Short-Time Dynamics and Electron-Lattice Interaction in Iron Based Superconductors

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Discovered in 2008, iron-based superconductors show superconductivity at temperatures as high as 55 K, and are currently being studied intensely worldwide. Their temperature-doping phase diagram is rich, having magnetic and nematic phases in addition to the superconducting one. It is now well-known that the magnetism and the superconductivity, arising from the electronic degrees of freedom, are sensitive to various deformations of the crystalline lattice. This electron-lattice coupling is an additional complexity, on top of that due to the electron-electron interaction, which makes a microscopic comprehension of these systems a formidable challenge. The aim of the proposal IRONIC is to perform very well-defined theoretical projects in order to quantify the different types of electron-lattice couplings in these materials, and to understand better the role of these couplings in establishing magnetic and superconducting order. A long-term benefit of research such as that proposed here is that, once well-understood, the electron-lattice coupling can, in principle, be tuned by material engineering to raise the superconducting transition temperature Tc.The scientific goals of the proposal are the following. (i) We note that, while there have been severalinteresting experimental breakthroughs in the use of pump-probe technique to study these systems, theoretically this area is currently not as well-developed. One aim will be to advance our theoretical understanding of such pump probe results, and to extract microscopic information such as those related to electron-lattice coupling, and those related to the exotic magnetic C4 phase. This will be achieved through a mix of symmetry-based Landau-Ginzburg approach as well as a more microscopic one where short timedynamics is captured using dynamical Bloch equations. (ii) The second goal is to study how unconventional electron-lattice coupling affects magnetic and nematic properties of these systems, as well as to theoretically understand how such couplings affect superconductivity.The originality of the proposal is its subject matter. There has been very little theoretical work in modeling pump-probe data in this field of research. Similarly, the effect of unusual electron-lattice coupling on the magnetic, nematic and superconducting phases of the FeSC remains largely unexplored theoretically, even though there are ample experimental evidences of such interactions.The project is divided into three tasks and six sub-tasks. Each of the two coordinators is in charge of three sub-tasks, and the tasks will be executed with the participation of two PhD students financed by the project, one each from the French and the German sides. The project requires several types of expertise, and the skills of the two partners are well complimented to meet the requirements. Furthermore, they have proven record of joint research. This ensures good synergy between them.