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 $\ensuremath{\kappa}\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl}$ reveal a phonon anomaly in a wide temperature range. Starting from ${T}_{\mathrm{ins}}\ensuremath{\sim}50--60\text{ }\text{ }\mathrm{K}$ where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature ${T}_{\mathrm{N}}=27\text{ }\text{ }\mathrm{K}$, where also a ferroelectric ordering at ${T}_{\mathrm{FE}}\ensuremath{\approx}{T}_{\mathrm{N}}$ occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below ${T}_{\mathrm{N}}$, while no change of the lattice symmetry is observed across ${T}_{\mathrm{N}}$ 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.