Topological advantage for adsorbate chemisorption on conjugated chains

TL;DR

The paper investigates how topology in a one-dimensional SSH chain affects adsorption of a molecule with an empty frontier $LUMO$, focusing on two observables: adsorbate occupancy $n_0$ and electronic friction $\gamma(R)$. Using a minimal Fano–Anderson–SSH framework, it compares edge/bulk adsorption across trivial, metallic, and topological phases and analyzes how midgap boundary states and solitons influence charge transfer and vibrational damping. Key findings show that symmetry-protected midgap states at edges and domain walls substantially boost $n_0$, while metallic phases, despite higher DOS, yield weaker local hybridization; electronic friction is maximal in the metal and intermediate at topological edges due to level splitting, with signatures persisting under disorder. The results highlight design principles for leveraging topological boundary modes to enhance adsorption energetics and vibrational dissipation, and they suggest extending these concepts to higher dimensions and engineered domain-wall patterns for catalysis and sensing applications.

Abstract

Topological matter offers opportunities for control of charge and energy flow with implications for chemistry still incompletely understood. In this work, we study an ensemble of adsorbates with an empty frontier level (LUMO) coupled to the edges, domain walls (solitons), and bulk of a Su-Schrieffer-Heeger polyacetylene chain across its trivial insulator, metallic, and topological insulator phases. We find that two experimentally relevant observables, charge donation into the LUMO and the magnitude of adsorbate electronic friction, are significantly impacted by the electronic phase of the SSH chain and show clear signatures of the topological phase transition. Localized, symmetry-protected midgap states at edges and solitons strongly enhance electron donation relative to both the metallic and trivial phases, whereas by contrast, the metal's extended states, despite larger total DOS near the Fermi energy, hybridize more weakly with a molecular adsorbate near a particular site. Electronic friction is largest in the metal, strongly suppressed in gapped regions, and intermediate at topological edges where hybridization splits the midgap resonance. These trends persist with disorder highlighting their robustness and suggest engineering domain walls and topological boundaries as pathways for employing topological matter in molecular catalysis and sensing.

Figures (10)

• Figure 1: Schematic Fano--Anderson SSH setup. White and orange circles denote sublattices A and B, respectively, each with a single $p_z$ orbital. Intra- and intercell hoppings ($v$ and $w$) are tunable. A diatomic adsorbate couples either near an edge ($x_M=x_E$) or at the chain center ($x_M=x_B$). (a) Hybridization with edge/bulk modes in the topological phase ($v<w$). (b) Hybridization with a trivial insulator ($v> w$) or with the metallic limit ($v=w$), where states are delocalized.
• Figure 2: LUMO occupation near the (a) edge $(T_{1,A}=T_{1,B}/3=0.1)$ and (b) bulk center $(T_{N/2,A}=T_{N/2+1,B}/3=T_{N/2-1,B}/3=0.1)$ of the chain for trivial $(r<1)$, metallic $(r\approx 1)$, and topological $(r>1)$ phases. Parameters: $N=800$, $v=10$, $\mu=0$.
• Figure 3: LUMO occupation vs. adsorbate level $\varepsilon_0$: topological edge ($w=15$, solid lines) compared with metallic bulk (colors denote $r=1.00814$ [purple], $1.00597$ [blue], $1.00465$ [blue–green]) for several $T_{m,A}$. Parameters: $N=800$, $v=10$, $\mu=0$.
• Figure 4: Schematic hybridization between the adsorbate LUMO and SSH states in the trivial (left), topological (center), and metallic (right) regimes.
• Figure 5: Resonant case $\varepsilon_0=0$. (a) $\braket{n_0}$ vs. number of adsorbates (defects) for soliton ensembles at three dimerizations ($r=1.5,1.1,1.0001$), compared to (i) metal ($r=1$ with the same number of on-site defects and (ii) clean metal ($r=1$, no defects). Shaded regions represent fluctuations over realizations of the disordered systems, and we have set the number of adsorbates to be equal the number of domain walls $N_{ad} = N_{DW}$. (b) $\braket{n_0}$ vs. $N_{\text{ad}}$ for a fixed set of 40 domain walls (topological) or none (clean metal), and adsorbates placed at random sites. Parameters: $\xi=7$, $N=3000$, $v=10$, $T_m=0.1$, and 25 realizations.