Observation of closed Fermi-pockets in the cuprate pseudogap phase above the superconducting critical temperature

The anomolous nature of pseudogap electronic structure of the cuprates is typified by the observation of truncated Fermi-arcs with photoemission spectroscopy. We report on angular-dependent magnetoresistance oscillations which reveal the existence of closed small Fermi-pockets in the pseudogap phase above the superconducting critical temperature T_c=74 K in the underdoped cuprate HgBa₂CuO_4+δ. The use of high temperatures avoids the widely examined subsequent Fermi-surface reconstruction at low-temperatures and high-fields that obscures the true pseudogap phase. Salient features of the data are reproduced by a Fermi-surface comprising nodal elliptical pockets. Notably, long-ranged broken translational symmetry, such as antiferromagnetism and charge-density wave, is known to be absent in the pseudogap phase of HgBa₂CuO_4+δ above T_c, thus suggesting a different mechanism for producing a pseudogap electronic structure.