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

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

2008-06-24

arxiv.org/abs/0806.3962v1

Phys. Rev. B 79, 014503 (2009)

Physics

Condensed Matter

Superconductivity

Scientific paper

10.1103/PhysRevB.79.014503

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 (refs 1-4). Early work has provided compelling evidence that the undoped parent materials exhibit spin-density-wave (SDW) antiferromagnetic order and undergo a structural phase transition from tetragonal to orthorhombic crystal symmetry upon cooling.5 Both the magnetic and structural instabilities are suppressed upon doping with fluoride ions before the appearance of superconductivity.6,7 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 ~ 0.20.8 We find that orthorhombic symmetry survives through the metal-superconductor boundary well into the superconducting regime 2 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 coefficient8 and a switch of the pressure coefficient of Tc from positive to negative,9 thereby implying that the low-temperature structure plays a key role in defining the electronic properties of these superconductors.

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