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Spin-polaron band of heavy carriersin the heavy-fermion ferromagnetic superconductor UGe2

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.