NESTING INSTABILITY IN A FAMILY OF ALPHA-(BEDT-TTF)(2)MHG(SCN)(4) [M=K, RB, NH4] STUDIED BY C-13-NMR

We have investigated low-temperature electronic states of the organic conductors, alpha-(BEDT-TTF)(2)MHg(SCN)(4) [M=K, Rb and NH4] in terms of C-13-NMR and de susceptibility measurements. For all the compounds, the overall temperature dependence of II(T1T) scales to that of chi(x )(2) except below the nesting instability. For M=Rb and K salts, the 1 /(T1T) decreased below the transition temperatures without line-broade ning or relaxation enhancement. The spectra changed from complicated l ines to an apparently simple Fake doublet below the nesting temperatur es. All of the NMR data are successfully explained by reduction of the electronic density-of states below the transition but do not have any sign of magnetic order.