Interplay between superconductivity and the strange-metal state in FeSe

In contrast to conventional Fermi liquids where resistivity scales quadratically with temperature, unconventional superconductors usually exhibit a normal-state resistivity that varies as a linear function of temperature ( T -linear) in the low-temperature limit. This phenomenon, termed as the strange metal, has been extensively studied in cuprates and is thought to be intimately linked to superconductivity. A quantitative description of its relationship with superconductivity is, therefore, of great importance for developing further theoretical understanding; however, so far, comprehensive studies are scarce due to the difficulties in realizing a systematic control of the superconducting state. Here we report the observation of a typical strange-metal behaviour in FeSe, namely, T -linear resistivity, linear-in-field magnetoresistance and universal scaling of magnetoresistance. More importantly, when we tune the superconductivity by ionic-liquid gating, the superconducting transition temperature increases from approximately 10 to 45 K, and the T -linear resistivity coefficient exhibits a quadratic dependence on the critical temperature. This is a ubiquitous feature that describes the relation between these parameters in various systems including overdoped cuprates and Bechgaard salts. This suggests that there may be a universal mechanism underlying the T -linear resistivity and unconventional superconductivity.