Strained Bilayer Graphene, Emergent Energy Scales, and Moire Gravity

Abstract

Twisted bilayer graphene is a rich condensed matter system, which allows one to tune energy scales and electronic correlations. The low-energy physics of the resulting moir\'e structure can be mathematically described in terms of a diffeomorphism in a continuum formulation. We point out that twisting is just one example of moir\'e diffeomorphisms. Another particularly simple and experimentally relevant transformation is a homogeneous isomorphic strain of one of the layers, which gives rise to a nearly identical moir\'e pattern (rotated by $90^\circ $ relative to the twisted structure) and potentially flat bands. We further observe that low-energy physics of the strained bilayer graphene takes the form of a theory of fermions tunneling between two curved space-times. Conformal transformation of the metrics results in emergent "moir\'e energy scales," which can be tuned to be much lower than those in the native theory. This observation generalizes to an arbitrary space-time dimension with or without an underlying lattice or periodicity and suggests a family of toy models of "moir\'e gravity" with low emergent energy scales. Motivated by these analogies, we present an explicit toy construction of moir\'e gravity, where the effective cosmological constant can be made arbitrarily small. We speculate about possible relevance of this scenario to the fundamental vacuum catastrophe in cosmology.