  The magnetic flux periodicity in superconducting loops is reviewed. Whereas
quantization of the magnetic flux with hc/2e prevails in sufficiently thick
loops with current free interior, the supercurrent in narrow loops is either
hc/2e or hc/e periodic with the external magnetic flux. The periodicity depends
on the properties of the condensate state, in particular on the Doppler shift
of the energy spectrum. For an s-wave superconductor in a loop with diameter
larger than the coherence length, the Doppler shift is small with respect to
the energy gap, and the hc/2e periodic behavior of its flux dependent
thermodynamic properties is maintained. However, for smaller s-wave loops and,
more prominently, narrow d-wave loops of any diameter R, the Doppler shift has
a strong effect on the supercurrent carrying state; as a consequence, the
fundamental flux periodicity is in fact hc/e. It is shown analytically and
numerically that the hc/e periodic component in the supercurrent decays only
algebraically as 1/R for large d-wave loops. For nodal superconductors the
discrete nature of the eigenergies close to the Fermi energy has to be
respected in the evaluation of the Doppler shift. Furthermore, we investigate,
whether the Doppler shift modifies the supercurrent through Josephson junctions
with d-wave superconductors. For transparent junctions, the Josephson current
behaves similar to the persistent supercurrent in a loop. These distinct
physical phenomena can be compared, if the magnetic flux in units of hc/e is
identified with the phase variation of the order parameter divided by 4 pi.
