Theory Meets Experiment in Low-Dimensional Structures with Correlated Electrons
Prague, Czech Republic, July 1 – 4, 2019
Observation of electronic end-states in hydrogen-bonded organic 1D molecular chains on Au(111)
One-dimensional structures offer a rich ecosystem for realizing quantum states with potential application for advanced information technologies. Surface confined molecular self-assembly is one avenue for creating 1D systems, where the extant structure is controlled by the precursor shape, and functional group interlinking chemistry. As an example, recent studies of properly designed graphene nanoribbons demonstrated the presence of topologically protected end-states .
Here we study self-assembled 1D chains of zwitterionic molecule 2,5-diamino-1,4-benzoquinonediimines (DABQDI)  on Au(111) in ultrahigh vacuum at 5K using combined scanning tunneling and non-contact atomic force microscopies (STM/nc-AFM) supported by theoretical analysis. Sub-molecular resolution achieved with a CO-functionalized tip hints structural information, specifically regarding the hydrogen bonds linking the precursor units. This is further supported by density functional theory (DFT) simulations and calculations of proton tunneling in between the unit cells, that enhances electronic communication across the chain.
On top of this, scanning tunneling spectroscopy (STS) measurements reveal a presence of in-gap electronic states near the Fermi energy, strongly localized to the chain ends. To rationalize the presence of these states, we propose an extension of the seminal Su, Schrieffer, Heeger (SSH)  model by mimicking the chemical structure of 1D DABQDI chain. A detailed analysis of the proposed model shows possible solutions with non-trivial topological phase, that exist within intuitively reasonable range of model parameters.
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 W. P. Su, J. R. Schrieffer, and A. J. Heeger, Phys. Rev. Lett. 42, 1698, (1979).