13C-NMR study on α-(BEDT-TTF)2MHg(SCN)4

The low temperature electronic states of the organic conductors, α-(BEDT-TTF)2MHg(SCN)4 [MK, Rb], have been studied by 13C-NMR with the use of 13C labeled samples. We observed anomalies in the nuclear spin-lattice relaxatin rate, T1, and shift of the NMR line below 8 K for the K salt and below 12 K for the Rb salt. These indicate some phase transition. However, absence of critical fluctuation near the transition temperature and absence of splitting or broadening of the NMR line is against the static magnetic ordering as found in usual SDW systems such as the TMTSF compounds. The present results imply that the electronic density of states decreases to a half below the transition.