Some unusual properties of layered graphite, including a linear energy dependence of the quasiparticle damping and weak ferromagnetism at low doping, are explained as a result of the proximity of a single graphene sheet to the excitonic insulator phase which can be further stabilized in a doped system of many layers stacked in the staggered ( $\mathrm{ABAB}\dots$) configuration.

• Received 30 January 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.246802