  We report quantum oscillations in the underdoped high Tc YBCO over a wide
range in magnetic field 28<B<85 T corresponding to ~12 oscillations, enabling
the Fermi surface topology to be mapped to high resolution. As earlier reported
by Sebastian et al., 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 535 T to be clearly resolved into three separate
peaks at 460 T, 532 T and 602 T. 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 (\alpha) that is almost
ideally two-dimensional in form. In contrast, the newly resolved weaker
adjacent spectral features originate from a deeply corrugated pocket (\gamma).
On comparison with band structure, the d-wave symmetry of the interlayer
dispersion locates the minimally corrugated \alpha pocket at the 'nodal' point
where holes are located in a translational symmetry-broken scenario, and the
significantly corrugated \gamma pocket at the 'antinodal' point in the
Brillouin zone, where electrons are located in a translational symmetry-broken
scenario. Translational symmetry breaking by an SDW is suggested from the
strong suppression of Zeeman splitting for the spectrally dominant pocket,
additional evidence for which is provided from the harmonics we resolve in the
present experiments. Given the similarity in \alpha and \gamma pocket sizes,
their opposite carrier type and the previous report of a diverging effective
mass, we discuss the possibility of a secondary Fermi surface instability at
low dopings of the excitonic insulator type, associated with the
metal-insulator QCP.
