Lattice Dynamics Coupled to Charge and Spin Degrees of Freedom in the Molecular Dimer-Mott Insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl reveal a phonon anomaly in a wide temperature range. Starting from T-ins similar to 50-60 K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature T-N = 27 K, where also a ferroelectric ordering at T-FE approximate to T-N occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below T-N, while no change of the lattice symmetry is observed across T-N in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.