Electron Pairing in Ferroelectric Niobium and Niobium Alloy Clusters

Cryogenic molecular beam experiments show that the ferroelectric dipole moments of small niobium clusters with an even number of valence electrons n are typically greater than those with odd n. This is verified in alloy clusters NbNXM where X = Al, Au, O, Mn, Fe, and Co; N ≤ 100;M ≤ 3. Like in superconducting alloys, Mn doping quenches the effect while Al and Au enhance it, suggesting a relation between cluster ferroelectricity and bulk superconductivity. A correlated ground state is proposed where the even-odd effect is caused by the depolarizing effect of a single unpaired electron.