  After three decades of enormous scientific inquiry, the emergence of
superconductivity in the cuprates remains an unsolved puzzle. One major
challenge has been to arrive at a satisfactory understanding of the unusual
metallic normal state from which the superconducting state emerges upon
cooling. A second challenge has been to achieve a unified understanding of
hole- and electron-doped compounds. Here we report detailed magnetoresistivity
measurements for the archetypal electron-doped cuprate
Nd$_{2-x}$Ce$_x$CuO$_{4+{\delta}}$ that, in combination with prior data,
provide crucial links between the normal and superconducting states and between
the electron- and hole-doped parts of the phase diagram. The characteristics of
the normal state (magnetoresistivity, quantum oscillations, and Hall
coefficient) and those of the superconducting state (superfluid density and
upper critical field) consistently indicate two-band (electron and hole)
features and clearly point to hole-pocket-driven superconductivity in these
nominally electron-doped materials. We show that the approximate Uemura scaling
between the superconducting transition temperature and the superfluid density
found for hole-doped cuprates also holds for the small hole component of the
superfluid density in the electron-doped cuprates.
