Phase separation at the magnetic–superconducting transition in La_0.7Y_0.3FeAsO_1−xF_x

Abstract

In this paper we report a detailed µ + 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 ≤ x ≤ 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.

Onset of magnetism in optimally electron-dopedLFe1−xRuxAsOSanna1, Carretta2, Renzi3 et al. 2013Phys. Rev. B

“…27), but notably not in La1111. In the latter the two order parameters are mutually exclusive at ambient pressure 4 and are observed to coexist in mesoscopically separated regions under both external 10 and chemical 31 pressures. Here, the detection of nanoscopic coexistence not only in Sm11Ru11 and Nd11Ru11, but also in La11Ru11, suggests that the substitution for Fe of the isovalent nonmagnetic Ru induces a static SR magnetic order.…”

Role of in-plane and out-of-plane dilution in CeFeAsO: Charge doping versus disorder

“…39,40 This scenario is completely different from what is reported for La-based 1111 materials. 2,41,42 It should be stressed that such nanoscopic coexistence is detected in CeFeAsO 1−x F x and CeFe 1−x Co x AsO only when the magnetic phase is short ranged. Remarkably, this is qualitatively different from what is obtained in the electron-doped 122 materials such as Ba(Fe 1−x Co x ) 2 As 2 , where the x range of coexistence is typically much wider, 11,43 superconductivity being reported to emerge still in the presence of clear coherent oscillations for the SDW phase.…”

Common effect of chemical and external pressures on the magnetic properties ofRCoPO (R=La,Pr,Nd,Sm).

“…While it is excluded that this enhancement is associated with 4f orbitals and with the localized magnetic moment at the R site, 20 no theory can still fully account for the experimental observations. The structural origin (i. e., chemical pressure) seems to be a more adequate explanation for the observed effects, 21 however in this case it would be naively expected that external pressure in optimally-doped systems could enhance T c as well, which is not the case experimentally. 22,23 Chemical dilutions and quenched disorder are expected indeed to crucially drive the superconducting properties as well, 24 invalidating this way a full analogy between the effects of external and chemical pressures on T c .…”