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

TL;DR

This NMR study reveals a unique, non-magnetic insulating phase in (NH3)xNaK2C60 with charge disproportionation, challenging previous beliefs about cation off-centering effects on superconductivity.

Contribution

The paper demonstrates a charge disproportionation mechanism leading to an insulating phase, contrasting with prior assumptions about cation off-centering in fullerides.

Findings

Insulating phase is non magnetic with a spin gap.

Charge disproportionation from C60^(3-) to C60^(2-) and C60^(4-) drives the transition.

Na+ cation re-centers in the insulating phase, refuting previous theories.

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.