▎ 摘　　要

The electrons in undoped graphene behave as massless Dirac fermions. Therefore, graphene can serve as a unique condensed-matter laboratory for the study of various relativistic effects, including quantum electrodynamics (QED) phenomena. Although theoretical models describing electronic properties of graphene have been elaborated in details, the QED effects were usually neglected. In this paper, we demonstrate theoretically that QED can drastically modify electronic properties of graphene. We predict the following QED effect: the opening of the band gap in a graphene monolayer placed inside a planar microcavity filled with an optically active media. We show that this phenomenon occurs due to the vacuum fluctuations of the electromagnetic field and is similar to such a well-known phenomenon as a vacuum-induced splitting of atomic levels (the Lamb shift). We estimate the characteristic value of the band gap and find that it can sufficiently exceed the value of the Lamb shift.