The long search for insulating materials that possess low-energy quasiparticles carrying electron’s quantum numbers except charge—inspired by the neutral spin-$1/2$ excitations, the so-called spinons, exhibited by Anderson’s resonating-valence-bond state—seems to have reached a turning point after the discovery of several Mott insulators displaying the same thermal and magnetic properties as metals, including quantum oscillations in a magnetic field. Here, we show that such anomalous behavior is not inconsistent with Landau’s Fermi liquid theory of quasiparticles at a Luttinger surface. That is the manifold of zeros within the Brillouin zone of the single-particle Green’s function at zero frequency, and which thus defines the spinon Fermi surface conjectured by Anderson.

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• Quasiparticles & collective excitations