Phonon renormalization effects accompanying the 6 K anomaly in the quantum spin liquid candidate κ−(BEDT-TTF)2Cu2(CN)

Inelastic neutron scattering measurements have revealed an abrupt change in the phonon damping at the 6 K anomaly in the quantum spin liquid candidate $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$. This is interpreted as a phase transition to the valence bond solid state on the basis of an effective model describing the spin-charge coupling in the dimer-Mott system.