Compensated electron and hole pockets in an underdoped high-Tc superconductor

We report quantum oscillations in the underdoped high-temperature superconductor YBa2Cu3O6+x over a wide range in magnetic field 28 <= mu H-0 <= 85 T corresponding to approximate to 12 oscillations, enabling the Fermi surface topology to be mapped to high resolution. As earlier reported by Sebastian et al. [Nature (London) 454, 200 (2008)], we find a Fermi surface comprising multiple pockets, as revealed by the additional distinct quantum oscillation frequencies and harmonics reported in this work. We find the originally reported broad low-frequency Fourier peak at approximate to 535 T to be clearly resolved into three separate peaks at approximate to 460, approximate to 532, and approximate to 602 T, in reasonable agreement with the reported frequencies of Audouard et al. [Phys. Rev. Lett. 103, 157003 (2009)]. However, our increased resolution and angle-resolved measurements identify these frequencies to originate from two similarly sized pockets with greatly contrasting degrees of interlayer corrugation. The spectrally dominant frequency originates from a pocket (denoted alpha) that is almost ideally two-dimensional in form (exhibiting negligible interlayer corrugation). In contrast, the newly resolved weaker adjacent spectral features originate from a deeply corrugated pocket (denoted gamma). On comparison with band structure, the d-wave symmetry of the interlayer dispersion locates the minimally corrugated alpha pocket at the nodal point k(nodal) = (pi/2, pi/2), and the significantly corrugated gamma pocket at the antinodal point k(antinodal) = (pi, 0) within the Brillouin zone. The differently corrugated pockets at different locations indicate creation by translational symmetry breaking-a spin-density wave has been suggested from the suppression of Zeeman splitting for the spectrally dominant pocket. In a broken-translational symmetry scenario, symmetry points to the nodal (alpha) pocket corresponding to holes, with the weaker antinodal (gamma) pocket corresponding to electrons-likely responsible for the negative Hall coefficient reported by LeBoeuf et al. [Nature (London) 450, 533 (2007)]. Given the similarity in alpha and gamma pocket volumes, their opposite carrier type and the previous report of a diverging effective mass in Sebastian et al. [Proc. Nat. Am. Soc. 107, 6175 (2010)], we discuss the possibility of a secondary Fermi surface instability at low dopings of the excitonic insulator type, associated with the metal-insulator quantum critical point. Its potential involvement in the enhancement of superconducting transition temperatures is also discussed.