Anomalous chiral transport with vorticity and torsion: Cancellation of two mixed gravitational anomaly currents in rotating chiral $p+ip$ Weyl condensates

TL;DR

This paper explores the relationship between chiral vortical and torsional effects in relativistic fermions, showing how their associated gravitational anomalies can cancel in rotating chiral superfluids, with implications for condensed matter systems.

Contribution

It establishes the connection between CVE, CTE, and gravitational anomalies, demonstrating their cancellation in rotating chiral $p+ip$ Weyl superfluids and clarifying the role of torsion and Nieh-Yan anomaly.

Findings

CVE and CTE currents cancel in equilibrium for rotating chiral superfluids.

Gravitational anomalies depend on torsion and are well-defined with temperature or chemical potential.

Anomalies are second order in gradients and contribute linearly in response.

Abstract

Relativistic gravitational anomalies lead to anomalous transport coefficients that can be activated at finite temperature in condensed matter systems with gapless fermions. The chiral vortical effect (CVE) is an anomalous chiral current along a rotation axis, expressed in terms of a gravimagnetic field and a gravitational anomaly. Another one, the chiral torsional effect (CTE), arises from hydrodynamically independent frame fields and connection. We discuss the relation of CVE, CTE and gravitational anomalies for relativistic fermions from the perspective of torsion and the Nieh-Yan anomaly. The DC transport coefficients of the two anomalies are found to be closely related depending whether or not torsion is non-zero in the hydrodynamics. The relativistic anomaly from torsion is well defined if instead of an ultraviolet divergent term, the chemical potential or temperature enter. The anomaly term is 2nd order in gradients and contributes in linear response for CTE, implying also the same for CVE. As an example, we consider chiral $p+ip$ Weyl superfluids/conductors. At low-energies, the system is effectively relativistic along a special anisotropy axis. The hydrodynamics is governed by two velocities, normal velocity $\boldsymbol{v}_n$ and superfluid velocity $\boldsymbol{v}_s$. The two anomalies follow from the normal component rotation and the dependence of the momentum density on the superfluid velocity (order parameter). In the CVE the chiral current is produced by solid body rotation of the normal component with (angular) velocity $\boldsymbol{v}_n= \boldsymbol\Omega \times \boldsymbol{r}$. In the CTE, a chiral current is produced the vorticity $\nabla \times \boldsymbol{v}_s$, which in the low-energy effective theory is torsion. In equilibrium, $\langle\langle \nabla \times \boldsymbol{v_s}\rangle\rangle= 2\boldsymbol\Omega$ on average and the two anomaly currents cancel.