Current and noise in a model of an AC-STM molecule-metal junction

TL;DR

This paper models the AC-STM transport properties of a molecule-metal junction, analyzing current response and shot noise to interpret experimental data and understand molecular energy levels.

Contribution

It introduces a theoretical framework for calculating AC response and shot noise in molecule-metal junctions using the Keldysh technique, aiding experimental interpretation.

Findings

Finite frequency current response computed for single and two-site molecules.

Zero frequency shot noise analyzed as a diagnostic for molecular energy levels.

Model applicable to interpreting AC-STM experiments on various surfaces.

Abstract

The transport properties of a simple model for a finite level structure (a molecule or a dot) connected to metal electrodes in an alternating current scanning tunneling microscope (AC-STM) configuration is studied. The finite level structure is assumed to have strong binding properties with the metallic substrate, and the bias between the STM tip and the hybrid metal-molecule interface has both an AC and a DC component. The finite frequency current response and the zero frequency photo-assisted shot noise are computed using the Keldysh technique, and examples for a single site molecule (a quantum dot) and for a two-site molecule are examined. The model may be useful for the interpretation of recent experiments using an AC-STM for the study of both conducting and insulating surfaces, where the third harmonic component of the current is measured. The zero frequency photo-assisted shot noise serves as a useful diagnosis for analyzing the energy level structure of the molecule. The present work motivates the need for further analysis of current fluctuations in electronic molecular transport.