Global Phase Diagram for Magnetism and Lattice Distortion of Fe-pnictide Materials

TL;DR

This paper presents a comprehensive phase diagram for magnetism and lattice distortions in Fe-pnictide materials, revealing different transition behaviors under doping and pressure, and highlighting the role of strain fluctuations.

Contribution

It introduces a unified theoretical framework describing magnetic and structural phase transitions in Fe-pnictides, including the effects of doping, pressure, and strain fluctuations.

Findings

Single transition at low doping and high pressure with weak fluctuations.

Split transitions at high doping and low pressure, including a quantum Ising transition.

Strain fluctuations influence the nematic transition, aligning with experimental observations.

Abstract

We study the global phase diagram of magnetic orders and lattice structure in the Fe-pnictide materials at zero temperature within one unified theory, tuned by both doping and pressure. On the low doping and high pressure side of the phase diagram, there is one single transition, which is described by a $z=2$ mean field theory with very weak run-away flows; on the high doping and low pressure side the transition is expected to split to two transitions, with one O(3) spin density wave transition followed by a z = 3 quantum Ising transition at larger doping. The fluctuation of the strain field fluctuation of the lattice will not affect the spin density wave transition, but will likely drive the Ising nematic transition more mean field like through a linear coupling, as observed experimentally in BaFe_2-xCo_xAs_2.