Persistent spin currents induced by a spatially-dependent magnetic field in a spin-1/2 Heisenberg antiferromagnetic ring

We show that a spatially-dependent magnetic field can induce a persistent spin current in a spin-1/2 Heisenberg antiferromagnetic ring, proportional to the solid angle subtended by the magnetic field on a unit sphere. The result is a direct consequence of Berry "parallel transport" in space. The magnitude of the spin current is determined by the ratio of longitudinal and transverse exchange interactions $J_\parallel/J_\perp$ and by the magnetic field. For large magnetic fields the Zeeman energy strongly renormalizes the Ising term giving rise to a maximum spin current. In the limit of $J_\parallel/J_\perp\ll1$ the amplitude of the current behaves like $1-(J_\parallel/J_\perp)^2$. In the opposite limit $\pi J_\perp> J_\parallel>J_\perp$ the amplitude scales as $\sqrt{J_\parallel/ J_\perp}\exp{(-\sqrt{J_\parallel/J_\perp})}$.