Quantum Monte Carlo study of magnetic correlations between adatom impurities in graphene

We study the interaction between two adatom impurity spins in graphene within the framework of the Anderson model. In particular, we calculate the inter-impurity magnetic correlations by using the quantum Monte Carlo (QMC) technique [1]. We find that, at high temperatures, the QMC results for the magnetic correlations between the impurities are in agreement with the Ruderman-Kittel-Kasuya-Yoshida (RKKY) predictions for graphene [1]. However, as the temperature is lowered, the inter-impurity magnetic correlations become strongly enhanced over the RKKY results, which points to the significance of the electronic correlations.\\[4pt] [1] R.M. Fye and J.E. Hirsch, PRB v38, 433 (1988).\\[0pt] [2] S. Saremi, PRB v76, 184430 (2007).