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 $T_{ \rm ins}\sim50$-$60$ K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature $T_\mathrm{ N} = 27$ K, where also a ferroelectric ordering at $T_{ \rm FE} \approx T_{ \rm N}$ 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.
Categories: cond-mat.str-el