Universal quantum oscillations in the underdoped cuprate superconductors

The metallic state of the underdoped high- T _c cuprates has remained an enigma: how may seemingly disconnected Fermi-surface segments, observed in zero magnetic field as a result of the opening of a partial gap (the pseudogap), possess conventional quasiparticle properties? How do the small Fermi-surface pockets evidenced by the observation of quantum oscillations emerge as superconductivity is suppressed in high magnetic fields? Such quantum oscillations, discovered in underdoped YBa ₂ Cu ₃ O _6.5 (Y123; ref.) and YBa ₂ Cu ₄ O ₈ (Y124; ref.), signify the existence of a conventional Fermi surface. However, owing to the complexity of the crystal structures of Y123 and Y124 (CuO ₂ double layers, CuO chains, low structural symmetry), it has remained unclear whether the quantum oscillations are specific to this particular family of cuprates. Numerous theoretical proposals have been put forward to explain the source of quantum oscillations, including materials-specific scenarios involving CuO chains and scenarios involving the quintessential CuO ₂ planes. Here we report the observation of quantum oscillations in underdoped HgBa ₂ CuO _4+δ (Hg1201), a model cuprate superconductor with individual CuO ₂ layers, high tetragonal symmetry and no CuO chains. This observation proves that quantum oscillations are a universal property of the underdoped CuO ₂ planes, and it opens the door to quantitative future studies of the metallic state and of the Fermi-surface reconstruction phenomenon in this structurally simplest cuprate.