Spin currents in superconductors

The possibility of the existence of macroscopic spin currents in the ground state of superconductors is raised. It is pointed out that such dissipationless currents are compatible with a slightly generalized form of the conventional BCS ground state, which breaks parity but is time-reversal invariant. It is shown that the magnitude of the measured magnetic field in rotating superconducting bodies can be understood if spin currents exist in the absence of body rotation, while it represents a puzzle otherwise. Concerning microscopic models of superconductivity it is shown that the attractive Hubbard model disfavors spin currents while the model of “hole superconductivity” favors them.