  Nematic elastomers with a locked-in anisotropy direction exhibit semi-soft
elastic response characterized by a plateau in the stress-strain curve in which
stress does not change with strain. We calculate the global phase diagram for a
minimal model, which is equivalent to one describing a nematic in crossed
electric and magnetic fields, and show that semi-soft behavior is associated
with a broken symmetry biaxial phase and that it persists well into the
supercritical regime. We also consider generalizations beyond the minimal model
and find similar results.
