Homogeneous vs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co and K doped iron pnictides

TL;DR

This study uses NMR to compare how magnetic order and superconductivity coexist in Co and K doped iron pnictides, revealing homogeneous coexistence in one compound and phase separation in another.

Contribution

It provides the first microscopic NMR evidence distinguishing homogeneous from inhomogeneous coexistence of magnetic order and superconductivity in doped iron pnictides.

Findings

Homogeneous coexistence of magnetic order and superconductivity in Ba(Fe0.95Co0.05)2As2.

Phase separation of magnetic and superconducting regions in Ba0.6K0.4Fe2As2.

Normal state exhibits spatial electronic and magnetic inhomogeneity.

Abstract

In Ba(Fe0.95Co0.05)2As2 all of the 75As NMR intensity at the paramagnetic resonance position vanishes abruptly below Tonset(SDW)=56 K, indicating that magnetic (spin density wave) order is present in all of the sample volume, despite bulk superconductivity below Tc=15 K. The two phases thus coexist homogeneously at the microscopic scale. In Ba0.6K0.4Fe2As2, on the other hand, the signal loss below Tonset(SDW)~75 K is not complete, revealing that magnetic order is bound to finite-size areas of the sample, while the remaining NMR signal shows a clear superconducting response below Tc=37 K. Thus, the two phases are not homogeneously mixed, at least for this potassium concentration. For both samples, spatial electronic and/or magnetic inhomogeneity is shown to characterize the NMR properties in the normal state.