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

In the pseudogap state of the high-transition-temperature (high-T-c) copper oxide superconductors(1), angle-resolved photoemission (ARPES) measurements have seen Fermi arcs-that is, open-ended gapless sections in the large Fermi surface(2-8)-rather than a closed loop expected of an ordinary metal. This is all the more puzzling because Fermi pockets (small closed Fermi surface features) have been suggested by recent quantum oscillation measurements(9-14). The Fermi arcs cannot be understood in terms of existing theories, although there is a solution in the form of conventional Fermi surface pockets associated with competing order, but with a back side that is for detailed reasons invisible to photoemission probes(15). Here we report ARPES measurements of Bi2Sr2-xLaxCuO6+delta (La-Bi2201) that reveal Fermi pockets. The charge carriers in the pockets are holes, and the pockets show an unusual dependence on doping: they exist in underdoped but not overdoped samples. A surprise is that these Fermi pockets appear to coexist with the Fermi arcs. This coexistence has not been expected theoretically.