Control of conductance in molecular junctions is of key importance in
the growing field of molecular electronics. The current in these
junctions is often controlled by an electric gate designed to shift
conductance peaks into the low-bias regime. Magnetic fields on the
other hand, have been rarely used due to the small magnetic flux
captured by molecular conductors (an exception is the Kondo effect in
single-molecule transistors). This is in contrast to a related field,
electronic transport through mesoscopic devices, where considerable
activity with magnetic fields has led to a rich description of
transport. The scarcity of experimental activity is due to the belief
that significant magnetic response is obtained only when the magnetic
flux is on the order of the quantum flux, while attaining such a flux
for molecular and nanoscale devices requires unrealistic magnetic
fields.
Here we review recent theoretical work regarding the essential
physical requirements necessary for the construction of nanometer
scale magnetoresistance devices based on an Aharonov-Bohm molecular
interferometer. We show that control of the conductance properties
using small fractions of a magnetic flux can be achieved by carefully
adjusting the lifetime of the conducting electrons through a
pre-selected single state that is well separated from other states due
to quantum confinement effects. Using a simple analytical model and
more elaborate atomistic calculations we demonstrate that magnetic
fields which give rise to a magnetic flux comparable to
10-3 of the quantum flux can be used to switch a class of
different molecular and nanometers rings, ranging from quantum
corrals, carbon nanotubes, and even a molecular ring composed of
polyconjugated aromatic materials.
The unique characteristics of the magnetic field as a gate is further
discussed and demonstrated in two different directions. First, a
three terminal molecular router devices that can function as a
parallel logic gate, processing two logic operations simultaneously,
is presented. Second, the role of inelastic effects arising from
electron-phonon couplings on the magnetoresistance properties is
analyzed. We show that a remarkable difference between electric and
magnetic gating is also revealed when inelastic effects become
significant. The inelastic broadening of response curves to electric
gates is replaced by narrowing of magnetoconductance peaks, thereby
enhancing the sensitivity of the device.
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