ESR and heat capacity studies of phase transition in Rb1C60

Rb1C60 is Studied by ESR as a function of frequency from several tenth MHz to 26 GHz and by heat capacity, to reveal the nature of the phase transitio n around 50 K. Above the transition temperature, the electronic states are consistent with a metallic state. Below it, the ESR linewidth increases lin early with decreasing T, which is ascribed to a development of short-range order (SRO) based on its frequency dependence. Furthermore, the present ana lysis of the EPR spectra suggested a distribution of the linewidth that was consistent with broadly distributed transition temperatures of the spin de nsity wave (SDW) or antiferromagnetism. The heat capacity shows a broad ano maly from 50 K down to 30 K. The apparent entropy change of such a magnetic anomaly suggests that only one fourth of spins are in the ordered state at 30 K, which is consistent with the reported intensity of AFMR.