When a small electric bias is applied to a single-molecule junction, current will flow through the molecule via a tunneling mechanism. In molecules with a cyclic or helical structure, or where the electronic structure is helical, there may be dominant circular currents, giving rise to a unidirectional magnetic field. Here, we implement the Biot-Savart law and calculate the magnetic field resulting from the ballistic current density for a selection of molecules. We show that archetypical cyclic and helical molecules, such as benzene and helicene, are not suitable candidates for the generation of a significant magnetic field. We find that three prerequisites are important for achieving a substantial magnetic field in a single-molecule junction. (1) The current must be high, (2) the ring current must be unidirectional within the bias window, and (3) the diameter of the ring current must be small. For example, the first two are not fulfilled for molecules with destructive quantum interference near the Fermi energy such as a meta-linked benzene. We identify both cyclic and linear molecules that fulfill the requirements. In cyclic annulenes with bond-length alternation the current can traverse the full length of the ring structure even in an ortho-linked configuration. Cumulenes and polyynes, both linear carbon chains which can have helical frontier orbitals, also achieve a significant magnetic field that points along the direction of the chain due to strong circular currents around the linear wire. Our results prove that a substantial magnetic field can, in theory, be induced even at low bias.

Substantial magnetic fields arising from ballistic ring currents in single-molecule junctions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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