Resonance Raman Investigations of Low Energetic Modes in the Organic Superconductors α$\text{}_{t}$-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ and β-(BEDT-TTF)$\text{}_{2}$IAuI

A study of low energetic modes in the resonance Raman spectra of the organic superconductors α$\text{}_{t}$-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ (T$\text{}_{c}$ = 8 K) and β-(BEDT-TTF)$\text{}_{2}$IAuI (T$\text{}_{c}$ = 4 K) in the temperature range of 1.5-100 K was performed. In both materials, a strong vanishing of low energetic phonon bands (at 30 cm$\text{}^{-1}$ and 40 cm$\text{}^{-1}$ for α$\text{}_{t}$-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ and at 27 cm$\text{}^{-1}$ and 33 cm$\text{}^{-1}$ for β-(BEDT-TTF)$\text{}_{2}$IAuI) below 71 was observed. For the symmetric stretching mode of the I¯$\text{}_{3}$ and the IAuI¯ anion at 120 cm$\text{}^{-1}$ no change below T, was found. In addition, wavelength dependent measurements on α-(BEDT-TTF)$\text{}_{2}$I$\text{}_{3}$ were performed, indicating that the modes around 30 cm$\text{}^{-1}$ are probably librational modes of the BEDT-TTF molecules.