Analog gravity and the continuum effective theory of the graphene tight binding lattice model

Abstract

We consider the tight-binding model of graphene with slowly spatially varying hopping functions. We develop a low energy approximation as a derivative expansion in a Dirac spinor that is perturbative in the hopping function deformation. The leading description is the Dirac equation in flat 2+1-d spacetime with (strain-)gauge field. Prior work considered subleading corrections written as non-trivial frame and spin connection terms. We previously argued that such corrections cannot be considered consistently without taking all the terms at the same order of approximation, which due to the unconventional power counting originating from the large gauge field, involve also higher covariant derivative terms. Here we confirm this, explicitly computing subleading terms. To the order we explore, the theory is elegantly determined by the gauge field and frame, both given by the hopping functions, the torsion free spin connection of the frame, together with coefficients for the higher derivative terms derived from lattice invariants. For the first time we compute the metric that the Dirac field sees - the `electrometric' - to quadratic order in the deformation allowing us to describe the subleading corrections to the dispersion relation for inhomogeneous deformations originating from corrections to the frame. Focussing on in-plane inhomogeneous strain, we use a simple model to relate the hopping functions to the strain field, finding the electrometric becomes curved at this quadratic order. Thus this lattice model yields an effective analog gravity description as a curved space Dirac theory, with large magnetic field, and Lorentz violating higher covariant derivative terms. We check this by comparison to numerical diagonalization. From this we conjecture a form for the effective theory for monolayer graphene in terms of the strain tensor, consistent up to quadratic order in the deformation.