Phase separation at the magnetic-superconducting transition in La${}_{0.7}$Y${}_{0.3}$FeAsO${}_{1-x}$F${}_x$

Abstract

In this paper we report a detailed ${\mu }^{+}$SR and {}${}^{19}$F-NMR study of the La${}_{0.7}$Y${}_{0.3}$FeAsO${}_{1-x}$F${}_x$ class of materials. Here, the diamagnetic La${}_{1-y}$Y${}_y$ substitution increases chemical pressure and, accordingly, sizeably enhances the optimal superconducting transition temperature. We investigate the magnetic-superconducting phase transition by keeping the Y content constant ($y=0.3$) and by varying the F content in the range $0.025\le x\le 0.15$. Our results show how magnetism and superconductivity coexist for $x=0.065$. Such coexistence is due to segregation of the two phases in macroscopic regions, resembling what was observed in LaFeAsO${}_{1-x}$F${}_x$ materials under applied hydrostatic pressure. This scenario is qualitatively different from the nanoscopic coexistence of the two order parameters observed when La is fully substituted by magnetic rare-earth ions like Sm or Ce.

arXiv commentProceedings of QCNP12 Conference

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• The onset of magnetism peaked around x=1/4 in optimally electron-doped LnFe(1-x)Ru(x)AsO(1-y)F(y) (Ln = La, Nd or Sm) superconductors
• Separation of antiferromagnetism and high-temperature superconductivity in Ca${}_{1-x}$La${}_x$Fe${}_2$As${}_2$ under pressure
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