Coexistence of static magnetism and superconductivity in SmFeAsO1−xFx as revealed by muon spin rotation

Abstract

The recent observation of superconductivity with critical temperatures (T c ) up to 55 K in the pnictide RFeAsO 1−x F x , where R is a lanthanide, marks the first discovery of a noncopper-oxide-based layered high-T c superconductor 1-3 . It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprate superconductors, as both families exhibit superconductivity following charge doping of a magnetic parent material. In this context, it is important to follow the evolution of the microscopic magnetic properties of the pnictides with doping and hence to determine whether magnetic correlations coexist with superconductivity. Here, we present a muon spin rotation study on SmFeAsO 1−x F x , with x = 0-0.30 that shows that, as in the cuprates, static magnetism persists well into the superconducting regime. This analogy is quite surprising as the parent compounds of the two families have rather different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the MottHubbard insulator in the cuprates. Our findings therefore suggest that the proximity to magnetic order and associated soft magnetic fluctuations, rather than strong electronic correlations in the vicinity of a Mott-Hubbard transition, may be the key ingredients of high-T c superconductors.Similar to the cuprates, the pnictide high-critical-temperature (T c ) superconductors (HTSCs) have a layered structure comprising alternating FeAs and LaO sheets, with the Fe arranged on a square lattice 1 . Theoretical calculations predict a quasi-twodimensional electronic structure, with LaO layers that mainly act as blocking layers and metallic FeAs layers that are responsible for superconductivity [4][5][6] , although these are multiband superconductors with up to five FeAs-related bands crossing the Fermi level [4][5][6][7] . Like the copper-oxide HTSCs, the superconducting state in the pnictides emerges on charge doping a magnetic parent compound [8][9][10] , with indications that the maximal T c occurs just as magnetism disappears [11][12][13] . The last point may well be of great significance, as the parent compounds in the two families are very different. For the pnictides, there are strong indications that they are itinerant systems with magnetism arising from a nesting-induced spin density wave (SDW). This is in contrast to the cuprates, where it is well established that the mother compounds are 'charge transfer insulators', where strongly repulsive electronic correlations yield an insulating and antiferromagnetic ground state despite a half-filled conduction band. It is therefore of great importance to obtain further insight into the differences and similarities of the pnictide and cuprate HTSCs. A particularly important question is how magnetism and superconductivity evolve on electron doping. In this context, muon spin rotation (μSR) is an ideal technique as it provides microscopic information corresponding to the bulk of a sample and there is a substantial body of μSR data that has been colle...

Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure

“…High temperature superconductivity is found only in the region of the phase diagram where the resistivity shows the pronounced anomalous resistivity above T c . Unlike the case of the arsenides however [3,22,23,28,29], the phase diagram does not have any region where a spin density wave or static magnetism is observed, and the increasing T c under pressure cannot in any way be attributed to the concurrent suppression of a magnetically ordered phase. The increase in T c , is, however, associated with a dramatic decrease in volume due to the collapse of the space between the Fe 2 Se 2 planes.…”

Muon spin rotation study of magnetism and superconductivity in Ba(Fe1−xCox)2Bernhard1, Wang2, Nuccio3 et al. 2012Phys. Rev. BSelf Cite5422871View 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.…”

“…A true coexistence and competition between short-ranged but bulk AF order and superconductivity has been established in underdoped samples of Sm-1111 11,12 and especially in single crystals of Ba(Fe 1−x Co x ) 2 As 2 . [13][14][15][16] Such a coexistence was not observed in La-1111.…”

Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe1−xCoxAs

“…A key question here is whether the magnetic order and superconductivity can coexist microscopically or mutually exclude each other. So far the available experimental methods used to answer this question are either spatially averaged probes or local ones without control of probing position, thus a consensus is still lacking [3][4][5][6][7][8][9] . Experiments on various iron pnictide compounds have revealed at least three categories of behaviour.…”

“…On the contrary, mSR on SmFeAsO 1 À x F x (ref. 5) and BaFe 2 À x Co x As 2 (ref. 8), together with nuclear magnetic resonance 6 and neutron scattering 7 on similar compounds, reveal the coexistence of the two phases.…”