Lower critical dimension of the random-field XY model and the zero-temperature critical line

The random-field XY model is studied in spatial dimensions d = 3 and 4, and in between, as the limit q -> infinity of the q-state clock models, by the exact renormalization-group solution of the hierarchical lattice or, equivalently, the Migdal-Kadanoff approximation to the hypercubic lattices. The lower critical dimension is determined between 3.81 < d(c) < 4. When the random field is scaled with q, a line segment of zero-temperature criticality is found in d = 3. When the random field is scaled with q(2), a universal phase diagram is found at intermediate temperatures in d = 3.