Current Noise and AC Response of a Back-Gated Single-Molecule Junction

Current noise and ac response properties of single-molecule junctions (SMJs) are essential for their potential application as nanoscale electron devices. We first study
$I{-}V$
characteristics of electronic transport through a back-gated benzene-type SMJ based on the nonequilibrium Green's function formalism within a tight-binding model, and then present analytical expressions and numerical calculations for current noises and ac conductances of the SMJ. We show that the noise power density in the high-frequency region is independent of the applied bias voltage, and will be significantly suppressed and have dip structures at the frequencies where electron transitions from the occupied molecular states to the unoccupied ones are excited. It is found that both the bias voltage and the back-gate voltage can affect current noise spectra and ac conductances drastically in the low-frequency region. The low-frequency part of the ac conductance can be enhanced by applying a bias voltage, but an opposite behavior with suppression of conductances is observed for the system without the particle-hole symmetry. AC conductances for the source and the drain leads also show distinct characteristics for this SMJ system. It is expected that our results will provide some bases for the ac application of SMJ nanodevices.