Coexistence of spin singlets and metallic behavior in simple cubic CsC60

Article

关键词: FULLERIDE SUPERCONDUCTORS;    PHASE-TRANSITIONS;    NMR;    RBC60;    C-60;    DISTORTIONS;    RESONANCE;    KC60;    GAP;   

DOI  :  10.1103/PhysRevB.66.155123

来源: SCIE

【 摘 要 】

We present a detailed nuclear magnetic resonance (NMR) study of simple cubic CsC60, the only metallic cubic fulleride known so far besides A(3)C(60), obtained by quenching from high temperature. Cs-133 NMR signals the presence of about 12-15 % of anomalous C-60 balls, characterized by a 15-meV gap. We present different experimental observations supporting the idea that a spin singlet (i.e., a C-60(2-)) is localized on these latter balls and stabilized by a Jahn-Teller distortion. A splitting of the Cs-133 spectrum into three different lines, with distinct electronic environments, is observed, which emerges naturally from such a situation. Quadrupole effects on Cs-133 confirm an inhomogeneous distribution of the charge between C-60 molecules. Analysis of the relative intensities of the Cs-133 lines show that the spin singlets do not form clusters at the local scale, but are diluted within the lattice. This is probably to avoid the occurrence of neighboring C-60(2-) and minimize electrostatic repulsion between these balls. Through spin-lattice relaxation measurements, we detect chemical exchange between the Cs sites above 100 K. From this, we deduce that the lifetime of a spin singlet on a given ball decreases exponentially with increasing temperature (from 15 s at 100 K to 3 ms at 130 K) with an activation energy of 320 meV. The implications of the presence of such spin singlets for the nature of the metallic state is discussed, in relation with a decrease of 1/T1T for C-13 at low temperatures, which indicates a deviation from a simple metallic model.

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