Symmetry-based theory of Dirac fermions on two-dimensional hyperbolic crystals: Coupling to the spin connection

Abstract: Discrete fermionic and bosonic models for hyperbolic lattices have attracted significant attention across a range of fields since the experimental realization of hyperbolic lattices in metamaterial platforms, sparking the development of hyperbolic crystallography. However, a fundamental and experimentally consequential aspect remains unaddressed: fermions propagating in curved space inherently couple to the underlying geometry via the spin connection, as required by general covariance - a feature not yet incorporated in studies of hyperbolic crystals. Here, we introduce a symmetry-based framework for Dirac fermions on two-dimensional hyperbolic lattices, explicitly incorporating spin-curvature coupling via a discrete spin connection. Starting from the continuous symmetries of the Poincar\'e disk, we classify the irreducible representations and construct a symmetry-adapted basis, establishing a direct correspondence to the continuum Dirac theory. We show that this continuum theory predicts a finite density of states at zero energy for any finite curvature in $D-$dimensional hyperbolic space with $2\leq D \leq 4$, suggesting enhanced susceptibility of Dirac fermions to interaction-driven instabilities at weak coupling. We then derive explicit forms of discrete translational and rotational symmetries for lattices characterized by Schl\"afli symbols ${p,q}$, and explicitly construct the discrete spin connection, represented as hopping phases, via parallel transport. Our results pave the way for experimental realization of spin-curvature effects in metamaterial platforms and systematic numerical studies of correlated Dirac phases in hyperbolic geometries.