  High-temperature superconductivity occurs as copper oxides are chemically
tuned to have a carrier concentration intermediate between their metallic state
at high doping and their insulating state at zero doping. The underlying
evolution of the electron system in the absence of superconductivity is still
unclear and a question of central importance is whether it involves any
intermediate phase with broken symmetry. The Fermi surface of underdoped
YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets in contrast
with the large cylinder characteristic of the overdoped regime1, pointing to a
topological change in the Fermi surface. Here we report the observation of a
negative Hall resistance in the magnetic field-induced normal state of
YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like. We
propose that electron pockets arise most likely from a reconstruction of the
Fermi surface caused by the onset of a density-wave phase, as is thought to
occur in the electron-doped materials near the onset of antiferromagnetic order
Comparison with materials of the La2CuO4 family that exhibit spin/charge
density-wave order suggests that a Fermi surface reconstruction also occurs in
those materials, pointing to a generic property of high-Tc superconductors.
