Emergent hall viscosity in the integer quantum hall phases of graphene-like systems

We explicitly distinguish Hall viscosity as defined relative to the strain field vs. relative to an emergent vielbein or metric field and discuss it for graphene-like systems. Aside from the gravitational or vielbein/metric related “geometric” Hall viscosity prevailing throughout the literature, a contribution proportional to the Hall conductivity, the “electronic” Hall viscosity, due to the emergent strain induced gauge field exists. We unify both contributions within the “emergent” Hall viscosity, determine it explicitly for graphene in the semimetal and Semenoff semiconducting phases, and in the latter case compare it to its non-relativistic limit. Under these circumstances two topological invariants enter the emergent Hall viscosity in the presence of translation and rotation symmetries. The Green function expression representing the electronic Hall viscosity is quantized in general, whereas that representing the geometric Hall viscosity is valid only under Lorentz (or, Galilean) symmetry. We discuss experimental perspectives for extracting the emergent Hall viscosity.