A self-consistent solution of the Ginzburg-Landau equations for a meso
scopic superconducting square loop has been obtained. It has been show
n that the inhomogeneous distribution of the amplitude of the order pa
rameter inside the loop leads to the appearance of certain areas where
it is much more difficult to rotate the superconducting condensate an
d which therefore sustain much higher applied magnetic fields. The int
erplay between the square symmetry of the loop and the cylindrical sym
metry of the magnetic field results in different oscillatory supercond
ucting phase boundaries which correspond to various phase boundary def
initions. The most ''realistic'' criterion to define the phase boundar
y magnetic field(H)-temperature(T) is formulated; it allows to obtain
a good agreement between the calculated H(T) curve and the experimenta
lly observed one. Copyright (C) 1996 Elsevier Science Ltd