Quantum magnetic excitations are generally thought to exist only in systems with small, isotropic magnetic moments which can be easily reversed. In metallic Yb2Pt2Pb, the Yb3+ ions form large, seemingly classical magnetic moments, with the spin-orbit coupling of the 4f-electrons leading to a J = ± 7/2 ground state doublet. Neutron scattering measurements reveal that from this unlikely host emerges a continuum of quantum excitations — spinons along one dimensional chains — in good agreement with the expectations for S = ± 1/2 moments governed by a remarkably straightforward Hamiltonian. In magnetic field the continuum is modified through the formation of a spinon Fermi surface and spinon bound states between the Yb chains, persisting up to the saturation field of 2.3 T. The Yb ground state doublet ensures that transverse excitations are virtually nonexistent, giving direct access to the longitudinal excitation channel without the presence of transverse damping.
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