selenium atoms. These compounds exhibit superconductivity (i.e. they lose all their electrical resistance) at relatively high temperatures (at up to 56 K as at November 2008). The current thinking is that the mechanism of superconductivity is not the same as that found in simple elements and compounds, such as lead and magnesium diboride, in which the vibrations of the crystal lattice cause the electrons to lower their energy by binding into pairs which can move through the crystal without resistance. Rather, the new iron arsenide and selenide superconductors exist in close proximity to magnetic phases, which suggests that magnetic interactions might be responsible for superconductivity.The purpose of the proposed work is to investigate the relationship between superconductivity and magnetism in selected members of the iron arsenide family of compounds. So far, all compounds found with a particular structural arrangement of iron and arsenic atoms have been successfully turned into superconductors through control of their electron count, even though there is considerable variation in the crystal structures. This is achieved by making small variations in the chemical composition through substitutions, e.g. by replacing some of the iron toms by cobalt. This chemical flexibility potentially makes it possible to tune the physical behaviour of the materials in many different ways, which may be exploited to perform systematic experiments to study the interplay between magnetism and superconductivity. The applicants have recently contributed new compositions to this field, such as LiFeAs and NaFeAs, and are well placed to embark on a systematic investigation of the magnetic and superconducting properties of these materials. In this project a graduate student will perform a series of studies using a range of neutron and X-ray scattering techniques to gain access to the fundamental magnetic behaviour on an atomic scale. The particular focus will be on the use of neutron spectroscopy to probe the magnetic dynamics in the non-superconducting compounds close in composition to the superconductors, and to probe the nature of the superconducting energy gap in the superconducting members. The proposed studies at the ISIS, ILL and Diamond facilities will build on the very recent work by the applicants in this field and will provide a coherent research programme for a graduate student who will gain experience of chemical synthesis, physical property measurements and extensive use of Central Facilities.