# Maximizing Tc by tuning nematicity and magnetism in FeSe1-xSx superconductors

## TL;DR

This study constructs a detailed phase diagram of FeSe$_{1-x}$S$_x$ superconductors, revealing how tuning nematicity and magnetism affects the superconducting transition temperature, and showing high $T_c$ can be achieved without nematic fluctuations.

## Contribution

It provides a complete three-dimensional phase diagram separating nematic and magnetic phases, uncovering two high-$T_c$ superconducting domes and clarifying the roles of nematicity and magnetism.

## Key findings

- Two superconducting domes with high $T_c$ on both ends of the antiferromagnetic phase.
- High $T_c$ achieved in a nonmagnetic tetragonal phase.
- Nematic fluctuations without magnetism are not essential for high-$T_c$ superconductivity.

## Abstract

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature ($T_{\rm c}$). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving nonmagnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe overlap with each other, the effects of two orders on superconductivity remain perplexing. Here we construct the three-dimensional electronic phase diagram, temperature ($T$) against pressure ($P$) and isovalent S-substitution ($x$), for FeSe$_{1-x}$S$_{x}$, in which we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended nonmagnetic tetragonal phase emerges, where we find a striking enhancement of $T_{\rm c}$. The completed phase diagram uncovers two superconducting domes with similarly high $T_{\rm c}$ on both ends of the dome-shaped antiferromagnetic phase. The $T_{\rm c}(P,x)$ variation implies that nematic fluctuations unless accompanying magnetism are not relevant for high-$T_{\rm c}$ superconductivity in this system.