  We study the XXZ Heisenberg model in a staggered magnetic field using the
HOTRG tensor renormalization method. Built into the tensor representation of
the XXZ model is the U(1) symmetry, which is systematically maintained at each
renormalization step. We determine the phase diagram of the model from the low
lying spectrum, and from the finite size dependence of the spectrum we extract
scaling dimensions, which are compared to predictions of low energy field
theory.
