Alternation of sign of magnetization current in driven XXZ chains with   twisted XY boundary gradients
  We investigate an open XXZ spin 1/2 chain driven out of equilibrium by coupling with boundary reservoirs targeting different spin orientations in XY plane. Symmetries of the model are revealed which appear to be different for spin chains of odd and even sizes. As a result, spin current is found to alternate with chain length, ruling out the possibility of ballistic transport. Heat transport is switched off completely by virtue of another global symmetry. Further, we investigate the model numerically and analytically. At strong coupling, we find exact nonequilibrium steady state using a perturbation theory. The state is determined by solving secular conditions which guarantee self-consistency of the perturbative expansion. We find nontrivial dependence of the magnetization current on the spin chain anisotropy $\Delta$ in the critical region $|\Delta|<1$, and a phenomenon of tripling of the twisting angle along the chain for narrow lacunes of $\Delta$. 
V. Popkov
cond-mat.stat-mech quant-ph