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-ethylenedithiotetrathiafulvalene] copper dicyanamide bromide (kappa-(BEDT-TTF)(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 features to the symmetric intramolecular modes of vibration, by comparison with a recently published study on neutral BEDT-TTF. We then used these data to help us make a more definitive assignment that has been previously possible of the features in the low-temperature IR conductivity spectra of (BEDT-TTF)(2)Cu[N(CN)(2)]Br. It was also necessary to measure the low-temperature IR conductivity of additional isotopic analogs of 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 electron-phonon activated totally symmetric (A(g)) modes, but also that the nature 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 most likely due to an activated non-totally-symmetric (B-3g) mode, instead of a normally IR-active mode, which had been previously assigned, and further that several other (B-3g) modes appear to be activated in the IR spectra.