Naslov
Persistence of molecular excitations in metallic fullerides and their role in a possible metal to insulator transition at high temperatures

Autori
Brouet, Veronique ; Alloul, Henry ; Garaj, Slaven ; Forro, Laszlo

Izvornik
Physical Review B (1098-0121) 66 (2002);

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
Electron-vibron interactions; NMR; Charged fullerenes; Phase-transitions; K_4C_60; Na_2CsC_60; Rb_3C_60; CsC_60

Sažetak
We present C-13 nuclear magnetic resonance (NMR) spin-lattice relaxation measurements (1/T_1) in Na_2CsC_60 and Rb_3C-60 from 10 to 700 K. The large temperature range of this measurement allows to define unambiguously an increase of 1/T_1T with increasing temperature, which is anomalous in a simple metallic picture, where the Korringa law predicts 1/T_1T=cst. From the analogy with the relaxation data in Na_2C_60 and K_4C_60, we suggest that this increase is associated with the existence of an additional relaxation channel related to singlet-triplet (ST) excitations of Jahn-Teller distorted C_60^(2-) and C_60^(4-). The amplitude of the ST component is found to depend directly on the density of states, which indicates an interplay between metallic and molecular excitations. We propose a phenomenological model to describe the correlation between the two phenomena. The C_60^(2-) and C_60^(4-) would be formed within the metal on very short time scales (10^(-14) s) that do not imply static charge segregation. The interaction between metallic and molecular properties is also revealed by the high-temperature behavior of Na_2CsC_60 and CsC_60, which we then discuss. A divergence between the behaviors of 1/T_1, the NMR shift, and the electron-spin resonance susceptibility is interpreted as the result of a rapid increase of the lifetime of the charge carriers, signaling a tendency to charge localization. In our analysis, the particular stability of C_60^(2n-) is then a common feature of all known metallic fullerides and allows to reconcile apparently contradicting properties of these systems.

Izvorni jezik
Engleski

Znanstvena područja
Fizika

POVEZANOST RADA