Insular superconductivity in Co-doped iron pnictide CaFe$_{1-x}$Co$_x$AsF

TL;DR

This study reveals phase separation and superconducting islands in Co-doped CaFeAsF, showing that superconductivity occurs in localized domains around Co ions, with a nearly proportional volume fraction to Co content.

Contribution

It provides evidence for insular superconductivity in CaFe$_{1-x}$Co$_x$AsF, highlighting phase separation and the formation of superconducting islands around Co ions.

Findings

Macroscopic phase separation between magnetic and superconducting phases.

Superconducting volume fraction proportional to Co content.

Superconducting islands indicate very short coherence length.

Abstract

The presence of macroscopic phase separation between the superconducting and magnetic phases in \cfcaf is demonstrated by muon spin rotation (muSR) measurements conducted across their phase boundaries (x=0.05-0.15). The magnetic phase tends to retain the high transition temperature (T_m > T_c), while Co-doping induces strong randomness. The volumetric fraction of superconducting phase is nearly proportional to the Co content $x$ with constant superfluid density. These observations suggest the formation of superconducting "islands" (or domains) associated with Co ions in the Fe$_2$As$_2$ layers, indicating a very short coherence length.