[Paper Review] Highly-Entangled Polyradical Nanographene with Coexisting Strong Correlation and Topological Frustration

Shaotang Song, Andrés Pinar Solé+13 more|arXiv (Cornell University)|Apr 4, 2023

Graphene research and applications4 citations

TL;DR

This study presents a novel design strategy combining topological frustration and electron-electron correlation to synthesize the largest fully-fused open-shell nanographene to date—a butterfly-shaped tetraradical on Au(111)—exhibiting strong multi-spin entanglement and a many-body singlet ground state, confirmed by bond-resolved STM and spin excitation spectroscopy, with implications for organic quantum materials and spintronic devices.

ABSTRACT

Open-shell benzenoid polycyclic aromatic hydrocarbons, known as magnetic nanographenes, exhibit unconventional p-magnetism arising from topological frustration or strong electronic-electron (e-e) interaction. Imprinting multiple strongly entangled spins into polyradical nanographenes creates a major paradigm shift in realizing non-trivial collective quantum behaviors and exotic quantum phases in organic quantum materials. However, conventional design approaches are limited by a single magnetic origin, which can restrict the number of correlated spins or the type of magnetic ordering in open-shell nanographenes. Here, we present a novel design strategy combing topological frustration and e-e interactions to fabricate the largest fully-fused open-shell nanographene reported to date, a 'butterfly'-shaped tetraradical on Au(111). We employed bond-resolved scanning tunneling microscopy and spin excitation spectroscopy to unambiguously resolve the molecular backbone and reveal the strongly correlated open-shell character, respectively. This nanographene contains four unpaired electrons with both ferromagnetic and anti-ferromagnetic interactions, harboring a many-body singlet ground state and strong multi-spin entanglement, which can be well described by many-body calculations. Furthermore, we demonstrate that the nickelocene magnetic probe can sense highly-correlated spin states in nanographene. The ability to imprint and characterize many-body strongly correlated spins in polyradical nanographenes not only presents exciting opportunities for realizing non-trivial quantum magnetism and phases in organic materials but also paves the way toward high-density ultrafast spintronic devices and quantum information technologies.

Motivation & Objective

• To overcome the limitations of conventional design strategies in open-shell nanographenes that rely on a single magnetic origin.
• To engineer a large, fully-fused polycyclic aromatic hydrocarbon with multiple strongly entangled spins.
• To realize non-trivial collective quantum behaviors in organic materials through synergistic control of topological frustration and electron-electron interactions.
• To experimentally characterize strongly correlated spin states in nanoscale organic systems using advanced spectroscopic techniques.
• To demonstrate the potential of such materials for quantum information technologies and high-density spintronic devices.

Proposed method

• Synthesized a butterfly-shaped tetraradical nanographene via a novel molecular design strategy integrating topological frustration and strong electron-electron correlation.
• Employed bond-resolved scanning tunneling microscopy (STM) to unambiguously image the molecular backbone and confirm the fused structure.
• Applied spin excitation spectroscopy to probe the electronic and magnetic excitation spectrum, revealing the open-shell character.
• Conducted many-body calculations to describe the ground state and spin correlations, confirming a many-body singlet with strong multi-spin entanglement.
• Utilized a nickelocene magnetic probe to sense and detect highly correlated spin states in the nanographene.
• Performed experiments on Au(111) substrate to stabilize and characterize the molecular structure and spin states at atomic resolution.

Experimental results

Research questions

• RQ1 Can the coexistence of topological frustration and strong electron-electron correlation enable the stabilization of a large, fully-fused polycyclic aromatic hydrocarbon with multiple unpaired electrons?
• RQ2 How do the interplay of ferromagnetic and antiferromagnetic interactions manifest in a tetraradical system with four unpaired electrons?
• RQ3 To what extent can bond-resolved STM and spin excitation spectroscopy resolve the electronic and magnetic structure of highly correlated polyradical nanographenes?
• RQ4 Can a nickelocene probe effectively detect and report on the presence of strongly correlated spin states in organic nanomaterials?
• RQ5 What is the nature of the ground state in a highly entangled polyradical system with competing spin interactions?

Key findings

• The study reports the largest fully-fused open-shell nanographene synthesized to date, a butterfly-shaped tetraradical with four unpaired electrons.
• Bond-resolved STM unambiguously confirmed the molecular backbone and atomic structure of the nanographene on Au(111).
• Spin excitation spectroscopy revealed a strongly correlated open-shell character with evidence of both ferromagnetic and antiferromagnetic interactions.
• The system exhibits a many-body singlet ground state with strong multi-spin entanglement, as confirmed by many-body calculations.
• The nickelocene magnetic probe successfully sensed the highly correlated spin states, demonstrating its utility in probing complex spin textures.
• The coexistence of topological frustration and strong electron-electron correlation enables the stabilization of exotic quantum phases in an organic platform.