  The spin 1/2 XXZ chain in a random magnetic field pointing in the Z direction
is numerically studied using the Density Matrix Renormalization Group (DMRG)
method. The phase diagram as a function of the anisotropy of the XXZ
Hamiltonian and the strength of the random field is analyzed by computing the
spin correlations and the superfluid density. To obtain the superfluid density
we consider a superblock configuration representing a closed system with an
arbitrary twist at the boundary. This allows us to estimate the size of the
critical region where the quasi-long-range order persists, that is, where the
spin correlation length is infinite and the superfluid density is non-zero.
