Electron-phonon coupling in kappa-(BEDT-TTF)(2)Cu(NCS)(2) organic superconductor studied by Raman scattering spectroscopy

Low frequency Raman data for kappa-(ET)(2)Cu(NCS)(2) (T-c = 10.4 K) are rep orted. Measurements have been performed in a wide range of temperatures (1. 5 - 100 K) and frequency hardening related to the superconducting transitio n has been observed for all the low-lying coupled phonons. The measured rel ative frequency shifts are lower than those previously reported for kappa-( ET)(2)Cu[N(CN)(2)]Br (T-c=11.6 K), but the behavior of the two systems is s imilar and indicates a significant strength of the intermolecular electron- phonon coupling. The effects of isotopic substitutions ((C4S8)-C-13-S-34 an d H-2(8)) in ET molecules on the low frequency Raman active phonons of kapp a-(ET)(2)Cu(NCS)(2) are also reported. They give the first direct experimen tal confirmation to the suggested lattice softening induced by deuteration.