  We investigate macroscopic entanglement in an infinite XX spin-1/2 chain with
staggered magnetic field, B_l=B+e^{-i\pi l}b. Using single-site entropy and by
constructing an entanglement witness, we search for the existence of
entanglement when the system is at absolute zero, as well as in thermal
equilibrium. Although the role of the alternating magnetic field b is, in
general, to suppress entanglement as do B and T, we find that when T=0,
introducing b allows the existence of entanglement even when the uniform
magnetic field B is arbitrarily large. We find that the region and the amount
of entanglement in the spin chain can be enhanced by a staggered magnetic
field.
