Phase transition beneath the superconducting dome in BaFe[subscript 2](As[subscript 1-x]P[subscript x])[subscript 2]

Abstract

We present a theory for the large suppression of the superfluid density ρ[subscript s] in BaFe[subscript 2](As[subscript 1−x]P[subscript x])[subscript 2] in the vicinity of a putative spin-density wave quantum critical point at a P doping, x = x[subscript c]. We argue that the transition becomes weakly first order in the vicinity of x[subscript c], and disorder induces puddles of superconducting and antiferromagnetic regions at short length scales; thus, the system becomes an electronic microemulsion. We propose that frustrated Josephson couplings between the superconducting grains suppress ρ[subscript s]. In addition, the presence of “normal” quasiparticles at the interface of the frustrated Josephson junctions will give rise to a highly nontrivial feature in the low-frequency response in a narrow vicinity around x[subscript c]. We propose a number of experiments to test our theory.