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

TL;DR

This NMR study of Na2C60 and K4C60 reveals deviations from expected electronic gap behavior at high temperatures and residual metallicity at low temperatures, highlighting complex structural and electronic interactions in fullerides.

Contribution

The paper provides extended temperature range NMR data revealing high-temperature deviations and low-temperature residual metallicity in fullerides, linking structural changes to electronic properties.

Findings

High-temperature deviation from activated behavior in 1/T1.

Detection of symmetry lowering via quadrupole effects.

Residual metallic character at low temperatures.

Abstract

We present an NMR study of Na2C60 and K4C60, two compounds that are related by electron-hole symmetry in the C60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or C60^{4-}. However, the extended temperature range of the measurements presented here (10 K to 700 K) allows to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T1 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 in Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60, modifications of the 13C spectra lineshapes 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/T$_1$T tends to a constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.