Ground-state properties of the narrowest zigzag graphene nanoribbon from quantum Monte Carlo and comparison with density functional theory

TL;DR

This study uses quantum Monte Carlo to analyze the correlated ground-state properties of the narrowest zigzag graphene nanoribbon, revealing antiferromagnetic order and providing benchmarks for density functional theory methods.

Contribution

First-principles QMC calculations elucidate the magnetic properties of a narrow zigzag graphene nanoribbon, establishing benchmarks for DFT approaches and estimating Hubbard U parameters.

Findings

AFM stabilization energy of 36 meV per carbon atom

Absolute magnetization of 1.13 μB per unit cell

Survival of antiferromagnetic correlations above room temperature

Abstract

By means of quantum Monte Carlo (QMC) calculations from first principles, we study the ground-state properties of the narrowest zigzag graphene nanoribbon, with an infinite linear acene structure. We show that this quasi-one-dimensional system is correlated and its ground state is made of localized $\pi$ electrons whose spins are antiferromagnetically (AFM) ordered. The AFM stablization energy (36(3) meV per carbon atom) and the absolute magnetization (1.13(1) $\mu_\textrm{B}$ per unit cell) predicted by QMC are sizable, and they suggest the survival of antiferromagnetic correlations above room temperature. These values can be reproduced to some extent by density functional theory (DFT) only by assuming strong interactions, either within the DFT+U framework or using hybrid functionals. Based on our QMC results, we then provide the strength of Hubbard repulsion in DFT+U suitable for this class of systems.