Magnetism, nematicity and superconductivity in 1111 oxypnictide single-crystals

F掺杂LaFeAsO多晶中超导电性的发现开启了高温超导电性的铁时代。这些材料通常被称为铁基超导体(IBS)。在IBS的低温相中表现出三种有序：(I)轨道或电子有序；(Ii)正交形变；(Iii)反铁磁自旋密度波有序。在长程有序态的先例中，出现了旋转对称性被破坏但时间反转和平移对称性保持不变的相，被称为向列态。磁性、向列性和超导电性紧密地交织在一起，但这三个有序中的哪一个是核心问题，仍然存在争议。虽然人们已经对许多家族的IBS进行了全面的研究，但由于缺乏合适的单晶，对稀土掺杂的REFeAsO(1111)化合物初始体系的研究还很少。这种对1111体系缺乏了解是非常令人遗憾的，因为这些化合物表现出最高的块状超导转变温度，并且与其他类别有显著的不同，如轨道简并度更高，垂直于层的电子色散减少。要深入理解IBS，特别是1111材料，需要具有合适的尺寸、质量和组成的单晶。由于晶体生长中的重大挑战，1111家族目前是所有IBS中最不被了解的。直到最近，在德累斯顿的IFW上，Pi Wurmehl组才使用固态单晶生长方法生长了具有相当大的c轴生长的大的、刻面良好的LaFeAsO单晶。这种方法并不广为人知，也没有完全被理解，这种生长是扩散控制的，晶体是通过异常的颗粒生长从多晶基质中生长出来的。这种生长LaFeAsO晶体的成功将为化学和物理领域的激动人心的实验奠定基础。我们希望通过将晶体生长工作与使用X射线和中子辐射的散射研究相结合，从1111晶体的首次上市提供的机会中获利。因此，我们想要澄清晶体结构，特别是寻找由NQR实验证明的结构扭曲。我们将表征磁序和短程磁关联作为组成、温度和单轴应变的函数。磁激发被认为是超导对的一个关键元素，当有更多的晶体可用时，可以用非弹性中子散射来研究。研究具有较高转变温度的材料中的超导自旋共振模和研究具有不同电子结构的1111材料的自旋空间各向异性似乎是最有趣的。

The discovery of superconductivity in F-doped LaFeAsO polycrystals initiated the so-called iron-age of high-temperature superconductivity. These materials are frequently referred to as Fe-based superconductors (IBS). Three types of order manifest in the low-temperature phases of IBS: (i) orbital or electronic ordering (ii) orthorhombic distortion, and (iii) the antiferromagnetic spin density wave order. Precedent to long-range ordered states, a phase with broken rotational symmetry but with preserved time-reversal and translation symmetries emerges and is referred to as nematic state. Magnetism, nematicity and superconductivity are intimately interwoven but the question, which of these three orderings is central, remains matter of controversy. While there have been comprehensive studies on many families of IBS, the initial system of doped REFeAsO (1111) compounds with RE a rare earth has been very little studied due to the absence of suitable single crystals. This lack of understanding the 1111 system is highly regrettable, as these compounds exhibit the highest bulk superconducting transition temperatures, and as there are significant differences with the other classes such as a higher degree of orbital degeneracy and a reduced electronic dispersion perpendicular to the layers.A deep understanding of IBS in general and of 1111 materials in particular requires single crystals of appropriate size, quality and composition. Due to the major challenges in crystal growth, the 1111 family is currently the least understood among all IBS. Only recently, large and well faceted LaFeAsO single crystals with considerable c axis growth were grown in the group of PI Wurmehl at the IFW in Dresden using solid-state single-crystal growth. This approach is not commonly known and not fully understood, the growth is diffusion controlled and crystals are grown from a polycrystalline matrix via abnormal grain growth. This success in growing LaFeAsO crystals will set the stage for exciting experiments both in the field of chemistry and physics. We wish to profit of the opportunities the first availability of the 1111 crystals offers by combining the crystal growth efforts with scattering studies using X-ray and neutron radiation. Thereby we want to clarify the crystal structure and in particular to search for the structural distortions evidenced by NQR experiments. We will characterize magnetic order and short-range magnetic correlations as function of composition, temperature and uniaxial strain. Magnetic excitations are considered as a key element of the superconducting pairing and can be studied by inelastic neutron scattering when larger amounts of crystals become available. It seems most interesting to study the superconducting spin-resonance modes in materials with higher transition temperatures and to study the spin-space anisotropies in 1111 materials, which exhibit a different electronic structure.