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An outstanding problem in the field of high-transition-temperature (high-Tc) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime. The normal state uncomplicated by thermal fluctuations can be studied using applied magnetic fields that are sufficiently strong to suppress long-range superconductivity at low temperatures. Proposals in which the normal ground state is characterized by small Fermi surface pockets that exist in the absence of symmetry breaking have been superseded by models based on the existence of a superlattice that breaks the translational symmetry of the underlying lattice. Recently, a charge superlattice model that positions a small electron-like Fermi pocket in the vicinity of the nodes (where the superconducting gap is minimum) has been proposed as a replacement for the prevalent superlattice models that position the Fermi pocket in the vicinity of the pseudogap at the antinodes (where the superconducting gap is maximum). Although some ingredients of symmetry breaking have been recently revealed by crystallographic studies, their relevance to the electronic structure remains unresolved. Here we report angle-resolved quantum oscillation measurements in the underdoped copper oxide YBa2Cu3O6 + x. These measurements reveal a normal ground state comprising electron-like Fermi surface pockets located in the vicinity of the nodes, and also point to an underlying superlattice structure of low frequency and long wavelength with features in common with the charge order identified recently by complementary spectroscopic techniques.

An enduring question in correlated systems concerns whether superconductivity is favored at a quantum critical point (QCP) characterized by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high T c cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultrastrong magnetic fields to measure quantum oscillations in underdoped YBa 2 Cu 3 O 6þx , revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in T c in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centered at a critical Fermi surface instability.fermi surface | high temperature superconductivity | metal-insulator transition | quantum oscillations | quantum critical point A continuous zero temperature instability between different ground states-termed as a quantum critical point-is characterized by a divergence in a relevant susceptibility (1-3). In strongly correlated systems (4), the influence of criticality on the entire body of itinerant electrons results in a global divergence of the effective mass-which is recognized as the key defining experimental signature of quantum criticality (4, 5). The growth of electronic correlations on the zero temperature approach to the critical instability can be experimentally accessed by the tuning of parameters such as pressure and doping. Quantum oscillation measurements are ideally suited to investigate the effects of such tuning due to the direct access they provide to the effective mass of the elementary fermionic excitations that can be traced across the quantum critical point (QCP) (6). Such a direct probe is crucial in superconducting materials, where bulk thermodynamic signatures of quantum critical behavior of the normal quasiparticles (3, 4, 7) are difficult to access due to the overlying superconducting dome.While the emergence of high T c superconductivity in the cuprate family is inextricably linked to the parent Mott insulating compound, remarkably little is known about the physics of the metal-insulator cross-over (8) and its relation to electronic correlations. By using quantum oscillation measurements in strong magnetic fields to access normal state quasiparticles in underdoped YBa 2 Cu 3 O 6þx , we uncover a striking doping-dependent upturn in the effective mass at the location of the metal-insulator cross-over (9-13). Our findings provide bulk thermodynamic evidence for a metal-insulator quantum critical point (QCP) in high T c cuprates (14-19), without requiring extrapolation below the superconducting dome. The effective mass divergence unaccompanied by a change in Fermi surface area away from half-filling signals a unique many-body mechanism (20) that drives insulating behavior in underdoped cuprates.We trace the...