When a superconducting sample is submitted to a sufficiently strong external
magnetic field, the superconductivity of the material is lost. In this paper we
prove that this effect does not, in general, take place at a unique value of
the external magnetic field strength. Indeed, for a sample in the shape of a
narrow annulus the set of magnetic field strengths for which the sample is
superconducting is not an interval. This is a rigorous justification of the
Little-Parks effect. We also show that the same oscillation effect can happen
for disc-shaped samples if the external magnetic field is non-uniform. In this
case the oscillations can even occur repeatedly along arbitrarily large values
of the Ginzburg--Landau parameter κ. The analysis is based on an
understanding of the underlying spectral theory for a magnetic Schr\"{o}dinger
operator. It is shown that the ground state energy of such an operator is not
in general a monotone function of the intensity of the field, even in the limit
of strong fields
