Dipolar interactions and constant dielectric loss spectra

Frequency-independent dielectric loss spectra constitute an ubiquitous phenomenon in strongly disordered materials. The degree of universality of this effect calls for a microscopic model that takes into account interactions and structural disorder, but otherwise requires only minimal assumptions. Under this point of view we review and further investigate the cooperative relaxation behaviour in an assembly of defect centres at frozen random positions. These centres carry an internal dipolar degree of freedom and interact by dipole-dipole forces. Such a description essentially amounts to studying the cooperative dynamics of interacting "asymmetric double well potential" (ADWP)-like centres, without invoking an ad hoc barrier distribution. Kinetic Monte Carlo simulations of an associated random dipolar lattice model show a transition from Debye to constant dielectric loss behaviour upon lowering the temperature. These results are interpreted with the help of analytic theories that focus on slow relaxation within strongly interacting nearest neighbour pairs of defects. Finally, a collective mode representation of the dielectric response derived from stochastic field theory is presented.