The metallic state of the underdoped high-Tc cuprates has remained an enigma:How may seemingly disconnected Fermi surface segments, observed in zeromagnetic field as a result of the opening of a partial gap (the pseudogap),possess conventional quasiparticle properties? How do the small Fermi-surfacepockets evidenced by the observation of quantum oscillations (QO) emerge assuperconductivity is suppressed in high magnetic fields? Such QO, discovered inunderdoped YBa2Cu3O6.5 (Y123) and YBa2Cu4O8 (Y124), signify the existence of aconventional Fermi surface (FS). However, due to the complexity of the crystalstructures of Y123 and Y124 (CuO2 double-layers, CuO chains, low structuralsymmetry), it has remained unclear if the QO are specific to this particularfamily of cuprates. Numerous theoretical proposals have been put forward toexplain the route toward QO, including materials-specific scenarios involvingCuO chains and scenarios involving the quintessential CuO2 planes. Here wereport the observation of QO in underdoped HgBa2CuO4+{\delta} (Hg1201), a modelcuprate superconductor with individual CuO2 layers, high tetragonal symmetry,and no CuO chains. This observation proves that QO are a universal property ofthe underdoped CuO2 planes, and it opens the door to quantitative futurestudies of the metallic state and of the Fermi-surface reconstructionphenomenon in this structurally simplest cuprate.
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