Origin of permanent electric dipoles in homonuclear niobium clusters

Kristopher Andersen, University of California, Davis

Evidence of a permanent electric dipole moment in free niobium clusters containing on the order of 10-100 atoms at temperatures below 100 K has been recently reported by Moro et al. (Science 300, 1265). The observed moments of 0-3 debye were measured by deflecting a cryogenically cooled cluster beam through an inhomogeneous electric field. The origin of this nonintuitive dielectric behavior is still unknown, and there is speculation that this “ferroelectric” transition bears some connection to the superconducting transition in crystalline niobium. Using first-principles density-functional calculations the electric dipole is analyzed in terms of the asymmetry of the cluster, state decomposition, and charge deformation. The magnitude of the calculated moments generally agree with experiment within the studied size range of 2-15 atoms. We have found that the origin of large dipoles lies in the formation of partial covalent bonds that drive the charge separation and encourage the formation of clusters with low structural symmetry.

Abstract Author(s): Kristopher E. Andersen, Vijay Kumar, and Warren E. Pickett