Slow dipole dynamics in organic charge transfer salts

At temperatures around 60 K, measurements of the dielectric constant in the κ-(BEDT-TTF)_2 X family of organic charge transfer salts have indicated the emergence of glassy dynamics in a relaxor ferroelectric phase. We propose that an extended Hubbard model on a triangular lattice of dimers is a minimal model for these systems to capture glassy dynamics. Allowing for disorder, we use a strong coupling expansion to second order in the Hubbard interaction to obtain a low-energy effective model in terms of spin and dipole degrees of freedom. By focusing on classical terms in the effective model we obtain a model amenable to classical Monte Carlo simulations. We perform equilibrium and out-of-equilibrium Monte Carlo simulations and calculate an analog of the Edwards-Anderson order parameter for dipoles and the two-time auto-correlation function for charge degrees of freedom. For appropriate parameters we find evidence for aging dynamics and a non-zero Edwards-Anderson order parameter, implying the emergence of glassiness in the charge degrees of freedom at low temperatures, as would be expected for a relaxor ferroelectric.