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

Abstract

In strongly correlated materials, cooperative behaviour of the electrons causes a variety of quantum ordered states that may, in some cases, coexist 1-3 . It has long been believed, however, that such coexistence among ferromagnetic ordering, superconductivity and heavy-fermion behaviour is impossible, as the first supports parallel spin alignment while the conventional understanding of the latter two phenomena assumes spin-singlet or antiparallel spins. This understanding has recently been challenged by an increasing number of observations in uranium intermetallic systems (UGe 2 , URhGe, UIr and UCoGe) 4-7 in which superconductivity is found within a ferromagnetic state and, more fundamentally, both ordering phenomena are exhibited by the same set of comparatively 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 8,9 . Within the heavy-fermion scenario, this alternative mechanism necessarily 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 remarkable phenomenaheavy-fermion behaviour, ferromagnetism and superconductivity although its nature and the nature of those heavy quasiparticles still remains unclear. Here we report spectroscopic evidence (from high-field muon spin rotation measurements) for the formation in UGe 2 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 behaviour.Within the BCS theory of superconductivity (SC), it became clear long ago 10 that pairing of electrons in the spin-singlet state is effectively destroyed by an exchange mechanism arising from strong Coulomb interactions between the valence electrons. In a ferromagnetically (FM) ordered state, this exchange interaction tends to align the spins of electrons within a Cooper pair in parallel, thereby effectively preventing the pairing. Likewise, within the standard heavy-fermion (HF) approach, the Kondo effect

Magnetocaloric and Magnetic Properties of Meta‐Magnetic Heusler Alloy Ni41Co9Mn31.5Ga18.5

“…UGe 2 also indicate small magnetic moment compared to the full moment of U 5f electron, 3.6 μ B /U. This is due to the large magnetic anisotropy, due to the spin polarization band [58][59][60]. The magnetic anisotropy energy is estimated as 6.17 meV = 107 T [58].…”

Ultra-fast electron capture by electrosterically-stabilized gold nanoparticles

“…A recent report has shown that bac-14 stabilizes the Au surface through-space by anion and cation interactions (Fig. 46 Aqueous suspensions of Au/bac-14 NPs were found to scavenge electrons at unprecedented rates, k e = 5 × 10 14 M −1 s −1 at μM concentrations. 44 In the present work, to determine the rate of electron capture by AuNP/bac-14 in solution as a function of temperature, positive anti-muons (µ + ) 45 were used as spin probes as well as the ionizing radiation.…”

“…in a slurry made by adding small amount of water to AuNP solid) it is possible that muons would cause radiation damage to the AuNPs during radiolysis process. 46 Aqueous suspensions of Au/bac-14 NPs were found to scavenge electrons at unprecedented rates, k e = 5 × 10 14 M −1 s −1 at μM concentrations. The rate constant reported here is considerably larger than rate constants of reactions involving equilibrated solvated electrons in water, suggesting ET to AuNPs occurs at a rate that competes with the solvation of the ionized electron ("dry electron"/non-equilibrium electrons before solvation).…”