Theory of Magnetic Order in $ Fe_{1+y}Te_{1-x}Se_x$

TL;DR

This paper introduces a local spin model to explain the complex magnetic phases in Fe-based superconductors, capturing transitions between magnetic orders and predicting quantum critical behavior influenced by composition.

Contribution

The study presents a novel local spin model that accounts for both commensurate and incommensurate magnetic orders and predicts a quantum critical region in Fe(Te,Se) superconductors.

Findings

Model reproduces magnetic order transitions

Calculates experimentally measurable spin-wave features

Predicts a large quantum critical region

Abstract

We develop a local spin model to explain the rich magnetic structures in the iron-based superconductors $Fe_{1+y}Te_{1-x}Se_x$. We show that our model exhibits both commensurate antiferromagnetic and incommensurate magnetic order along the crystal a-axis. The transition from the commensurate to the incommensurate phase is induced when the concentration of excess $Fe$ atoms is larger than a critical value. Experimentally measurable spin-wave features are calculated, and the mean-field phase diagram of the model is obtained. Our model also suggests the existence of a large quantum critical region due to strong spin frustration upon increasing $Se$ concentration.