  Electron-doped and hole-doped superconducting cuprates exhibit a symmetric
phase diagram as a function of doping. This symmetry is however only
approximate. Indeed, electron-doped cuprates become superconductors only after
a specific annealing process: This annealing affects the oxygen content by only
a tiny amount, but has a dramatic impact on the electronic properties of the
sample. Here we report the occurrence of superconductivity in oxygen-deficient
Nd$_{2-x}$Ce$_x$CuO$_4$ thin films grown in an oxygen-free environment, after
annealing in pure argon flow. As verified by x-ray diffraction, annealing
induces an increase of the interlayer distance between CuO$_2$ planes in the
crystal structure. Since this distance is correlated to the concentration of
oxygens in apical positions, and since oxygen content cannot substantially
increase during annealing, our experiments indicate that the superconducting
phase transition has to be ascribed to a migration of oxygen ions to apical
positions during annealing. Moreover, as we confirm via first-principles
density functional theory calculations, the changes in the structural and
transport properties of the films can be theoretically described by a specific
redistribution of the existing oxygen ions at apical positions with respect to
CuO$_2$ planes, which remodulates the electronic band structure and suppresses
the antiferromagnetic order, allowing the emergence of hole superconductivity.
