Circular Currents in Molecular Wires

TL;DR

This paper investigates circular currents in molecular wires with loop structures, defining the current precisely, analyzing dephasing effects, and demonstrating their occurrence and magnitude in various aromatic molecules.

Contribution

It introduces a unified definition of circular current, studies dephasing impacts, and shows that circular currents are common and significant in several molecular structures.

Findings

Circular currents can be larger than net currents in certain voltage ranges.

Circular currents are persistent despite thermal dephasing.

All three definitions of magnetic field yield identical measures of loop current.

Abstract

We consider circular currents in molecular wires with loop substructures studied within simple tight-binding models. Previous studies of this issue have focused on specific molecular structures. Here we address several general issues. First we consider the quantitative definition of a circular current and adopt a definition that identified the circular component of a loop current as the sole source of the magnetic field induced in the loop. The latter may be associated with the field at the loop center, with the magnetic moment associated with this field or with the total magnetic flux threading the loop. We show that all three definitions yield an identical measure of the loop current. Secondly, we study dephasing effects on the loop current and the associated induced magnetic field. Finally, we consider circular currents in several molecular structures: benzene, azulene, naphthalene and anthracene and show that circular currents occur generically in such structures and can be, in certain voltage ranges, considerably larger than the net current through the molecule, and are furthermore quite persistent to normal thermal dephasing.