Kubo formulas for viscosity: Hall viscosity, Ward identities, and the relation with conductivity

TL;DR

This work establishes a rigorous Kubo framework for the full, frequency-dependent viscosity tensor, including Hall viscosity, by formulating stress response to uniform strains via GL$(d,\mathbb{R})$ strain generators and Ward identities. It derives three equivalent forms of the viscosity response (stress-strain, stress-stress, strain-strain) and defines a finite, physically meaningful viscosity by subtracting the static inverse compressibility, accommodating both zero and nonzero magnetic fields. A key result is a general viscosity–conductivity relation that holds at all frequencies and in magnetic fields, enabling extraction of viscosity components from the $\mathbf{q}$-dependent conductivity, with the Hoyos–Son relation recovered as a special case. The authors validate the framework through concrete examples including free gases, integer/fractional quantum Hall systems, and complex $\ell$-wave paired superfluids, and they offer an electrodynamic interpretation of Hall viscosity in terms of orbital spin. The findings provide a unified, versatile toolkit for predicting and potentially measuring Hall viscosity in quantum fluids, linking macroscopic transport to microscopic orbital structure and topological properties.

Abstract

We derive from first principles the Kubo formulas for the stress-stress response function at zero wavevector that can be used to define the full complex frequency-dependent viscosity tensor, both with and without a uniform magnetic field. The formulas in the existing literature are frequently incomplete, incorrect, or lack a derivation; in particular, Hall viscosity is overlooked. Our approach begins from the response to a uniform external strain field, which is an active time-dependent coordinate transformation in d space dimensions. These transformations form the group GL(d,R) of invertible matrices, and the infinitesimal generators are called strain generators. These enable us to express the Kubo formula in different ways, related by Ward identities; some of these make contact with the adiabatic transport approach. For Galilean-invariant systems, we derive a relation between the stress response tensor and the conductivity tensor that is valid at all frequencies and in both the presence and absence of a magnetic field. In the presence of a magnetic field and at low frequency, this yields a relation between the Hall viscosity, the q^2 part of the Hall conductivity, the inverse compressibility (suitably defined), and the diverging part of the shear viscosity (if any); this relation generalizes a result found recently. We show that the correct value of the Hall viscosity at zero frequency can be obtained (at least in the absence of low-frequency bulk and shear viscosity) by assuming that there is an orbital spin per particle that couples to a perturbing electromagnetic field as a magnetization per particle. We study several examples as checks on our formulation.