Critical dielectric relaxation at a magnetic quantum phase transition

Slow relaxational dynamics of electric polarization is characteristic of ferroelectrics and reveals itself in dissipative anomalies of the dielectric constant. We report the observation of such anomalies in a nonpolar frustrated quasi-1D quantum ferro-antiferromagnet Cs₂Cu₂Mo₃O₁₂. The dielectric response shows two components: a critical divergence at the field-induced magnetic quantum critical point (QCP), and an unusual dissipative peak at ultra-low temperatures, present at all fields. The former has been also identified in a sister compound and stems from the inverse Dzyaloshinskii-Moriya mechanism in the strongly magnetized state. The latter manifests itself as a strong modulation in the complex permittivity with a marked field and temperature dependence, which also becomes critical at the magnetic transition. Analysis using a Cole-Cole relaxation model indicates that the dynamics is mediated by the soft magnon that softens at the QCP. These observations demonstrate the emergence of dynamical electric dipole moments in a frustrated spin system.