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

TL;DR

This study investigates the structural phase transitions in SmFeAsO1-xFx superconductors, revealing that orthorhombic symmetry persists into the superconducting phase and only transitions to tetragonal symmetry at higher doping levels, influencing electronic properties.

Contribution

It provides detailed high-resolution synchrotron X-ray diffraction data showing the persistence of orthorhombic symmetry into the superconducting regime, highlighting the crystal structure's role in electronic behavior.

Findings

Orthorhombic symmetry survives into the superconducting phase.

Structural distortion is suppressed at doping levels x > 0.15.

Structural crossover correlates with anomalies in resistivity and Hall coefficient.

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 (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.