  Weakly coupled superconducting layers are described by the three-dimensional
XY model with strong coupling in two directions and weak coupling in the third
direction. For the usual Josephson-type interplane coupling the coherence
between the layers is lost at the same temperature as that within the layers.
Thus a low-temperature layer decoupling due to a proliferation of fluxons
between planes, as proposed by Friedel, does not occur in this case. However,
for a modified interplane coupling there are two phase transitions, one of a
Kosterlitz-Thouless type from a disordered high-temperature phase to an
intermediate phase with phase coherence only parallel to the layers, the second
from this effectively two-dimensional phase to a three-dimensional phase with
coherence in all directions and a finite "n-state" order parameter. Thus we do
find a "Friedel transition" for this special class of models.
