Coincident structural and magnetic order in BaFe$_2$(As$_{1-x}$)P$_x$)$_2$ revealed by high-resolution neutron diffraction

TL;DR

This study uses high-resolution neutron diffraction to analyze the structural and magnetic phase transitions in BaFe$_2$(As$_{1-x}$)P$_x$)$_2$ across various temperatures and compositions, revealing coincident transitions and insights into quantum criticality.

Contribution

It provides detailed neutron diffraction data showing that structural and magnetic transitions are first order and coincident, and refines the understanding of the phase diagram and quantum criticality in this material.

Findings

Transitions are first order and coincident within 0.5 K.

Structural quantum criticality likely does not exist above x=0.28.

Spin-density wave order enhances the c/a ratio below transition.

Abstract

We present neutron diffraction analysis of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ over a wide temperature (10 to 300 K) and compositional ($0.11 \leq x \leq 0.79$) range, including the normal state, the magnetically ordered state, and the superconducting state. The paramagnetic to spin-density wave and orthorhombic to tetragonal transitions are first order and coincident within the sensitivity of our measurements ($\sim 0.5$ K). Extrapolation of the orthorhombic order parameter down to zero suggests that structural quantum criticality cannot exist at compositions higher than $x = 0.28$, which is much lower than values determined using other methods, but in good agreement with our observations of the actual phase stability range. The onset of spin-density wave order shows a stronger structural anomaly than the charge-doped system in the form of an enhancement of the $c/a$ ratio below the transition.