Spin dynamics near a putative antiferromagnetic quantum critical point in Cu substituted BaFe$_2$As$_2$ and its relation to high-temperature superconductivity

TL;DR

This study investigates spin fluctuations near a quantum critical point in Cu-doped BaFe$_2$As$_2$, revealing distinct paramagnon-like fluctuations and evidence of quantum critical behavior relevant to high-temperature superconductivity.

Contribution

It provides new insights into spin dynamics near a quantum critical point in Cu-doped BaFe$_2$As$_2$, highlighting the decoupling of quasiparticles and spin fluctuations and demonstrating $ ext{ω/T}$ scaling.

Findings

Paramagnon-like spin fluctuations are present in the antiferromagnetic state.

Cu substitution decouples quasiparticle interactions from spin fluctuations.

Evidence of $ ext{ω/T}$ scaling indicates quantum criticality.

Abstract

We present the results of elastic and inelastic neutron scattering measurements on non-superconducting Ba(Fe${_{0.957}}$Cu${_{0.043}}$)${_2}$As${_2}$, a composition close to a quantum critical point between AFM ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low Cu composition as well as the parent compound BaFe$_2$As$_2$ and superconducting Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe$_{0.957}$Cu$_{0.043}$)$_2$As$_2$, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spin-spin correlation length, ${\xi(T)}$, increases rapidly as the temperature is lowered and find ${\omega/T}$ scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.