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Metal-to-insulator evolution in (NH3)xNaK2C60: an NMR study

M. Ricco·G. Fumera·T. Shiroka·O. Ligabue·C. Bucci·F. Bolzoni
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Abstract

A singular evolution toward an insulating phase, shown by 23Na and 13C NMR, has been observed in the superconducting fullerides (NH3)xNaK2C60 for x>1. Unlike most common cases, this insulating phase is non magnetic and 13C spin lattice relaxation shows the presence of a spin gap. These two features suggest that a charge disproportion from C60^(3-) to C60^(2-) and C60^(4-) can drive the system from the metallic to the insulating state. The restoring of the Na+ cation in the center of the octahedral interstice in the insulating phase, as indicated by 23Na and 2H lineshape analysis, confute the current belief that the cation off-centering is effective in quenching the superconductivity.

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M. Ricco, G. Fumera, T. Shiroka, O. Ligabue, C. Bucci, F. Bolzoni. 2003-05-08. Metal-to-insulator evolution in (NH3)xNaK2C60: an NMR study. https://doi.org/10.1103/physrevb.68.035102

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