First-principles quantum Monte Carlo study of the density-density response in free-standing graphene*

Electrons in graphene behave like 2D massless Dirac fermions in low energy. According to RPA, the usual screening in a metal should be absent; however, experiments on free-standing graphene suggest that the electron interactions are screened and it is a weakly correlated semimetal. However, it is still unclear whether the screening effect is due to exciton effects, the sigma electrons or the core electrons. We will report progress on first-principles quantum Monte Carlo calculations of the density-density response of free-standing graphene. We will use this quantity to investigate the screening in graphene.