Cotunneling theory and multiplet excitations: emergence of asymmetric line shape in inelastic scanning tunneling spectroscopy of correlated molecules on surfaces

TL;DR

This paper extends cotunneling theory to multireference systems, revealing that asymmetric line shapes in inelastic scanning tunneling spectroscopy can arise from multireference character and asymmetric tip/substrate couplings, explaining experimental observations.

Contribution

It introduces a generalized cotunneling formalism for multireference systems, explaining asymmetric spectral features in inelastic STS of correlated molecules on surfaces.

Findings

Multireference character causes asymmetric line shapes in STS.

Asymmetric tip/substrate couplings contribute to spectral asymmetry.

The model explains experimentally observed asymmetric peaks and dips.

Abstract

Recent advances in on-surface chemistry, combined with scanning probe microscopy, have enabled the synthesis of correlated molecules on surfaces and the characterization of their chemical and electronic properties with unprecedented spatial resolution. Low-energy magnetic excitations of individual molecules are frequently investigated by scanning tunneling spectroscopy (STS) and often appear as symmetric step-like features in the differential conductance as a function of bias voltage. The interpretation of such steps is well established within cotunneling theory and effective model Hamiltonians (e.g., Hubbard- and spin-based models). Here, we extend the cotunneling formalism to general multireference systems. We show that multireference character, together with orbital-dependent and strongly asymmetric tip/substrate couplings, can produce pronounced asymmetric line shapes in inelastic STS. These results provide an alternative microscopic mechanism for the asymmetric peaks and dips near the Fermi level frequently observed in STS experiments.