Discrete superconducting phases in FeSe-derived superconductors

TL;DR

This paper reveals a series of discrete superconducting phases in FeSe-derived materials, mapped through carrier tuning, highlighting the importance of the iron site and providing a comprehensive phase diagram.

Contribution

It presents the first detailed phase diagram of FeSe-derived superconductors showing discrete phases, using ionic gating to tune carrier concentration.

Findings

Discrete superconducting phases are robust against Se substitution by S.

These phases are sensitive to Fe substitution by Cu.

A complete phase diagram for FeSe derivatives is established.

Abstract

A general feature of unconventional superconductors is the existence of a superconducting dome in the phase diagram as a function of carrier concentration. For the simplest iron-based superconductor FeSe (with transition temperature Tc ~ 8 K), its Tc can be greatly enhanced by doping electrons via many routes, even up to 65 K in monolayer FeSe/SiTiO3. However, a clear phase diagram with carrier concentration for FeSe-derived superconductors is still lacking. Here, we report the observation of a series of discrete superconducting phases in FeSe thin flakes by continuously tuning carrier concentration through the intercalation of Li and Na ions with a solid ionic gating technique. Such discrete superconducting phases are robust against the substitution of Se by 20% S, but are vulnerable to the substitution of Fe by 2% Cu, highlighting the importance of the iron site being intact. A complete superconducting phase diagram for FeSe-derivatives is given, which is distinct from other unconventional superconductors.