Filamentary superconductivity in antiferromagnetic iron arsenides

The interplay among competing ground states of correlated electron systems can give rise to a rich spectrum of emergent behavior. The iron-based superconductors are particularly noteworthy, and have attracted extensive interest since the discovery of La[O1−xFx]FeAs in 2008. Like the high temperature superconducting cuprates, superconductivity (SC) in the iron arsenides emerges from an antiferromagnetic (AFM) parent state upon doping with excess charge carriers, and the superconducting pairing mechanism may be related to the AFM instability of the parent state. In both cases the electronic degrees of freedom condense into an unusual coexistence of both AFM and SC order parameters for intermediate dopings. The nature of this coexistence is poorly understood.

In general, a subdominant order parameter can emerge locally in regions where the dominant order vanishes or is suppressed. Recent experiments in the iron arsenides suggest that SC and AFM order parameters are indeed spatially modulated on a microscopic scale. Among the iron based superconductors, the AFe2As2 (A = Ca, Sr and Ba) materials are of particular interest because large single crystals of these oxygen-free compounds can be easily synthesized. These materials undergo a tetragonal to orthorhombic transition followed by an AFM state upon cooling. Both chemical doping and applied pressure suppress the magnetostructural order of the parent compounds and give rise to SC, and the phase diagrams are similar to those of other unconventional superconductors.

The CaFe2As2 system is noteworthy because SC is induced at only 0.4 GPa (non-hydrostatic), whereas the Sr and Ba materials require 2.8 and 2.5 GPa, respectively. Recently we reported  evidence  for coexisting filamentary SC and AFM in the undoped parent compound CaFe2As2, in which the SC order remains localized within AFM domain walls (DWs). A similar phenomenon has been observed in heavy fermion materials, and enhanced superfluid density has been observed at twin boundaries in Ba(Fe1−xCox)2As2. CaFe2As2 is noteworthy, however, because the SC never achieves bulk long-range order, and surprisingly the filamentary SC appears to be related to the presence of low frequency spin fluctuations.