Coexistence of Fermi arcs and Fermi pockets in a high-Tc copper oxide superconductor

One outstanding puzzle in high-temperature cuprate superconductors relates to the 'Fermi surface pocket' that is expected to emerge when the material is slightly doped or underdoped. Such pockets were recently seen in quantum oscillation measurements. Angle-resolved photoemission (ARPES) measurements reveal Fermi arcs, that is, open-ended gapless sections in the large Fermi surface, rather than the closed loops (Fermi pockets) expected of an ordinary metal, contradicting the quantum oscillation measurements. Meng et al. report ARPES measurements of La-Bi2201 showing the presence of both Fermi pockets and Fermi arcs. This coexistence is surprising since it is not predicted by current theories.