  Newly-born pulsars offer favorable sites for the injection of heavy nuclei,
and for their further acceleration to ultrahigh energies. Once accelerated in
the pulsar wind, nuclei have to escape from the surrounding supernova envelope.
We examine this escape analytically and numerically, and discuss the pulsar
source scenario in light of the latest ultrahigh energy cosmic ray (UHECR)
data. Our calculations show that, at early times, when protons can be
accelerated to energies E>10^20 eV, the young supernova shell tends to prevent
their escape. In contrast, because of their higher charge, iron-peaked nuclei
are still accelerated to the highest observed energies at later times, when the
envelope has become thin enough to allow their escape. Ultrahigh energy iron
nuclei escape newly-born pulsars with millisecond periods and dipole magnetic
fields of ~10^(12-13) G, embedded in core-collapse supernovae. Due to the
production of secondary nucleons, the envelope crossing leads to a transition
of composition from light to heavy elements at a few EeV, as observed by the
Auger Observatory. The escape also results in a softer spectral slope than that
initially injected via unipolar induction, which allows for a good fit to the
observed UHECR spectrum. We conclude that the acceleration of iron-peaked
elements in a reasonably small fraction (< 0.01%) of extragalactic
rotation-powered young pulsars would reproduce satisfactorily the current UHECR
data. Possible signatures of this scenario are also discussed.
