We study the effects of an impurity on the magnetic response and the entanglement properties of the infinite $S=\frac{1}{2}$ $XX$ chain in a transverse field $h$ in its quasi-long-range ordered phase, i.e., below the critical point $h=1$. We show that regardless of the sign of the exchange coupling, and for whatever value of the applied field, the impurity generates an oscillating magnetization that displays a power-law decay with the distance from the impurity itself. This effect sensibly modifies the overall magnetic behavior of the chain, as testified by the observed rounding-off of the divergence of the longitudinal susceptibility. Moreover, the emergence of the above alternating structure in the spin configuration entails a substantial change in the pairwise entanglement distribution along the chain, so that concurrences are found either enhanced (up to a factor larger than 2) or quenched, depending on the distance of the spin pair from the impurity, on its strength, and on the value of the magnetic field. Acting on such control parameters, an adiabatic manipulation of both the magnetic and entanglement properties can be performed. We find these effects robust against thermal fluctuations, and we can therefore suggest realistic setups to experimentally probe the predicted behavior.

• Received 23 January 2008

DOI:https://doi.org/10.1103/PhysRevA.77.062314