Finite-temperature dynamics and the role of disorder in nearly critical Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$

TL;DR

This study uses inelastic neutron scattering to explore how disorder affects spin correlations and critical behavior in a nearly critical 3D XY antiferromagnet, revealing disorder-induced local excitations and deviations from theoretical expectations.

Contribution

It provides new insights into the role of disorder in quantum critical systems by analyzing spin dynamics and local excitations near a soft-mode transition in a disordered antiferromagnet.

Findings

Observation of $ ablarac{ ext{hbar}\omega}{T}$ scaling near the critical point

Identification of disorder-induced local excitations above the magnon band

Deviation of critical exponents from spin wave theory due to disorder

Abstract

Inelastic neutron scattering is used to investigate the temperature dependence of spin correlations in the 3-dimensional XY antiferromagnet Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$, $ x = 0.14(1)$, tuned close to the chemical-composition-induced soft-mode transition. The local dynamic structure factor shows $\hbar\omega/T$ scaling behavior characteristic of a quantum critical point. The deviation of the measured critical exponent from spin wave theoretical expectations are attributed to disorder. Another effect of disorder is local excitations above the magnon band. Their energy, structure factor and temperature dependence are well explained by simple strong-bond dimers associated with Br-impurity sites.