Coexistence and competition of magnetism and superconductivity on the nanometer scale in underdoped BaFe1.89Co0.11As2

TL;DR

This study demonstrates that in underdoped BaFe1.89Co0.11As2, bulk magnetism and superconductivity coexist and compete on the nanometer scale, with magnetic order developing below 32 K and superconductivity below 21.7 K, affecting each other's properties.

Contribution

It provides combined muSR and IR evidence showing coexistence and competition of magnetism and superconductivity at the nanometer scale in an iron-based superconductor.

Findings

Magnetic order develops below 32 K and is reduced but persists below Tc.

Superconductivity is bulk and involves a gapped low-energy state.

Magnetism and superconductivity coexist and influence each other.

Abstract

We report muon spin rotation (muSR) and infrared (IR) spectroscopy experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Our combined data reveal a bulk magnetic order, likely due to an incommensurate spin density wave (SDW), which develops below Tmag \approx 32 K and becomes reduced in magnitude (but not in volume) below Tc = 21.7 K. A slowly fluctuating precursor of the SDW seems to develop alrady below the structural transition at Ts \approx 50 K. The bulk nature of SC is established by the muSR data which show a bulk SC vortex lattice and the IR data which reveal that the majority of low-energy states is gapped and participates in the condensate at T << Tc.