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

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, 3.5%, and 7.5%), a prototypical quantum antiferromagnet with random bonds. The critical indexes and 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.