/sup 13/C-NMR study on /spl alpha/-(BEDT-TTF)/sub 2/MHg(SCN)/sub 4/

Summary form only given, as follows. NMR experiments of organic conductors have so far been performed extensively at the proton site. However, the protons are located at edges of the molecule and, in many cases, have so small spin density of the molecular orbital responsible for conduction electrons in question that the proton is not the best nuclear probe. Selective substitution of ¹³C isotope to the carbon sites where spin density is rich is a useful method for NMR study. In this study, we attack a problem encountered in a series of compounds, α-(BEDT-TTF)₂MHg(SCN)₄ [M = K,Rb,Tl] by ¹³C-NMR; these compounds undergo some phase transition around 10 K from metallic state to another metallic state, which is discussed in the context of spin density wave state in many literatures but is not yet clear. We introduced ¹³C isotope into the central carbon sites of BEDT-TTF molecule and performed ¹³C-NMR measurements at this sites for spl alpha/-(BEDT-TTF)₂KHg(SCN)₄ in the temperature range between 2 and 300 K. We observed an anomaly in the nuclear spin-lattice relaxatin rate, T₁, and small change of line shape below 8 K, indicating some phase transition. However, absence of critical fluctuation near the transition temperature and only subtle change of the lineshape seems to be against the usual static magnetic ordering as found in the TMTSF compounds. We present field and angular dependence of NMR parameters and give quantitative discussion on the transition in question.