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Abstract
A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (
T
c
). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains 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
. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where
T
c
shows a striking enhancement. The completed phase diagram uncovers that high-
T
c
superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas
T
c
remains low near the nematic critical point.