The mystery of high-temperature superconductivity has puzzled physicists for decades. In high-temperature superconductors, electrons form Cooper pairs and flow without resistance below a critical temperature T_c, while their behavior above T_c is even more enigmatic. Particularly, many unconventional superconductors exhibit “strange metal” behavior in the normal state, with electrical resistance increasing linearly with temperature, contrary to conventional theories. Understanding the connection between this strange metallicity and unconventional superconductivity is key to unlocking the mechanism of high-temperature superconductivity.

The group led by JIN Kui at the Institute of Physics, Chinese Academy of Sciences, had already laid a crucial foundation in previous work. Using high-throughput experimental techniques, the team discovered a universal scaling law between T_c and the strange-metal resistivity coefficient A₁ in both cuprate and iron-based superconductors, showing that T_c ∝ (A₁)^0.5. These pioneering findings provided direct evidence that the scattering mechanism in strange metals is fundamentally connected to electron pairing in high-temperature superconductors.

Based on these discoveries, JIN’s team has achieved another breakthrough. In their latest study published in Science Advances, they further revealed a linear relationship between superfluid density (a key parameter characterizing the superconducting state) and the strange-metal resistivity coefficient. This finding directly couples Cooper pair condensation to normal-state electron scattering, revealing a deeper connection between these two phenomena. It unambiguously demonstrates that the same interaction that underlies the strange-metal behavior gives rise to the electron pairing and condensation in high-temperature superconductors.

The research team overcame substantial technical challenges by developing a novel experimental approach that integrates ionic liquid gating with operando two-coil mutual inductance measurements. This innovative setup allowed them to precisely tune the superconducting properties of FeSe films while simultaneously measuring changes in superfluid density with high accuracy. With this capability, they systematically varied T_c from 11 K to 43 K in a single FeSe sample, observing a clear power-law relationship between T_c and zero-temperature superfluid density ρ_s0: T_c ∝$（{\rho }_{so}{）}^{0.55\pm 0.11}$.

Most remarkably, a linear scaling between ρ_s0 and A₁ emerged, derived from normal-state electrical transport data. This relationship holds true for both iron-based and cuprate superconductors, despite their different electronic structures and pairing symmetries. Theoretical collaboration with SACHDEV Subir’s group at Harvard University showed that this universal behavior could be quantitatively reproduced by the two-dimensional Yukawa-Sachdev-Ye-Kitaev model, which accounts for the cooperative effects of quantum critical fluctuations and disorder. These quantitative relationships suggest a common mechanism may govern both strange metallicity and unconventional superconductivity, representing a significant advance in understanding the mysterious mechanism behind high-temperature superconductivity.

This study, entitled “Correlation between unconventional superconductivity and strange metallicity revealed by operando superfluid density measurements” was published in Science Advances.

The study was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, the CAS Project for Young Scientists in Basic Research, the Key-Area Research and Development Program of Guangdong Province, and the Beijing Nova Program of Science and Technology.