Crystal structure and phase transitions across the metal-superconductor boundary in the SmFeAsO1−xFx (0≤x≤0.20) family

Abstract

The fluorine-doped rare-earth iron oxyarsenides REFeAsO1−xFx (RE=rare earth) have recently emerged as a new family of high-temperature superconductors with transition temperatures (Tc) as high as 55 K. Here we use high-resolution synchrotron x-ray diffraction to study the structural properties of SmFeAsO1−xFx (0≤x≤0.20) in which superconductivity emerges near x∼0.07 and Tc increases monotonically with doping up to x∼0.20. We find that orthorhombic symmetry survives through the metal-superconductor boundary well into the superconducting regime and the structural distortion is only suppressed at doping levels, x≥0.15, when the superconducting phase becomes metrically tetragonal. Remarkably this crystal symmetry crossover coincides with reported drastic anomalies in the resistivity and the Hall coefficient, and a switch of the pressure coefficient of Tc from positive to negative, thereby implying that the low-temperature structure plays a key role in defining the electronic properties of these superconductors.

Source-reported materials — not catalogue approval

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