LETTER TO THE EDITOR Infrared studies of the organic superconductor

Abstract. The organic superconductor κ-(BEDT-TTF)2Cu(SCN)2 has been investigated at room temperature by polarized mid-infrared (600–7000 cm−1) reflectivity measurements at pressures of up to 1.7 GPa. The optical effective mass, mopt, decreases linearly with pressure, in contrast to the pressure dependence of the effective mass, m∗, determined by magnetic quantum oscillation measurements (Caulfield J et al 1994 J. Phys.:Condens. Matter 6 2911–24; Caulfield J et al 1995 Synth. Met. 70 815–8). Most phonon modes are seen to exhibit a linear pressure dependence of 0.5–1%/GPa. The stronger pressure dependence of the central C=C mode of the BEDT-TTF molecule is discussed: it is thought to be due to the pressure dependence of the effective Coulomb repulsion in κ-(BEDT-TTF)2Cu(SCN)2. These measurements suggest that a change in the electron–electron interaction under pressure could be the relevant factor for the suppression of superconductivity under pressure in κ-(BEDT-TTF)2Cu(SCN)2. κ-(BEDT-TTF)2Cu(SCN)2 is one of the best characterized organic molecular metals. At ambient pressure it is a superconductor with a superconducting transition temperature of Tc 10.4 K, which decreases upon the application of pressure, until at pressures exceeding 0.5 GPa superconductivity is fully suppressed [1, 2]. Magnetic quantum oscillation measurements hav