Lattice vibrations of the charge-transfer salt   $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$: novel interpretation of the   electrodynamic response in a spin-liquid compound
  The dimer Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ exhibits unusual electrodynamic properties. Numerical investigations of the electronic ground state and the molecular and lattice vibrations reveal the importance of the Cu$_2$(CN)$_3^-$ anion network coupled to the BEDT-TTF molecules: The threefold cyanide coordination of copper and linkage isomerism in the anion structure cause a loss of symmetry, frustration, disorder, and domain formation. Our findings consistently explain the temperature and polarization-dependent THz and infrared measurements, reinforce the understanding of dielectric properties and have important implications for the quantum spin-liquid state, which should be treated beyond two-dimensional, purely electronic models. 
M. Dressel, P. Lazic, A. Pustogow, E. Zhukova, B. Gorshunov, J.A.   Schlueter, O. Milat, B. Gumhalter, and S. Tomic
cond-mat.str-el