Low-frequency raman spectra in /spl kappa/-(BEDT-TTF)/sub 2/Cu(NCS)/sub 2/ and /spl kappa/-(BEDT-TTF)/sub 2/Cu[N(CN)/sub 2/]Br

Summary form only given. The ordering of the terminal ethylene groups of the ET molecules in /spl kappa/-(BEDT-TTF)/sub 2/Cu[N(CN)/sub 2/]Br and in their deuterated compounds was studied between 18 and 295 K by Raman scattering. It is believed that they become ordered at low temperature; the Cu(NCS)/sub 2//sup -/ salt has a staggered conformation, while the Cu[N(CN)/sub 2/]Br/sup -/ salt has an eclipsed one. The degree of disorder of the ethylene groups affects the superconducting transition temperature. Raman spectra of these four compounds were similax to one another. The low-frequency spectra changed slowly with decreasing temperature. A broad peak at about 50 cm/sup -1/ extremely broadened and its intensity became weak with decreasing temperature. We consider that it is due to the effect of ordering of the terminal ethylene groups. The anomalous behavior was interpreted in terms of critical dynamics of the pseudospin-phonon coupled system, where the up-spin state and the down-spin state represent the staggered conformation and the eclipsed one. We found that the ordered state is formed at around the superconducting transition temperature.