Gaps and excitations in fullerides with partially filled bands: NMR study of Na2C60 and K4C60

Article

关键词: NUCLEAR-MAGNETIC-RESONANCE;    PHASE-TRANSITION;    MOLECULAR-DYNAMICS;    K3C60;    C-60;    RB4C60;    A(4)C(60);    NA2CSC60;    RELAXATION;    C-13-NMR;   

DOI  :  10.1103/PhysRevB.66.155122

来源: SCIE

【 摘 要 】

We present a nuclear magnetic resonance (NMR) study of Na2C60 and K4C60, two compounds that are related by electron-hole symmetry in the C-60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T-1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C-60(2-) or C-60(4-). However, the extended temperature range of the measurements presented here (10 K to 700 K) allows one to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T-1 deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected by the appearance of quadrupole effects on the Na-23 spectra of Na2C60. In K4C60, modifications of the C-13 spectra line shapes also indicate a structural modification. We discuss this high-temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, 1/T1T tends to a constant value for Na2C60, both for C-13 and Na-23 NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.

【 授权许可】

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