A strongly inhomogeneous superfluid in an iron-based superconductor

TL;DR

This study reveals a highly inhomogeneous superfluid density in an iron-based superconductor using atomic-resolution Josephson STM, showing local quasiparticle strength correlates with superfluid density, independent of structural disorder.

Contribution

It demonstrates that superfluid in FeTe0.55Se0.45 varies spatially and correlates with quasiparticle properties, challenging previous assumptions about disorder effects.

Findings

Superfluid density is strongly inhomogeneous.

Inhomogeneity is not due to structural disorder.

Superfluid strength correlates with quasiparticle sharpness.

Abstract

Among the mysteries surrounding unconventional, strongly correlated superconductors is the possibility of spatial variations in their superfluid density. We use atomic-resolution Josephson scanning tunneling microscopy to reveal a strongly inhomogeneous superfluid in the iron-based superconductor FeTe0.55Se0.45. By simultaneously measuring the topographic and electronic properties, we find that this inhomogeneity in the superfluid density is not caused by structural disorder or strong inter-pocket scattering, and does not correlate with variations in Cooper pair-breaking gap. Instead, we see a clear spatial correlation between superfluid density and quasiparticle strength, putting the iron-based superconductors on equal footing with the cuprates and demonstrating that locally, the quasiparticles are sharpest when the superconductivity is strongest. When repeated at different temperatures, our technique could further help elucidate what local and global mechanisms limit the critical temperature in unconventional superconductors.