Metal-insulator transition at 50 K inNa2C60

Abstract

Temperature dependence of electron spin resonance in Na 2 C 60 was studied in a temperature range from 2 to 350 K. It was shown that Na 2 C 60 was metallic above 50 K and had a metal-insulator transition at 50 K. The center frequency for the Hg͑2͒ Raman mode in Na 2 C 60 at 298 K was close to those in the metallic Rb 3 C 60 , K 3 C 60 , and Cs 3 C 60 , while the linewidth was close to that in the metallic but nonsuperconducting Cs 3 C 60. The Hg͑2͒ 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 C 60 and Rb 6 C 60 , which showed the metal-insulator transition at 50 K in Na 2 C 60. The origin of this metal-insulator transition was discussed in terms of the electron-phonon interaction ͑Jahn-Teller effect͒ and the electron-electron interaction ͑Mott-Hubbard picture͒. ͓S0163-1829͑99͒04123-5͔

Role of Dynamic Jahn-Teller Distortions inNa2C60andNaBrouet1, Alloul2, Thiên-Nga3 et al. 2001Phys. Rev. Lett.428400View full textAdd to dashboardCite

“…In particular, we do not observe any anomaly at 50 K that might be assigned to a metalinsulator transition as claimed in ref. [8]. Hence, the ground state is non-magnetic, which is confirmed by the absence of magnetic broadening of the NMR spectra at low T [18].…”

“…Na 2 C 60 is the only compound with n=2 known so far, but experimental studies are still limited and controverted. The first ESR studies [7] concluded that its electronic properties were identical to K 4 C 60 but more recently, another ESR investigation claimed that it was metallic with a metalinsulator transition at 50 K [8]. We present the first NMR study of Na 2 C 60 , which allows to detect singlet-triplet excitations of JTD C 60 balls, as found in A 4 C 60 [9][10][11].…”

Phase segregation on the nanoscale inNa2C60

“…The spin-lattice relaxation at low temperature indicated that the band degeneracy is not completely lifted and the ground state might be weakly metallic. Kubozono et al 19 interpret a sudden change at 50 K in the susceptibility and the Raman spectrum as a sign of a metal-insulator transition. Reference 20 reports hysteretic behavior in the ESR spectra in the temperature range 300-400 K.…”

Mixed valence model for superconductivity

“…Probably K 3 C 60 is delocalised on account of the disproportionation mechanism eq. (2), while K 4 C 60 (and N a 2 C 60 [42,44]) remains a semiconductor. The latter mechanism also explains why K 3 C 60 is a superconductor.…”