Origin of spatial inhomogeneity in superfluid stiffness and transition temperature

Determine the physical origin of the micrometer-scale spatial variations in superfluid stiffness and superconducting transition temperature in stoichiometric FeTe thin films, including whether they arise from structural defects, strain, or nonuniform distributions of residual interstitial Fe.

Background

Scanning SQUID measurements reveal nearly twofold spatial variations in the superfluid stiffness and correlated variations in the local superconducting transition temperature over characteristic length scales of approximately 10–20 micrometers. Atomic-force microscopy finds no correlation between the local surface topography and the superfluid stiffness, leaving structural defects, strain, and nonuniform interstitial-Fe distributions as possible explanations.

The microscopic source of this inhomogeneity is unresolved because the observed length scale is substantially larger than the characteristic surface morphology and the measurements do not independently identify the relevant subsurface or through-thickness disorder.

References

The physical origin of the micrometer-scale variations in $K_s$ and $T_c$ is currently unknown. We examined the surface morphology of sample A using atomic force microscopy but found no correlation between the topography and the local $K_s$ (see Supplementary Fig. S5). The characteristic lateral size of the surface features is $0.6 m, much smaller than the length scale discussed above. Other possibilities include structural defects, strain, or nonuniform distribution of residual interstitial Fe both in the lateral directions and through the film thickness. Further studies are required to clarify this aspect.

— Signatures of nodal superconductivity in stoichiometric FeTe  (2609.08116 - Li et al., 8 Sep 2026) in Section 2, paragraph following Fig. 2