Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe2

It has long been believed that coexistence among ferromagnetic ordering, superconductivity or heavy-fermion behaviour is impossible, as the former supports parallel spin alignment while the latter two phenomena assume a spin-singlet configuration. This understanding has recently been challenged by a number of observations in uranium intermetallic systems where superconductivity (SC) is found within a ferromagnetic state and both ordering phenomena are facilitated by the same set of comparatively heavy quasiparticles which bind into spin-triplet pairs in the SC state. Within the heavy-fermion scenario, this mechanism necessarily assumes that the magnetism has a band character. This band is expected to be responsible for all three phenomena – heavy-fermion behaviour, ferromagnetism and superconductivity – although its nature and the nature of the heavy quasiparticles have so far remained unclear. Our high-field muon spin rotation measurements are indicative of spin polarons of subnanometer size in UGe2. These spin polarons behave as heavy quasiparticles made of 5f electrons. Once coherence is established, they may form a narrow spin-polaron band which thus may provide a natural reconciliation of itinerant ferromagnetism with spin-triplet superconductivity and heavy-fermion behaviour.