We revisit the Coleman-Hill theorem in the context of reduced planar QED. Using the global U(1) Ward identity for this non-local but still gauge invariant theory, we can confirm that the topological piece of the photon self-energy at zero momentum does not receive further quantum corrections apart from the potential one-loop contribution, even when considering the Lorentz non-invariant case due to the Fermi velocity v F < c. This is of relevance to probe possible time parity odd dynamics in a planar sheet of graphene which has an effective description in terms of (2 + 1)-dimensional planar reduced QED.Keywords: Reduced QED, dynamical Chern-Simons term. * david.dudal@kuleuven.be † ana.mizher@kuleuven.be ‡ pablo.pais@kuleuven.be
I. CONTEXT AND MOTIVATIONQuantum Electrodynamics in (2 + 1) dimensions (QED 3 ) has been widely used as a toy model for Quantum Chromodynamics (QCD). This is due to the fact that although being Abelian, QED 3 exhibits similar features as non-Abelian gauge theories, making it possible, for instance, to map and investigate chiral symmetry breaking and confinement into it [1][2][3][4][5]. The similarity is reinforced by the fact that a non-Abelian gauge theory at high temperature suffers a dimensional reduction and, if coupled to N f fermion families, the non-Abelian interactions are suppressed by a factor of N −1 f , so that in the large N f limit the theory can be considered approximately Abelian.Recently, the emergence of the so-called Dirac and Weyl planar materials [6], converted QED 3 into a playground in which a potential link between high energy physics (including quantum fields in curved spacetimes) and condensed matter can emerge [7][8][9][10][11][12][13]. Those are materials in which, due to the specific structure of their underlying lattice, the charge carriers present a relativistic-like behavior, being correctly described by a Dirac-like equation in some regimes. Particularly, the physical realization of graphene and other materials in two space dimensions, that are proved to contain a priori massless Dirac spinors, naturally yields the fermionic part of QED 3 [14, 15] through the continuum limit of the tight-binding theory, usually applied to describe their conduction electrons, which in turn implies a direct connection to QCD, as discussed above.Nevertheless, even though in these systems the fermions are constrained to remain in-plane and therefore are correctly described by a theory in (2 + 1) dimensions, the gauge fields responsible for the interaction between these electrons are not subject to the same constraint. One of the most remarkable consequences of this fact is that the interaction between electrons remains the familiar ∼ 1/r potential rather than the logarithmic one that would take place if the gauge fields were also restricted to the plane. Therefore, it is convenient and necessary to modify QED 3 in order to merge the desired features of the two sectors of the theory, starting with a general (3 + 1) theory and dimensionally reducing it to a non-local effective (2 ...