Low temperature Raman scattering of κ-(ET)2X superconductors
Résumé
Single crystals of organic superconductors have been studied with micro-Raman scattering above and below Tc. The two phases investigated are κ-(ET)2Cu(NCS)2 (where ET = BEDT-TTF bis-ethylenedithiotetrathiafulvalene, Tc= 10.4K) and κ-(ET)2Cu[N(CN)2]Br (Tc= 11.6K). The Raman lineshapes have been calculated from the experimental spectra with a Green's function model involving a coupling between the phonons and a common continuum of electronic excitations. Renormalized frequencies, electron-phonon coupling constants, dampings, scattering amplitude ratios of the continuum and the phonons are derived. Satisfactory reproduction of the low frequency spectrum is achieved and shows that low-energy electronic excitations may be present in these systems. It is found that some low frequency phonons characteristic of intermolecular coupling harden slightly below Tc (1 to 2 cm-1) whereas the high frequency C=C vibrations are not significantly changed with the temperature. This possibly indicates a correlation between low-energy phonons and the superconducting mechanism.