Non-thermodynamic nature of the orbital angular momentum in neutral fermionic superfluids

TL;DR

This paper explores the non-thermodynamic nature of orbital angular momentum in neutral fermionic superfluids, showing it depends on boundary conditions and is not an equilibrium property, supported by mean field and RDMG analyses.

Contribution

It clarifies the boundary sensitivity of OAM in superfluids and introduces the concepts of unpaired fermions and fermionic Landau criterion to explain this behavior.

Findings

OAM depends on boundary conditions in superfluids.

Mean field theory accurately describes OAM sensitivity.

RDMG calculations support the mean field conclusions.

Abstract

We discuss the orbital angular momentum (OAM) and the edge mass current in neutral fermionic superfluids with broken time reversal symmetry. Recent mean field studies imply that total OAM of a uniform superfluid depends on boundary conditions and is not a thermodynamic quantity. We point out that this does not conflict with thermodynamics, because there is no intensive external field conjugate to OAM with which a uniform superfluid is stable in the thermodynamic limit, in sharp contrast to the orbital magnetization in a non-superfluid system. We establish a simple physical picture for the sensitivity of OAM to boundaries by introducing the notion of "unpaired fermions" and "fermionic Landau criterion" within a mean field description. In order to go beyond the mean field approximation, we perform a density matrix renormalization group calculation and conclude that the mean field understanding is essentially correct.