Correlated Trends of Coexisting Magnetism and Superconductivity in Optimally Electron-Doped Oxypnictides

Abstract

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe1−xRuxAsO0.85F0.15 for 0.1 ≤ x 0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and finally disappear around a common critical threshold xc ∼ 0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold xc.The appearance of high-T c superconductivity (SC) close to the disruption of static magnetic (M) order is a general feature of the Fe-based superconductors either as a function of doping or external pressure. In the REFeAsO family (RE1111) it is found that SC and M strongly compete and hardly coexist simultaneously [1, 2], apart for RE=Sm and Ce [3,4] within a small doping range where both order parameters are depressed. Coexistence implies short range magnetic order, that is detected only by local probes such as muon-spin rotation (µSR) [4] or nuclear quadrupole resonance (NQR) [5], since it eludes long coherence probes such as powder diffraction [6]. The competition between the superconducting and magnetic ground-states must be reconciled with the prevailing models of pairing mediated by spin fluctuations [7]. These models are seemingly in contradiction with the evidence that the two mutually excluding orders coexist only when phase separation occurs.Here we show, by means of µSR and 75 As NQR, that magnetism is surprisingly still at play in optimally F-doped SmFe 1−x Ru x AsO 0.85 F 0.15 . The isoelectronic Fe:Ru substitution is found to deteriorate the superconducting state in optimally electron-doped SmFeAsO 0.85 -F 0.15 samples and simultaneously to recover static magnetism within the FeAs layers, for 0.1 ≤ x 0.5. This is accompanied by a local electronic rearrangement within the FeAs layers. When Ru doping approaches the critical threshold x c = 0.6, corresponding to percolation of a magnetic square lattice with nearest neighbor (n.n.) and next-nearest neighbor hopping, both magnetism and SC vanish.The investigated polycrystalline SmFe 1−x Ru x AsO 0.85 -F 0.15 samples are the same of Ref. 8. From 19 F nuclear magnetic resonance the relative fluorine content was found to be constant within ∆ 0.01 in the whole set of samples investigated. To investigate the bulk character of the superconducting state we carried out transverse field (TF)-µSR measurements, where a sample is fieldcooled (FC) in a magnetic field larger than the lower superconducting critical field H c1 , applied perpendicular to the initial muon-spin orientation (H ⊥ S µ ). A flux-line lattice (FLL) i...

Unconventional Disorder Effects in Correlated Superconductors

“…We now turn to the discussion of non-magnetic disorder, and again motivate the study by a set of puzzling experimental findings from FeSCs summarized in Figs. 1(c,d), which compare the effect on T c and T m of Ru ions substituting for Fe in OD La-1111 and Sm-1111 [13,14,28]. Ru is isovalent to Fe, and therefore expected to be a source of weak disorder, consistent with the huge amount of ∼ 60% of Ru required to suppress T c , as seen in Fig.…”

“…Specifically, T m is defined identically to the experimental µSR definition by the highest T exhibiting a 50% magnetic volume fraction. A site is defined to contribute to the volume fraction if its internal dipolar local field exceeds |0.5|mT [13,14]. In the case of 15% disorder discussed in Fig.…”

“…[13,14] The resulting complex inhomogeneous phases and their properties in terms of thermodynamics and transport constitute an important open problem in the field.…”

“…[6] Scanning tunneling spectroscopy found a plethora of exotic atomicsized impurity-generated states, [7][8][9][10] NMR and neutrons observed clear evidence of glassy magnetic behavior, [11,12] and µSR discovered magnetic phases generated by non-magnetic disorder. [13,14] The resulting complex inhomogeneous phases and their properties in terms of thermodynamics and transport constitute an important open problem in the field.…”

Muon spin rotation study of magnetism and superconductivity in Ba(Fe1−xCox)2Bernhard1, Wang2, Nuccio3 et al. 2012Phys. Rev. B5422871View full textAdd to dashboardCite

“…It has previously been very successfully applied to study the coexistence of magnetism and superconductivity in a variety of unconventional superconductors such as the underdoped cuprates, [37][38][39][40] the ruthenate cuprates, 41 the triplet superconductor Sr 2 RuO 4 (Ref. 42), or more recently the iron arsenides 11,12,15,17,19,20,[43][44][45][46][47] and iron selenides. [48][49][50] The neutron diffraction experiments were performed at the thermal four-circle single-crystal diffractometer TRICS at the spallation neutron source SINQ at PSI.…”