Metal-insulator transition at 50 K inNa2C60

Temperature dependence of electron spin resonance in ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ was studied in a temperature range from 2 to 350 K. It was shown that ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ was metallic above 50 K and had a metal-insulator transition at 50 K. The center frequency for the Hg(2) Raman mode in ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ at 298 K was close to those in the metallic ${\mathrm{Rb}}_3{\mathrm{C}}_{60},$ ${\mathrm{K}}_3{\mathrm{C}}_{60},$ and ${\mathrm{Cs}}_3{\mathrm{C}}_{60},$ while the linewidth was close to that in the metallic but nonsuperconducting ${\mathrm{Cs}}_3{\mathrm{C}}_{60}.$ The Hg(2) mode showed a large blueshift and narrowing at 50 K. The center frequency and the linewidth in the low-temperature region from 50 K were almost the same as those in the insulating ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_6{\mathrm{C}}_{60},$ which showed the metal-insulator transition at 50 K in ${\mathrm{Na}}_2{\mathrm{C}}_{60}.$ The origin of this metal-insulator transition was discussed in terms of the electron-phonon interaction (Jahn-Teller effect) and the electron-electron interaction (Mott-Hubbard picture).