A singular evolution toward an insulating phase, shown by /sup 23/Na and /sup 13/C NMR, has been observed in the superconducting fullerides (NH/sub 3/)/sub x/NaK/sub 2/C/sub 60/ for x>1. Unlike most common cases, this insulating phase is nonmagnetic and /sup 13/C spin lattice relaxation shows the presence of a spin gap. These two features suggest that a charge disproportion from C/sub 60//sup 3-/ to C/sub 60//sup 2-/ and C/sub 60//sup 4-/ can drive the system from the metallic to the insulating state. The restoring of the Na/sup +/ cation in the center of the octahedral interstice in the insulating phase, as indicated by /sup 23/Na and /sup 2/H line shape analysis, confutes the current belief that the cation off-centering is effective in quenching the superconductivity.
Metal-to-insulator evolution in (NH/sub 3/)/sub x/NaK/sub 2/C/sub 60/: an NMR study
Bolzoni F
2003
Abstract
A singular evolution toward an insulating phase, shown by /sup 23/Na and /sup 13/C NMR, has been observed in the superconducting fullerides (NH/sub 3/)/sub x/NaK/sub 2/C/sub 60/ for x>1. Unlike most common cases, this insulating phase is nonmagnetic and /sup 13/C spin lattice relaxation shows the presence of a spin gap. These two features suggest that a charge disproportion from C/sub 60//sup 3-/ to C/sub 60//sup 2-/ and C/sub 60//sup 4-/ can drive the system from the metallic to the insulating state. The restoring of the Na/sup +/ cation in the center of the octahedral interstice in the insulating phase, as indicated by /sup 23/Na and /sup 2/H line shape analysis, confutes the current belief that the cation off-centering is effective in quenching the superconductivity.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
