Isotropic transverse XY chain with energy and magnetization currents

The ground-state correlations are investigated for an isotropic transverse XY chain that is constrained to carry either a current of magnetization (J) or a current of energy (J). We find that the effect of JÞ0 on the large-distance decay of correlations is twofold: ~i! Oscillations are introduced and ~ii! the amplitude of the power-law decay increases with increasing current. The effect of energy current is more complex. Generically, correlations in current carrying states are found to decay faster than in the J50 states, contrary to expectations that correlations are increased by the presence of currents. However, increasing the current, one reaches a special line where the correlations become comparable to those of the J50 states. On this line, the symmetry of the ground state is enhanced and the transverse magnetization vanishes. A further increase of the current destroys the extra symmetry, but the transverse magnetization remains at the high-symmetry zero value. @S1063-651X~98!13405-0#