Protonation-induced discrete superconducting phases in bulk FeSe single crystals

TL;DR

This study reveals discrete superconducting phases with up to 44 K in H$^+$-intercalated FeSe crystals, linked to Fermi surface topology changes and nematic phase suppression, advancing understanding of high-$T_c$ mechanisms.

Contribution

It uncovers discrete superconducting phases in FeSe with high $T_c$, associated with Fermi surface topology changes, using ionic liquid gating.

Findings

Maximum $T_c$ of 44 K observed.

Suppression of nematic phase with increasing $T_c$.

Abrupt Fermi surface topology change linked to discrete phases.

Abstract

The superconducting transition temperature, $T_{\rm{c}}$, 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-$T_{\rm{c}}$ superconductivity. Investigation on the evolution of the electronic structure with $T_{\rm{c}}$ is essential to understanding electronic behavior and high-$T_{\rm{c}}$ superconductivity in FeSe and its derived superconductors. In this report, we have found a series of discrete superconducting phases, with a maximum $T_{\rm{c}}$ up to 44 K, in H$^+$-intercalated FeSe single crystals using an ionic liquid gating method. Accompanied with the increase of $T_{\rm{c}}$, 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-$T_{\rm{c}}$ superconducting phase was also revealed.