Protonation-induced discrete superconducting phases in bulk FeSe single crystals

The superconducting transition temperature, T-c, of FeSe can be significantly enhanced several-fold by applying pressure, electron doping, intercalating spacing layer, and reducing dimensionality. Various ordered electronic phases, such as nematicity and spin density waves, have also been observed accompanying high-T-c superconductivity. In this paper, we have found a series of discrete superconducting phases, with a maximum T-c up to 44 K, in H+-intercalated FeSe single crystals using an ionic liquid gating method. Accompanied with the increase of T-c, suppression of the nematic phase and evolution from non-Fermi-liquid to Fermi-liquid behavior was observed. An abrupt change in the Fermi surface topology was proposed to explain the discrete superconducting phases. A band structure that favors the high-T-c superconducting phase was also revealed.