Dielectric relaxation in the quantum multiferroics Rb₂Cu₂Mo₃O₁₂ and Cs₂Cu₂Mo₃O₁₂

Motivated by the recent discovery of dielectric relaxation by quantum critical magnons in Cs₂Cu₂Mo₃O₁₂, we conduct a detailed analysis of its dielectric response and compare it to that in the isostructural compound Rb₂Cu₂Mo₃O₁₂. Measurements in the vicinity of the field-induced magnon softening show that its description in terms of 3D Bose-Einstein condensation of magnons quantum critical point is unaltered by the presence of dielectric relaxation. We also demonstrate the existence of dielectric relaxation anomalies at 19 K in Rb₂Cu₂Mo₃O₁₂ and discuss the implications for the microscopic origin of dielectric activity in two compounds.