Novel Dipole-Lattice coupling in the Quantum-Spin-Liquid Material κ-Cu-CN

A family of molecular Mott insulators on triangular lattice provided a few S=1/2 triangular quantum spin liquid candidates, with κ-(BEDT-TTF)₂Cu₂(CN)₃ being the most studied material of this group. The large number experimental work presented a conflicting set of evidence, some suggesting spin liquid behavior, while other pointing on deceased magnetization reminding of a valence bond solid with orphan spins. In this work we use Raman scattering spectroscopy to probe both local charge on molecular sites and lattice phonons as a function of temperature down to 6K. Based on the analysis of the increasing width of Gaussian line shape of the BEDT-TTF charge sensitive vibration ν₂ on colling below 40 K we suggest a development of disordered fluctuating charge disproportionation on (BEDT-TTF)₂ dimers of amplitude as small as 0.06e. The lattice phonons show strong anomalous broadening on cooling only in (c,c) scattering channel, associated with the developing charge disproportionation. We suggest an interpretation, where the coupling of disordered charge dipoles on dimers to the lattice results in anisotropic modulation of charge transfer integrals between dimer lattice sites. Such fluctuation would results in fluctuations of magnetic coupling between spins which can produce short lived charge ordered spin-singlet pairs.