Concurrent conductance and transition voltage spectroscopy study of scanning tunneling microscopy vacuum junctions. Does it unravel new physics?

TL;DR

This study investigates conductance and transition voltage spectroscopy in vacuum nanogaps, revealing that an additional 'ghost' current is necessary to explain experimental data, which may also be relevant in molecular junctions.

Contribution

The paper introduces the concept of a 'ghost' current to reconcile experimental data with theoretical models in vacuum nanogaps and suggests its possible presence in molecular junctions.

Findings

Standard models cannot explain the data without the ghost current.

The ghost current becomes significant at larger nanogap sizes.

Similarities between vacuum and molecular junction data suggest a common ghost contribution.

Abstract

We show that the conductance ($G$) and transition voltage ($V_t$) spectroscopy data for standard vacuum nanogaps reported in the preceding paper by Sotthewes \emph{et al} cannot be understood within the framework of the existing theories/models whatever realistic effects are incorporates. However, if we include an additional ("ghost") current, the trends of the dependencies of $G$ and $V_t$ on the nanogap size $d$ can be explained. The ghost current is very small. Therefore, effects related to the ghost current can only be revealed at larger $d$'s, where the ghost current becomes comparable to or overcomes the tunneling current. Although we are not able to unravel the origin of this ghost contribution, we can and do refer to experimental $G$- and $V_t$-data reported for molecular junctions, which exhibit similarities to the presently considered vacuum nanojunctions. Analyzing notable difficulties related to the regime assigned as transport via hopping, we speculate that a ghost contribution may also exist in molecular junctions.