Metal-insulator transition inRbC60polymer fulleride studied by ESR and electron-spin relaxation

The ESR intensity, line shape, and longitudinal electron-spin relaxation in the polymer phase of the ${\mathrm{RbC}}_{60}$ fulleride are investigated in the temperature range $4.2<T<\mathrm{}300\mathrm{}\hspace{0.5em}\mathrm{K}.\mathrm{}$ Most attention is focused on the metal-insulator transition region (25–50 K). It is found that below 50 K the ESR line can be separated into two Lorentzian components ascribed to conduction electrons and some localized paramagnetic centers (with concentration of about 0.03 per formula unit) with allowance made for the relaxation bottleneck. The decrease of the conduction-electron susceptibility obeys an activation law with the characteristic energy $\Delta {/k}_B=80\mathrm{}\pm 10\hspace{0.5em}\mathrm{K}$ related to the opening of a gap $2\Delta \approx 100\hspace{0.5em}{\mathrm{cm}\mathrm{}}^{\mathrm{-}1}.$ The same quantity is found by analyzing both longitudinal and transverse relaxation caused by fluctuations of internal fields with correlation time ${\tau }_c\propto \exp \left(2\mathrm{}\Delta {/k}_{B}T\right).\mathrm{}$ Below 25 K, the temperature dependencies of the linewidth and the relaxation times change abruptly, revealing the development of a new ordered state. The nature of this state is discussed.