Strongly correlated physics in organic open-shell quantum systems

TL;DR

This work identifies a nonperturbative, many-body splitting of the SOMO in open-shell organic radicals as a hallmark of strong electronic correlations in molecular junctions. By integrating ab initio descriptions with beyond-DFT many-body techniques (ED and real-space DMFT) in a LO-based active-space framework and coupling to NEGF transport, the authors demonstrate a Mott-like scattering mechanism that splits the SOMO and suppresses conductance, with strong dependence on the SOMO’s spatial distribution. A minimal three-orbital model reproduces the key physics and clarifies the role of quantum interference in transport, predicting distinct I–V_b signatures across linear and cyclic radicals. The results underscore the necessity of nonperturbative correlation treatments to interpret transport in radical junctions and point to experimental probes (nonlinear I–V or differential conductance) to detect strong correlation effects in open-shell molecular devices.

Abstract

Strongly correlated physics arises from electron-electron scattering within partially filled orbitals. Organic molecules in open-shell configurations are therefore good candidates to exhibit many-body effects. We focus on electron transport in a two-terminal single-molecule junction setup, in which the molecular bridge consists of an organic radical with a molecular orbital hosting a single unpaired electron (SOMO). We perform beyond state-of-the-art numerical simulations combining an ab-initio description of the chemical environment, with quantum field-theoretical techniques that account for many-body effects. The key observation is that the SOMO resonance is prone to splitting and we identify a giant electronic scattering rate as the driving many-body mechanism, akin to that of the Mott metal-to-insulator transition. By comparing linear and cyclic radicals, we show that the spatial distribution of the SOMO and its projection on the molecular backbone have dramatic consequences for the transport properties of the junction. We argue that the phenomenon and the underlying microscopic mechanism apply to a broad family of open-shell molecular systems, and can explain puzzling experimental observations such as suppressed conductance in radical junctions.

Figures (10)

• Figure 1: (a) Schematics of the scattering region of the single-molecule junction, consisting of the molecular bridge and the Au electrodes. The screening space (R) and active space (A) intended as a subspace of the orbitals of the molecule, are highlighted. (b) Structure of the pentadienyl and benzyl radicals, substituted with the amino anchoring groups, and of the Au(111) electrodes.
• Figure 2: (a,d) Resonant structures of the pentadienyl and benzyl radicals in the gas phase. (b,e) Structure of the radical molecules substituted with amino anchoring groups. (c,f) SOMO isosurfaces projected on the p$_z$ LOs of the substitued molecules. Isovalues: $\pm 0.03$ au. See text for a discussion.
• Figure 3: Partially screened Coulomb parameters $U_{ij} = \mathbf{U}_{A;ij}$ in the LO basis for (a) pentadienyl and the (b) benzyl radicals.
• Figure 4: Electron transmission function through the pentadienyl radical junction. DFT predicts a SOMO resonance close to $E_F$. Taking into account the Coulomb repulsion beyond restricted DFT yields: (a) a splitting of the resonance into $\downarrow$-SOMO and $\uparrow$-SUMO due to spin-symmetry breaking; (b) a splitting of the resonance due to many-body effects (without spin symmetry breaking) revealing a transmission node close to the Fermi energy.
• Figure 5: Electron transmission function through the pentadienyl radical junction. Both the $G_0W_0$ and the self-consistent $GW$ approximations fail to predict the splitting of the SOMO feature, as described within ED and R-DMFT, cfr. Fig. \ref{['fig:Te_pentadienyl']}.