Circular currents in molecular wires

We consider circular currents driven by voltage bias 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 identifies 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 measures yield an identical definition of the loop current. Second, 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; can be, in certain voltage ranges, considerably larger than the net current through the molecule; and are furthermore quite persistent to normal thermal dephasing.