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

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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.