  In strongly correlated materials, cooperative behavior of the electrons
causes a variety of quantum ordered states that may, in some cases, coexist. It
has long been believed, however, that such coexistence among ferromagnetic
ordering, superconductivity and heavy-fermion behavior is impossible, as the
first supports parallel spin alignment while the conventional understanding of
the latter two phenomena assumes spin-singlet or anti-parallel spins. This
understanding has recently been challenged by an increasing number of
observations in uranium systems (UGe2, URhGe, UIr and UCoGe) in which
superconductivity is found within a ferromagnetic state and, more
fundamentally, both ordering phenomena are exhibited by the same set of heavy
5f electrons. Since the coexistence of superconductivity and ferromagnetism is
at odds with the standard theory of phonon-mediated spin-singlet
superconductivity, it requires an alternative pairing mechanism, in which
electrons are bound into spin-triplet pairs by spin fluctuations. Within the
heavy-fermion scenario, this alternative mechanism assumes that the magnetism
has a band character and that said band forms from heavy quasiparticles
composed of f electrons. This band is expected to be responsible for all three
phenomena although its nature and the nature of those heavy quasiparticles
still remains unclear. Here we report spectroscopic evidence for the formation
in UGe2 of subnanometer-sized spin polarons whose dynamics we follow into the
paramagnetic and ferromagnetic phases. These spin polarons behave as heavy
carriers and thus may serve as heavy quasiparticles made of 5f electrons; once
coherence is established, they form a narrow spin-polaron band which thus
provides a natural reconciliation of itinerant ferromagnetism with spin-triplet
superconductivity and heavy-fermion behavior.
