Quest for nematicity in iron-based superconductors

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One of the fundamental new discoveries on superconductors in the recent years is the experimental observation of electronic nematicity, i.e., a spontaneous symmetry breaking, which is crucial for understanding superconductivity in pnictide superconductors as well as for many other materials. It is associated with the evolution of short-range electronic order, however, the mechanisms of its formation as well as its interdependencies with superconductivity are not yet clear. The current proposal asks for extension of the ongoing project on the quest for nematicity in in iron-based superconductors which yielded a variety of important results and has raised further relevant questions. The main and unique experimental technique will still be the dilatometric bending method to investigate electronic nematic order in iron based superconductors. The very successful collaboration between Heidelberg University and KIT which includes sharing of labs for the project will be extended. During the past project year, our main focus was on 1111 and 122 systems where successful studies on nematicity have been done and published. Notably, our analysis of 1111 and 122 data in the first project phase has shown that a more thorough comparison of elastoresistivity and 3-point bending data is needed as compared to the previous literature. In the remaining time of the first project phase, we will finish studies on LiFeAs and NaFeAs as outlined in the work plan. The third year which funding is applied for in the proposal at hand will particularly focus on 11 and further 122 systems, i.e., on Eu(Fe1−xCox)2As2 and FeSe1-xTex with various doping levels as well as some rare-earth 1111 samples. All these materials have not yet studied by means of the bending method. In case of the new Eu(Fe1−xCox)2As2 and FeSe1-xTex single crystals, the bending data will be corroborated by thermal expansion studies which also do not exist in the literature. The dilatometric studies in the project will be accompanied by relevant experimental and theory studies by our collaborators, including, e.g., anisotropic resistivity, specific heat, and numeric analysis for nematic order-parameter using DFT calculation.