Correlation between unconventional superconductivity and strange metallicity revealed by operando superfluid density measurements

Strange-metal behavior has been observed in superconductors ranging from cuprates to pressurized nickelates, but its relationship to unconventional superconductivity remains elusive. Here, we perform operando superfluid density measurements on ion-gated FeSe films. We observe a synchronized evolution of the superconducting condensate and the strange-metal phase with electron doping, from which a linear scaling between zero-temperature superfluid density and strange-metal resistivity coefficient is further established. The scaling also applies to different iron-based and cuprate superconductors despite their distinct electronic structures and pairing symmetries. Such a correlation can be reproduced in a theoretical calculation on the two-dimensional Yukawa-Sachdev-Ye-Kitaev model by considering a cooperative effect of quantum critical fluctuation and disorder. These findings suggest that a common mechanism may govern both the Cooper pair condensation and the normal-state strange metallicity in unconventional superconductors.