C-13-NMR STUDY ON ALPHA-(BEDT-TTF)(2)MHG(SCN)(4)

The low temperature electronic states of the organic conductors, alpha -(BEDT-TTF)(2)MHg(SCN)(4) [M=K, Rb], have been studied by C-13-NMR wit h the use of C-13 labeled samples. We observed anomalies in the nuclea r spin-lattice relaxatin rate, T-1, and shift of the NMR line below 8 K for the K salt and below 12 K for the Rb salt. These indicate some p hase transition. However, absence of critical fluctuation near the tra nsition temperature and absense of splitting or broadening of the NMR line is against the static magnetic ordering as found in usual SDW sys tems such as the TMTSF compounds. The present results imply that the e lectronic density of states decreases to a half below the transition.