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 for integer quantum Hall states 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 translational and rotational symmetry which we derive in the Green function representation of Wigner-Weyl calculus. We discuss experimental perspectives for extracting the emergent Hall viscosity.