Structural and magnetic phase transitions near optimal superconductivity in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

TL;DR

This study investigates structural and magnetic phase transitions in phosphorus-doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$, revealing that these transitions are coupled and vanish abruptly near optimal superconductivity, indicating a weakly first order transition.

Contribution

The paper provides evidence that in BaFe$_2$(As$_{1-x}$P$_x$)$_2$, magnetic and structural transitions are coupled and disappear abruptly near optimal doping, challenging the existence of a quantum critical point.

Findings

Transitions are always coupled and approach $T_N \\approx T_s \\ge T_c$ for $x=0.29$.

Transitions vanish abruptly for $x \\ge 0.3$, indicating a weakly first order transition.

Magnetic order disappears near optimal doping, similar to electron-doped iron pnictides.

Abstract

We use nuclear magnetic resonance (NMR), high-resolution x-ray and neutron scattering to study structural and magnetic phase transitions in phosphorus-doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at $x=0.3$. However, we show that the tetragonal-to-orthorhombic structural ($T_s$) and paramagnetic to antiferromagnetic (AF, $T_N$) transitions in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ are always coupled and approach to $T_N\approx T_s \ge T_c$ ($\approx 29$ K) for $x=0.29$ before vanishing abruptly for $x\ge 0.3$. These results suggest that AF order in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ disappears in a weakly first order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.