Abstract:We present the results of Synchrotron XRD measurements on powdered single crystal samples of BaFe2-xRuxAs2, as a function of Ru content, and as a function of temperature, across the spin density wave transition in BaFe1.9Ru0.1As2. The Rietveld refinements reveal that with Ru substitution, while the a-axis increases, the c-axis decreases. In addition the variation of positional co-ordinates of As (zAs), the Fe-As bond length and the As-Fe-As bond angles have also been determined. In the sample with x=0.1, temperature dependent XRD measurements, indicate that the orthorhombicity shows the characteristic increase with decrease in temperature, below the magnetic transition. It is seen that the c-axis, the As-FeAs bond angles, Fe-As bond length and positional co-ordinate of the As show definite anomalies close to the structural transition. The observed anomalies in structural parameters are analysed in conjunction with geometric optimization of the structure using ab-initio electronic structure calculations.

Using the experimentally measured temperature and doping dependent structural parameters on Ru doped BaFe2As2, orbital-dependent reconstruction of the electronic structure across the magnetostructural transition is found, through first principle simulations. Below structural transition there exists two distinct Fe -Fe bond distances which modifies the Fe-dxy orbital largely due to its planar spatial extension leading to Lifshitz transition, while the otherwise degenerate Fe-dxz and dyz orbitals become non-degenerate, giving rise to orbital order. The orbital order follows the temperature dependence of orthorhombocity and is also the cause of two distinct Fe -Fe bond distances. Doping dependent Fermi surfaces show nearly equal expansion of both the electron and hole like Fermi surfaces whereas the hole Fermi surface shrinks with temperature but the electron Fermi surface expands comparatively slowly. The observed structural transition in this compound is electronic in origin, occurs close to the Lifshitz transition whereas the suppression of the concurrent magnetic transition is due to loss of temperature dependent nesting of Fermi surface.

Fermiology of various 122 systems are studied through first principles simulation. Electron doping causes expansion of electron and shrinkage of hole Fermi pockets. Isovalent Ru substitution (up to 35%) makes no visible modification in the electron-and hole-like Fermi surfaces (FSs) providing no clue regarding the nature of charge carrier doping. However, in case of 32% P doping there are considerable changes in the hole FSs. From our calculations, it is very clear that two-dimensionality of FSs may favour electron pair scattering between quasi-nested FSs which has important bearings in various orders (magnetic, orbital, superconducting) present in Fe-based superconductors.

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