Field-induced criticality in a gapped quantum magnet with bond disorder

TL;DR

This study investigates the effects of bond disorder on the field-induced quantum phase transition in a gapped quantum magnet, revealing critical behavior and disorder effects on magnetic excitations, challenging some existing theoretical predictions.

Contribution

It provides experimental measurements of critical indices in a disordered quantum magnet and compares findings with theoretical models, highlighting the impact of disorder on quantum criticality.

Findings

Critical indices $\phi$ and $eta$ determined.

Disorder affects magnetic gap excitations.

Results challenge some Bose Glass predictions.

Abstract

Neutron diffraction and calorimetric measurements are used to study the field-induced quantum phase transition in piperazinium-Cu$_2$(Cl$_{1-x}$Br$_x$)$_6$ ($x=0$, x=3.5% and x=7.5%), a prototypical quantum antiferromagnet with random bonds. The critical indexes $\phi$ and $\beta$ are determined. The findings contradict some original predictions for Bose Glass based on the assumption $z=d$, but are consistent with recent theoretical results implying $z<d$. Inelastic neutron experiments reveal that disorder has a profound effect on the lowest-energy magnetic gap excitation in the system.