Exotic Heavy-Fermion State in Filled Skutterudite SmOs₄Sb₁₂

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Specific heat and transport measurements have revealed an unconventional heavy-fermion (HF) state in SmOs₄Sb₁₂ single crystals. The electronic specific-heat coefficient (γ=0.82 J/K² mol) and the coefficient (A) of the quadratic temperature dependence of electrical resistivity are largely enhanced, although the ratio Aγ⁻² is reduced from the Kadowaki–Woods ratio of HF materials. Both γ and A do not show any significant decrease in applied field in contrast with Ce-based HF compounds, suggesting an unconventional origin of the heavy quasiparticles. A weak ferromagnetic ordering sets in below ∼3 K, probably originating in the itinerant quasiparticles.