Spin liquid polymorphism in a correlated electron system on the threshold of superconductivity

TL;DR

This study reveals spin-liquid polymorphism in a FeTe-based superconductor, showing a temperature-dependent competition between two magnetic phases, which suggests a liquid-liquid phase transition relevant to understanding unconventional superconductivity.

Contribution

It uncovers the existence of two competing spin liquid phases in a correlated electron system near superconductivity, demonstrating a first-order liquid-liquid phase transition.

Findings

Identification of ferromagnetic and antiferromagnetic plaquette phases

Observation of temperature-dependent phase competition

Evidence for a liquid-liquid phase transition in the spin system

Abstract

We report neutron scattering measurements, which reveal spin-liquid polymorphism in a '11' iron chalcogenide superconductor, a poorly-metallic magnetic FeTe tuned towards superconductivity by substitution of a small amount of Tellurium with iso-electronic Sulphur. We observe liquid-like magnetic dynamics, which is described by a competition of two phases with different local structure, whose relative abundance depends on temperature. One is the ferromagnetic (FM) plaquette phase observed in the non-superconducting FeTe, which preserves the C$_4$ symmetry of the underlying square lattice and is favored at high temperatures. The other is the antiferromagnetic plaquette phase with broken C$_4$ symmetry, which emerges with doping and is predominant at low temperatures. These findings suggest a first-order liquid-liquid phase transition in the electronic spin system of FeTe$_{1-x}$(S,Se)$_x$. We thus discover remarkable new physics of competing spin liquid polymorphs in a correlated electron system approaching superconductivity. Our results facilitate an understanding of large swaths of recent experimental data in unconventional superconductors.