Nanoscopic coexistence of magnetic and superconducting states within the FeAs layers ofCeFeAsO1−xFx

Abstract

We report on the coexistence of magnetic and superconducting states in CeFeAsO 1−x F x for x = 0.06͑2͒, characterized by transition temperatures T m = 30 K and T c = 18 K, respectively. Zero-field and transverse-field muon-spin-relaxation measurements show that below 10 K the two phases coexist within a nanoscopic scale over a large volume fraction. This result clarifies the nature of the magnetic-to-superconducting transition in the CeFeAsO 1−x F x phase diagram, by ruling out the presence of a quantum critical point which was suggested by earlier studies.The recent discovery of high-T c superconductivity ͑SC͒ close to the disruption of magnetic ͑M͒ order in Fe-based compounds has stimulated the scientific community to further consider the role of magnetic excitations in the pairing mechanism. In order to address this point it is necessary to understand how the ground state evolves from the M to the SC phase within each family of Fe-based superconductors. In the REFeAsO 1−x F x family ͑hereafter RE1111, with RE =La or a rare earth͒ early experiments have suggested that the M-SC crossover is RE dependent. For instance, a smooth reduction in the magnetic and superconducting ordering temperatures, T m and T c , respectively, was found for RE = Ce, 1 suggesting the presence of a quantum critical point. 2 For RE = Sm a partial coexistence of the M and SC states was found 3 while a first-order transition seems to occur for RE = La. 4 Successive studies 5-7 have shown that the doping region where T m and T c are both nonzero is virtually pointlike in Sm1111, demonstrating that the cases of RE = La and Sm can be reconciled under a unique behavior. 5 Recently, nanoscale electronic inhomogeneities have been shown to be present in both RE = La and Sm in a wide range above the crossover region. 8 Actually also the case of RE = Ce is susceptible to further investigation concerning the presence of electronic inhomogeneities in the superconducting dome or even the possible microscopic coexistence of magnetic ordering and superconductivity in the FeAs layers, 2 which might have eluded previous neutron-diffraction studies. 1 In fact, contrary to diffraction techniques, which cannot detect short-range magnetic order, muons act as local magnetic probes, hence making muon spectroscopy ͑SR͒ an ideal tool for this sort of investigations. For this reason SR has long been employed to study the M-SC coexistence in cuprates 9-13 as well as in other superconducting compounds, such as the ruthenocuprates, 14 or the heavy-fermion superconductors. [15][16][17] Here we report on zero-field ͑ZF͒-and transverse-field ͑TF͒-SR measurements on a sample of CeFeAsO 1−x F x which unambiguously show the coexistence of superconductivity and short-range magnetic order on a nanoscopic length scale. While in contradiction with previous experimental findings on the same compound, 1 this result closely resembles the behavior of Sm1111 at the M-SC crossover. 3,5-7

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

“…2). This is a typical Fe dipolar field value at the muon site in F-doped 1111 close to a M-SC crossover [3,4]. The transverse component is overdamped down to 1.5 K (no asymmetry oscillations in Fig.…”

“…2, inset) and its fast relaxation is partially quenched in fields of order B i ≈ 50 mT applied along the initial muon-spin direction (not shown), as expected for a static B i . Therefore the overdamped muon-spin precessions are due to inhomogeneous short-range order [4]. The fraction of the sample volume where muons experience a net field B i is calculated as [4].…”

“…Therefore the overdamped muon-spin precessions are due to inhomogeneous short-range order [4]. The fraction of the sample volume where muons experience a net field B i is calculated as [4]. Its dependence on temperature is shown in Fig.…”

A view from inside iron-based superconductors

“…Still, µ + are able to feel the dipolar field generated by the uncompensated magnetic moments on the domain walls (see red arrows in the figure under the magnifying lens). In the case of the dipolar field generated by Fe in the case of 1111 oxy-pnictide materials, one can roughly deduce that the minimal distance required in order to probe magnetism under these conditions for µ + implanted out of the domains is of the order of 1 nm [94]. In the case of the mesoscopic segregation of the order parameters, µ + implanted out of the magnetic domains (and conventionally labelled as µ + 2 ) are on the average too far away from the domains themselves to probe static magnetism and only µ + implanted into the domains (and conventionally labelled as µ + 1 ) give rise to a magnetic signal.…”

s-wave pairing in the optimally doped LaO0.5F0.5BiSLamura1, Shiroka2, Bonfà3 et al. 2013Phys. Rev. BSelf Cite

“…Magnetism coexisting with superconductivity is observed in underdoped pnictides, 12,16,17 but magnetic fields may also appear in superconductors with spin-triplet pairing, as observed in Sr 2 RuO 4 . 18,19 In Fig.…”