Quantum Electrical Dipole in Triangular Systems: a Model for Spontaneous Polarity in Metal Clusters

TL;DR

This paper explores how quantum dipole moments arise in symmetric triangular molecules with degenerate ground states, examining their behavior under distortions, external fields, and correlations, revealing conditions for observable Stark effects.

Contribution

It introduces a model showing the persistence of quantum dipole moments in triangular molecules despite Jahn-Teller distortions and other effects.

Findings

Quantum dipole moments exist in symmetric molecules with degenerate ground states.

Jahn-Teller distortions do not eliminate the quantum dipole if distortions are small.

Linear Stark effects are observable at moderate fields and low temperatures.

Abstract

Triangular symmetric molecules with mirror symmetry perpendicular to the 3-fold axis are forbidden to have a fixed electrical dipole moment. However, if the ground state is orbitally degenerate and lacks inversion symmetry, then a ``quantum'' dipole moment does exist. The system of 3 electrons in D_3h symmetry is our example. This system is realized in triatomic molecules like Na_3. Unlike the fixed dipole of a molecule like water, the quantum moment does not point in a fixed direction, but lies in the plane of the molecule and takes quantized values +/- mu_0 along any direction of measurement in the plane. An electric field F in the plane leads to a linear Stark splitting +/- mu_0 F}. We introduce a toy model to study the effect of Jahn-Teller distortions on the quantum dipole moment. We find that the quantum dipole property survives when the dynamic Jahn-Teller effect is included, if the distortion of the molecule is small. Linear Stark splittings are suppressed in low fields by molecular rotation, just as the linear Stark shift of water is suppressed, but will be revealed in moderately large applied fields and low temperatures. Coulomb correlations also give a partial suppression.