Crystal growth and thermodynamic investigations in isovalently doped iron-based superconductor

铁粒子呈现出丰富的相图，其中超导性与自旋密度波（SDW）和向列有序共存和竞争，从而形成了非常规的配对机制。超导性可以通过载流子掺杂（杂价）、施加压力或等价掺杂来获得。尽管人们普遍认为自旋涨落在库珀对的形成中起着重要作用，但许多方面，如磁性的作用、化学调谐的性质以及由此产生的对对称性仍然未知。SrFe2(As1-xPx)2, （Sr122）和LaFeAs1-xPxO （La1111）是典型的等价掺杂超导体，因为P具有与as相似的电子构型。因此，预计不会引入额外的电子或空穴。因此，掺杂P比在电子掺杂化合物中观察到的更缓慢地抑制静态磁序。Ba(Fe1-xPx)2As2的等价取代已被很好地记录下来，并在后者体系中发现了量子临界点（QCP）的清晰特征。因此，QCP需要在另一个同价体系中进行测试，即SrFe2(As1-xPx)2。此外，Sr122和La1111体系中P掺杂的超导性质仍存在争议。本提案的目的是专注于122和1111材料的单晶生长，以及使用x射线方法，电阻率，磁化率，比热和低温下的μ子自旋共振测量对晶体进行全面表征，以深入了解上述问题。在一个实验室中使用晶体生长和物理表征为全面探索122和1111化合物的物理特性提供了一个很好的结合。研究的中心目标是：(i)利用法兰克福歌德大学物理研究所Cornelius Krellner教授实验室的各种设备合成单晶，如铁基超导体。对于Sr122，我将使用自通量法；而对于La1111，将使用sn通量。对于La1111化合物，到目前为止还没有生长出单晶。但是，学习培养这些1111体系的单晶将对更好地理解各种1111化合物背后的物理学有很大的科学意义。（ii）将探讨等价掺杂超导体的超导性和磁性以及相图。（iii）有充分的文献证明，不同材料的氨基酸的间隙对称性是不同的。此外，实验证实了SC序参量的精确对称性；同时，随着兴奋剂的使用，它的演变仍然充满争议。因此，了解SC基态的对称性，将为化合物的微观配对机制提供线索，并将对高温超导现象有更深入的理解。

Iron pnictides present a rich phase diagram wherein superconductivity coexists and competes with the Spin Density Wave (SDW) and nematic order, resulting in unconventional pairing mechanisms. Superconductivity can be obtained through carrier doping (heterovalent), the application of pressure, or isovalent doping. Although there is a general consensus that spin fluctuations play an important role in the formation of Cooper pairs, much aspects such as the role of magnetism, the nature of chemical tuning, and the resultant pairing symmetry remain unknown. SrFe2(As1-xPx)2, (Sr122) and LaFeAs1-xPxO (La1111) are prototypical isovalently doped superconductors, as P has a similar electronic configuration to that of As. Therefore, it is not expected to introduce extra electrons or holes. Hence, doping P suppresses a static magnetic order much more gradually than observed in electron doped compounds. Isovalent substitution has been well documented in Ba(Fe1-xPx)2As2 with clear signatures of a Quantum Critical Point (QCP) found in the latter system. Thus, the QCP needs to be tested in another isovalent system i.e., SrFe2(As1-xPx)2. In addition, the nature of superconductivity in Sr122 and in La1111 systems with P doping remains under debate. The aim of this proposal is to focus on the single crystal growth of 122 and 1111 materials, together with a thorough characterization of crystals using x-ray methods, resistivity, magnetic-susceptibility, specific-heat, and Muon Spin Resonance measurements at low temperatures, to gain insights into the questions raised above. To use crystal growth and physical characterization in one laboratory provides an excellent combination to explore the physics of the 122 and 1111 compounds in a comprehensive way. Central objects of the research are: (i) To use diverse facilities in the laboratory of Prof. Dr. Cornelius Krellner at the Physics Institute at the Goethe University Frankfurt to synthesize single crystals such as Fe-based superconductors. For Sr122, I will use a self-flux method; whereas for La1111, the Sn-flux will be employed. For La1111 compounds, so far no single crystals have been grown. But learning to grow these single crystals of the 1111 systems will be of great scientific interest to better understand the physics behind various 1111 compounds. (ii) Superconductivity and magnetism in isovalently doped superconductors as well phase diagrams will be explored. (iii) It is well documented that the gap symmetry in pnictides differs from material to material. Moreover, experimental confirmations of the precise symmetry of the SC order parameter; as well its evolution with doping remains highly controversial. Therefore, understanding the symmetry character of SC ground states should provide clues to microscopic pairing mechanisms in pnictides and will give a deeper understanding of the phenomenon of high-temperature superconductivity.