Infrared studies of the organic superconductor κ-(BEDT-TTF)2Cu(SCN)2 under pressure

The organic superconductor kappa-(BEDT-TTF)(2)Cu(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, m(opt), decreases linearly with pressure, in contrast to the pressure dependence of the effective mass, m*, determined by magnetic quantum oscillation measurements (Caulfield J er 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 kappa-(BEDT-TTF)(2)Cu(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 kappa-(BEDT-TTF)(2)Cu(SCN)(2).