Fermi-resonance and pressure-induced mode stiffening in the organic superconductor κ-(BEDT-TTF)2Cu(SCN)2

Hydrostatic pressure has a drastic effect on the Tc of the organic superconductor κ-(BEDT-TTF)2Cu(SCN)2, completely suppressing superconductivity by 0.5 GPa. Several molecular vibrations, whose strong infrared activity is attributed to electron–phonon coupling, have been measured at room temperature in the pressure range, 0–1.7 GPa. However, resolving the modes observed between 1100 and 1400 cm−1 is complicated due to the mixing of several modes, with different pressure dependences. In this paper we use comparison of the infrared reflection spectrum from deuterated and protonated samples to aid the deconvolution of these overlapping phonon modes. This Fermi-resonance is modeled with a Green’s function to extract the linear pressure dependence of the individual modes and the inter-mode coupling strengths.