Lattice dynamics coupled to charge and spin degrees of freedom in the molecular dimer-Mott insulator $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Cl

Abstract

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 $T_{\mathrm{ins}}\sim 50$-$60$ 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 $T_{\mathrm{FE}}\approx T_{\mathrm{N}}$ 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.

6 pages, 3 figures with Erratum ( 4 pages, 2 figures) and supplemental material (4 pages, 4 figures)