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

The organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2) has been investiga ted at room temperature by polarized mid-infrared (600-7000 cm(-1)) reflect ivity measurements at pressures of up to 1.7 GPa. The optical effective mas s, m(opt), decreases linearly with pressure, in contrast to the pressure de pendence of the effective mass, m*, determined by magnetic quantum oscillat ion 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 pressur e 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 Coulom b repulsion in kappa-(BEDT-TTF)(2)Cu(SCN)(2). These measurements suggest th at a change in the electron-electron interaction under pressure could be th e relevant factor for the suppression of superconductivity under pressure i n kappa-(BEDT-TTF)(2)Cu(SCN)(2).