ELECTRON-PHONON EFFECTS IN THE ORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)(2)CU[N(CN)(2)]BR

We report on room-temperature Raman measurements on kappa-di[bis-ethyl enedithiotetrathiafulvalene] copper dicyanamide bromide (kappa-(BEDT-T TF)(2)Cu[N(CN)(2)]Br) and the isotopic analog C-13(6)-(BEDT-TTF)(2)Cu[ N(CN)(2)]Br, in which the six central carbon atoms in the inner rings of the BEDT-TTF molecule are replaced with C-13 and assign the feature s to the symmetric intramolecular modes of vibration, by comparison wi th a recently published study on neutral BEDT-TTF. We then used these data to help us make a more definitive assignment that has been previo usly possible of the features in the low-temperature IR conductivity s pectra of (BEDT-TTF)(2)Cu[N(CN)(2)]Br. It was also necessary to measur e the low-temperature IR conductivity of additional isotopic analogs o f BEDT-TTF, namely those containing C-13(6), C-13(2), S-34(8) and d(8) . We found, as expected, that the spectra are dominated by the electro n-phonon activated totally symmetric (A(g)) modes, but also that the n ature of the strongest of these, nu(3)(A(g)), is changed dramatically in (BEDT-TTF)(2)Cu[N(CN)(2)]Br from what it is in neutral BEDT-TTF. We also found, unexpectedly, that one of the stronger IR features is mos t likely due to an activated non-totally-symmetric (B-3g) mode, instea d of a normally IR-active mode, which had been previously assigned, an d further that several other (B-3g) modes appear to be activated in th e IR spectra.