2002/04/13 by V. Brouet, H. Alloul, S. Garaj +3
Chemistry · Earth and Planetary Sciences · Materials Science · Physics and Astronomy · #Fullerene Chemistry and Applications #High-pressure geophysics and materials #Thermal Expansion and Ionic Conductivity #cond-mat.str-el
paper · pdf · doi:10.1103/physrevb.66.155124
10 pages, 7 figures
arxiv created 2002/04/13 · openalex publication_date 2002/10/31 · arxiv updated 2009/11/30 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
We present 13C nuclear magnetic resonance (NMR) spin-lattice relaxation measurements (1/T1) in Na2CsC60 and Rb3C60 from 10 to 700 K. The large temperature range of this measurement allows to define unambiguously an increase of 1/T1T with increasing temperature, which is anomalous in a simple metallic picture, where the Korringa law predicts 1/T1T=cst. From the analogy with the relaxation data in Na2C60 and K4C60, 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 C60^2\ensuremath- and C60^4\ensuremath-. 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 C60^2\ensuremath- and C60^4\ensuremath- would be formed within the metal on very short time scales (10^\ensuremath-14s) that do not imply static charge segregation. The interaction between metallic and molecular properties is also revealed by the high-temperature behavior of Na2CsC60 and CsC60, which we then discuss. A divergence between the behaviors of 1/T1, 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 C60^2n\ensuremath- is then a common feature of all known metallic fullerides and allows to reconcile apparently contradicting properties of these systems.