A strongly inhomogeneous superfluid in an iron-based superconductor

Among the mysteries surrounding unconventional superconductors is the possibility of spatial variations in their superfluid density. We use Josephson scanning tunneling microscopy (JSTM) to reveal a strongly inhomogeneous superfluid in an iron-based superconductor. JSTM is based on the phenomenon of Josephson tunneling; Cooper-pairs tunnel through the vacuum barrier between two superconductors. One can extract the superconducting order parameter from the current-voltage spectra. Here we will present our JSTM results on FeTe_0.55Se_0.45 acquired with a Pb-coated tip [1]. By simultaneously measuring the topographic and electronic properties, we find that the inhomogeneity in the superfluid density is not caused by structural disorder or inter-band scattering, and is not correlated with the variations in the pair-breaking gap. Importantly, we reveal a spatial correlation between the inhomogeneous order parameter and the quasiparticle strength defined as the height of the coherence peak. This relation has been observed in copper oxide superconductors on the macroscopic scale. When repeated at different temperatures, our technique could help to elucidate what mechanisms limit the critical temperature in unconventional superconductors.
[1] Cho, D. et al. Nature 571, 541–545 (2019)