铁基超导材料的晶体生长与中子散射实验研究

The study of physical properties and superconducting mechanism of Fe-based superconductors has become an important research field in condensed matter physics　since the year 2008. In this field, for one thing the high-quality crystal growth is the basis for further research. For another,neutron scattering technique has become a critical experimental methods to solve the mystery of superconducting mechanism in Fe-based superconductors because of its unique advantages on probing magnetic structures or spin excitations. So far, large amounts of Fe-based superconductors with different cystal structures have been discovered. However, for some of these materials, no successful single crystal growth methods have been reported. Also the superconducting mechanism and pairing symmetry in Fe-based superconductors are still controversial with no consistent physical understanding. In this programme, we are planning to carry out research on two closely related aspects. Fristly, through the exploring for new methods of crystal growth, we would fabricate high quality single crystals or polycrystallines of Fe-based superconducting materials which have not yet been thoroughly studied or with difficulties in making single crystals; In the second aspect, we would carry out neutron scattering studies on these Fe-based superconducting samples with neutron triple-axies spectrometer, with the purpose of exploring the magnetic structures or spin excitations which are closely related to the Fe-based supercondcutivity.

2008年开始的铁基超导材料的物性与机理研究是凝聚态物理领域的一个重要研究方向。这其中，生长出高质量的铁基超导单晶或多晶样品是深入研究这一体系的基础，而另一方面中子散射技术在探测物质磁结构以及各类磁性和自旋激发方面的独特优势使得它成为公认的解决铁基超导机理之谜的关键实验工具。铁基超导家族拥有数量庞大且结构各异的超导体，这其中许多材料目前仍然没有成功生长出单晶体的报道，而对于铁基超导体的超导机制与配对对称性仍然没有定论和统一的物理图像。基于此，本项目将重点开展相辅相成的两个方面工作：首先通过晶体生长方法的探索与研究，在目前晶体生长困难和未被人们深入研究的铁基材料中，合成出高质量的铁基超导单晶或多晶样品；第二个方面，我们将在这些铁基超导样品上，利用中子三轴谱仪开展中子散射的实验研究，探索这些材料中与铁基超导机制密切相关的磁有序结构以及自旋和磁性激发。

按照申请书所制定的研究计划，本项目首先开展了铁基超导材料的晶体生长探索，成功生长出了包括NdFeAsOF、过渡金属离子杂质掺杂铁基-122体系以及Sr2VO3FeAs等高质量的单晶或多晶样品。在这些样品上利用包括中子散射在内的多种实验手段进行了深入的物理研究：通过对Ba0.6K0.4Fe(2-2x)TM2xAs2(TM=Cu,Ni)系列样品的结构、磁化以及输运性质的测量，发现铁基超导体Ba0.6K0.4Fe2As2可能同时存在多种配对对称性通道；通过对铁基超导材料NdFeAsO1-xFx样品的非弹性中子散射谱的测量，得到了该样品中的晶体场激发态的信息，发现F离子的引入对于Nd离子的局域晶场对称性产生了显著影响，并引入了额外的晶场能级，这与铜氧化物氧缺位样品中的行为非常相似；此外我们还通过模拟计算的方法给出了不同类型的中子散射谱仪在能量分辨率上的差异表现，优化了相关中子散射实验中准直器的选择方法。项目发表科研论文三篇，并完成了项目申请书所预期的科学目标。