Dimensional reduction in superconducting arrays and frustrated magnets

Some frustrated magnets and superconducting arrays possess unusual symmetries that cause the free energy or other physics of a $D$-dimensional quantum or classical problem to be that of a different problem in a reduced dimension $d<D$. Examples in two spatial dimensions include the square-lattice $p+ip$ superconducting array, the Heisenberg antiferromagnet on the checkerboard lattice (studied by a combination of 1/S expansion and numerical transfer matrix), and the ring-exchange superconducting array. Physical consequences are discussed both for ``weak'' dimensional reduction, which appears only in the ground state degeneracy, and ``strong'' dimensional reduction, which applies throughout the phase diagram. The ``strong'' dimensional reduction cases have the full lattice symmetry and do not decouple into independent chains, but their phase diagrams, self-dualities, and correlation functions indicate a reduced effective dimensionality. We find a general phase diagram for quantum dimensional reduction models in two quantum dimensions with $N$-fold anisotropy, and obtain the Kosterlitz-Thouless-like phase transition as a deconfinement of dipoles of 3D solitons.