  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 same thermal
and magnetic properties as metals, including quantum oscillations in a magnetic
field. Here, we show that such anomalous behaviour 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.
