Avoided quantum criticality and magnetoelastic coupling in BaFe$_{2-x}$Ni$_{x}$As$_{2}$

TL;DR

This study investigates the structural and magnetic phase transitions in electron-doped BaFe2-xNixAs2, revealing an avoided quantum critical point due to magnetic phase formation, which impacts the material's superconducting properties.

Contribution

It provides new insights into the suppression and separation of structural and magnetic transitions, and the emergence of an avoided quantum critical point in BaFe2-xNixAs2.

Findings

T_s and T_N transitions are suppressed and separated with Ni doping.

A secondary incommensurate magnetic phase forms near optimal doping.

An avoided quantum critical point influences superconducting and normal states.

Abstract

We study the structural and magnetic orders in electron-doped BaFe2-xNixAs2 by high-resolution synchrotron X-ray and neutron scatterings. Upon Ni-doping x, the nearly simultaneous tetragonal-to-orthorhombic structural T_s and antiferromagnetic (T_N) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in T_s>T_N with increasing x as was previously observed. However, the temperature separation between T_s and T_N decreases with increasing x for x> 0.065$, tending towards a quantum bi-critical point near optimal superconductivity at x=0.1. The zero-temperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088< x < 0.104, resulting in a finite value of T_N \approx T_c+10$ K above the superconducting dome around $x\approx 0.1$. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.