Raman studies of BEDT-TTF molecule-based organic superconductors

Abstract

The temperature dependence of the frequencies of the Raman features due to ${\ensuremath{\nu}}_{9}$ ${(A}_{g})$ and ${\ensuremath{\nu}}_{60}$ ${(B}_{3g})$ modes has been measured in the BEDT-TTF (ET) molecule-based organic superconductors $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br},$ $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl},$ ${\ensuremath{\alpha}}_{t}\ensuremath{-}(\mathrm{ET}{)}_{2}{\mathrm{I}}_{3},$ and $\ensuremath{\beta}\ensuremath{-}(\mathrm{ET}{)}_{2}{\mathrm{AuI}}_{2}.$ The frequency of the ${\ensuremath{\nu}}_{9}$ ${(A}_{g})$ mode was observed to soften in $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}$ and $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl}$ at low temperatures where NMR data indicated antiferromagnetic fluctuations, providing evidence of spin-phonon interactions. The result is also compared with a very recent theory which deals with the effect of electronic correlations on the frequencies of the molecular phonon modes. We observed a peak splitting for the ${\ensuremath{\nu}}_{60}$ ${(B}_{3g})$ mode in ${\ensuremath{\alpha}}_{t}\ensuremath{-}(\mathrm{ET}{)}_{2}{\mathrm{I}}_{3}$ below 190 K, where an incommensurate superstructure was reported, and a frequency decrease of $1.7\ifmmode\pm\else\textpm\fi{}0.4 {\mathrm{cm}}^{\ensuremath{-}1}$ for the upper component of the ${\ensuremath{\nu}}_{60}$ ${(B}_{3g})$ mode in the superconducting compound, demonstrating further the unusual properties of this mode.