Polarization dependence of wide-range Raman scattering spectra in $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$

We measured Raman scattering spectra of $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$, which is a quantum spin-liquid candidate, over a wavenumber range, 25-3200 cm$^{-1}$. This compound shows a relaxor-like dielectric response that could originate in the charge disproportionation within each BEDT-TTF dimer. The Raman spectra are composed of not only sharp vibrational modes but some broad structures. One broad component is observed below 700 cm$^{-1}$ in cross polarization configuration. It is assigned to a magnetic excitation and has a significant intensity at as low as 25 cm$^{-1}$ at 10 K, which results from a spin frustration effect. Moreover, some of the vibrational Raman lines below 120 cm$^{-1}$, which are assigned to intermolecular vibration, show a similar temperature dependence with the dielectric constant. This might indicate that the charge instability leading to the relaxor-like dielectric behaviors affects the Raman signals for molecular motions. We will discuss the magnetic excitation and the charge disproportionation in $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ in terms of wide-range Raman scattering spectra.