[Paper Review] Building spin-1/2 antiferromagnetic Heisenberg chains with diaza-nanographenes

Xiaoshuai Fu, Li Huang+14 more|arXiv (Cornell University)|Jul 30, 2024

Magnetism in coordination complexesMaterials Science3 citations

TL;DR

This study demonstrates the on-surface synthesis of spin-1/2 antiferromagnetic Heisenberg chains using diaza-hexa-peri-hexabenzocoronene (diaza-HBC) units on Au(111), achieving parity-dependent magnetic coupling through electron donation from the antiaromatic core. The chains exhibit gapped excitations in even-numbered chains and enhanced Kondo resonance in odd-numbered chains due to unpaired spin redistribution, confirming robust S = 1/2 spin per unit with tunable quantum magnetic behavior.

ABSTRACT

Understanding and engineering the coupling of spins in nanomaterials is of central importance for designing novel devices. Graphene nanostructures with π-magnetism offer a chemically tunable platform to explore quantum magnetic interactions. However, realizing spin chains bearing controlled odd-even effects with suitable nanographene systems is challenging. Here, we demonstrate the successful on-surface synthesis of spin-1/2 antiferromagnetic Heisenberg chains with parity-dependent magnetization based on antiaromatic diaza-hexa-peri-hexabenzocoronene (diaza-HBC) units. Using distinct synthetic strategies, two types of spin chains with different terminals were synthesized, both exhibiting a robust odd-even effect on the spin coupling along the chain. Combined investigations using scanning tunneling microscopy, non-contact atomic force microscopy, density functional theory calculations, and quantum spin models confirmed the structures of the diaza-HBC chains and revealed their magnetic properties, which has an S = 1/2 spin per unit through electron donation from the diaza-HBC core to the Au(111) substrate. Gapped excitations were observed in even-numbered chains, while enhanced Kondo resonance emerged in odd-numbered units of odd-numbered chains due to the redistribution of the unpaired spin along the chain. Our findings provide an effective strategy to construct nanographene spin chains and unveil the odd-even effect in their magnetic properties, offering potential applications in nanoscale spintronics.

Motivation & Objective

• To engineer nanographene-based spin chains with controlled quantum magnetic interactions for spintronic applications.
• To realize parity-dependent magnetic coupling in one-dimensional spin systems using chemically tunable diaza-nanographenes.
• To demonstrate the emergence of distinct magnetic excitations—gapped in even chains and Kondo-enhanced in odd chains—through structural and electronic control.
• To establish a platform for probing quantum spin phenomena in atomically precise, surface-supported nanostructures.

Proposed method

• On-surface synthesis of diaza-HBC-based spin chains on Au(111) using distinct synthetic strategies to tune chain terminals.
• Scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) for atomic-resolution structural characterization.
• Density functional theory (DFT) calculations to model electronic structure and spin distribution.
• Quantum spin models to interpret magnetic coupling and predict spin excitation spectra.
• Analysis of electronic transport and Kondo resonance to probe unpaired spin localization and spin correlation.
• Systematic comparison of even- and odd-numbered chains to identify odd-even effects in magnetic behavior.

Experimental results

Research questions

• RQ1 Can diaza-nanographenes be used to construct one-dimensional spin-1/2 antiferromagnetic Heisenberg chains with tunable magnetic coupling?
• RQ2 How does the chain length parity (odd vs. even) influence the magnetic excitation spectrum in these nanographene systems?
• RQ3 What is the role of electron donation from the diaza-HBC core to the Au(111) substrate in stabilizing S = 1/2 spins?
• RQ4 How do spin-polarized edge states and unpaired electron distribution affect Kondo resonance in odd-numbered chains?
• RQ5 To what extent do gapped excitations and Kondo resonances serve as signatures of antiferromagnetic order and spin frustration?

Key findings

• Two distinct diaza-HBC spin chains were synthesized with different terminal groups, both exhibiting robust odd-even effects in spin coupling.
• Each unit in the chains hosts an S = 1/2 spin due to electron donation from the antiaromatic diaza-HBC core to the Au(111) substrate.
• Even-numbered chains display gapped spin excitations, indicating a spin-Peierls-like distortion or dimerization.
• Odd-numbered chains show enhanced Kondo resonance, indicating localized unpaired spins due to spin redistribution along the chain.
• STM and nc-AFM confirmed the atomic structure and spin arrangement, with DFT calculations supporting the magnetic ground state.
• The combination of experimental and theoretical methods confirms the realization of a tunable, atomically precise spin chain system with controllable quantum magnetic properties.