Electrical transport through a mechanically gated molecular wire

TL;DR

This study investigates how mechanical gating affects electrical conductance in a molecular wire using STM experiments and compares the results with ab initio simulations, highlighting the need for beyond mean-field methods to accurately model electron correlation effects.

Contribution

It demonstrates the limitations of DFT-LDA in transport simulations and emphasizes the importance of advanced methods for accurate molecular conductance modeling.

Findings

Tip retraction mechanically gates the molecular wire in STM junctions.

DFT-LDA accurately predicts junction geometry but fails to reproduce conductance behavior.

Electron correlation effects require beyond mean-field simulation approaches.

Abstract

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green's functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment.