How Metallic are Small Sodium Clusters?

TL;DR

This study investigates the metallic properties of small sodium clusters, revealing that their electric screening abilities are consistent with metallic behavior, contrary to some theoretical predictions, through high-precision electric deflection experiments.

Contribution

The paper provides experimental evidence that small Na clusters behave as metals in terms of electric screening, challenging previous theories predicting large dipole moments.

Findings

Electric dipole moments are much smaller than predicted, consistent with zero.

Na cluster polarizabilities exhibit metal spheroid behavior.

Size oscillations in polarizability are linked to shell structure.

Abstract

Cryogenic cluster beam experiments have provided crucial insights into the evolution of the metallic state from the atom to the bulk. Surprisingly, one of the most fundamental metallic properties, the ability of a metal to efficiently screen electric fields, is still poorly understood in small clusters. Theory has predicted that many small Na clusters are unable to screen charge inhomogeneities and thus have permanent dipole moments. High precision electric deflection experiments on cryogenically cooled Na$_N$ ($N<200$) clusters show that the electric dipole moments are at least an order of magnitude smaller than predicted, and are consistent with zero, as expected for a metal. The polarizabilities of Na clusters also show metal spheroid behavior, with fine size oscillations caused by the shell structure.