We study the interfaces separating different phases of 3D systems by means of
the Reflection Positivity method. We treat discrete non-linear sigma-models,
which exhibit power-law decay of correlations at low temperatures, and we prove
the rigidity property of the interface.
Our method is applicable to the Ising and Potts models, where it simplifies
the derivation of some known results. The method also works for large-entropy
systems of continuous spins.Comment: 48 pages, 4 figures; updated for publication (to appear in CMP