Title: Protonation induced high-Tc phases in iron-based superconductors evidenced by NMR and magnetization measurements,
Abstract: Chemical substitution during growth is a well-established method to
manipulate electronic states of quantum materials, and leads to rich spectra of
phase diagrams in cuprate and iron-based superconductors. Here we report a
novel and generic strategy to achieve nonvolatile electron doping in series of
(i.e. 11 and 122 structures) Fe-based superconductors by ionic liquid gating
induced protonation at room temperature. Accumulation of protons in bulk
compounds induces superconductivity in the parent compounds, and enhances the
Tc largely in some superconducting ones. Furthermore, the existence of proton
in the lattice enables the first proton nuclear magnetic resonance (NMR) study
to probe directly superconductivity. Using FeS as a model system, our NMR study
reveals an emergent high-Tc phase with no coherence peak which is hard to
measure by NMR with other isotopes. This novel electric-field-induced proton
evolution opens up an avenue for manipulation of competing electronic states
(e.g. Mott insulators), and may provide an innovative way for a broad
perspective of NMR measurements with greatly enhanced detecting resolution.