# 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

## TL;DR

This study uses inelastic neutron scattering to investigate phonon anomalies in a molecular dimer-Mott insulator, revealing coupling between lattice vibrations, charge, and spin degrees of freedom across various phase transitions.

## Contribution

It provides new insights into the coupling of phonons with charge and spin fluctuations in a molecular Mott insulator, highlighting the role of structural fluctuations in phase transitions.

## Key findings

- Phonon modes become overdamped near the charge gap opening temperature.
- Phonon damping decreases below the antiferromagnetic ordering temperature.
- No change in lattice symmetry observed across magnetic transition.

## 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 $T_{\rm ins}\sim50$-$60$ 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.