Visualizing electron correlation by means of ab-initio scanning tunneling spectroscopy images of single molecules

TL;DR

This paper develops a many-body tunneling theory and computational approach to visualize electron correlation effects in single molecules using ab-initio scanning tunneling spectroscopy images, revealing how correlation influences molecular images.

Contribution

It introduces a novel theoretical and computational framework for analyzing electron correlation in STM images of molecules decoupled from substrates, combining quantum chemical methods with tunneling theory.

Findings

Correlation causes a significant squeezing of STM images.

Correlated images involve contributions from multiple orbitals beyond HOMO and LUMO.

Correlation effects are currently too small to be experimentally detected in studied molecules.

Abstract

Scanning tunneling microscopy (STM) has been a fundamental tool to characterize many-body effects in condensed matter systems, from extended solids to quantum dots. STM of molecules decoupled from the supporting conductive substrate has the potential to extend STM characterization of many body effects to the molecular world as well. In this article, we describe a many-body tunneling theory for molecules decoupled from the STM substrate, and we report on the use of standard quantum chemical methods to calculate the quantities necessary to provide the 'correlated' STM molecular image. The developed approach has been applied to eighteen different molecules, to explore the effects of their chemical nature and of their substituents, as well as to verify the possible contribution by transition metal centers. Whereas the bulk of calculations have been performed with CISD because of the computational cost, some tests have been also performed with the more accurate CCSD method to quantify the importance of the computational level on many-body STM images. We have found that correlation induces a remarkable squeezing of the images, and that correlated images are not derived from Hartree-Fock HOMO or LUMO alone, but include contributions from other orbitals as well. Although correlation effects are too small to be resolved by present STM experiments for the studied molecules, our results provide hints for seeking out other species with larger, and possibly experimentally detectable, correlation effects.