Phase transitions in a frustrated biquadratic Heisenberg model with coupled orbital degree of freedom for iron-based superconductors

TL;DR

This study uses Monte Carlo simulations to explore phase transitions in a frustrated biquadratic Heisenberg model with coupled orbital degrees of freedom, shedding light on nematicity and phase separation in iron-based superconductors.

Contribution

It introduces a model incorporating orbital interactions to explain simultaneous and separated phase transitions observed experimentally in iron pnictides.

Findings

Orbital and nematic transitions occur at the same temperature.

Orbital interaction can cause separation of structural and magnetic transitions.

Antiferro-quadrupolar state is stabilized by anisotropic biquadratic interaction.

Abstract

In this work, we study a biquadratic Heisenberg model with coupled orbital degree of freedom using Monte Carlo simulation in order to investigate the phase transitions in iron-based superconductors. The antiferro-quadrupolar state, which may be related to the magnetism of FeSe [Phys. Rev. Lett. 115, 116401 (2015)], is stabilized by the anisotropic biquadratic interaction induced by a ferro-orbital-ordered state. It is revealed that the orbital and nematic transitions occur at the same temperature for all the cases, supporting the mechanism of the orbital-driven nematicity as revealed in most recent experiments [Nat. Mater. 14, 210 (2015)]. In addition, it is suggested that the orbital interaction may lead to the separation of the structural and magnetic phase transitions as observed in many families of iron pnictides.