The pressure dependence of many-body interactions in the organic superconductor kappa-(BEDT-TTF)₂Cu(SCN)₂: A comparison of high pressure infrared reflectivity and Raman scattering experiments

We determine the pressure dependence of the electron-phonon coupling in $\kappa$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$ by comparison of high pressure Raman scattering and high pressure infrared (IR) reflectivity measurements. The Raman active molecular vibrations of the BEDT-TTF dimers stiffen by 0.1-1 %GPa$^{-1}$. In contrast, the corresponding modes in the IR spectrum are observed at lower frequency, with a pressure dependence of 0.5-5.5 %GPa$^{-1}$, due to the influence of the electron-phonon interaction. Both dimer charge-oscillation and phase-phonon models are employed to extract the pressure dependence of the electron molecular-vibration coupling for these modes. Analysis of our data suggests that the reduction of electron-phonon coupling under pressure does not account for the previously observed suppression of superconductivity under pressure and that electron-electron interactions may contribute significantly to the pairing mechanism.