  An understanding of the missing antinodal electronic excitations in the
pseudogap state is essential for uncovering the physics of the underdoped
cuprate high temperature superconductors. The majority of high temperature
experiments performed thus far, however, have been unable to discern whether
the antinodal states are rendered unobservable due to their damping, or whether
they vanish due to their gapping. Here we distinguish between these two
scenarios by using quantum oscillations to examine whether the small Fermi
surface pocket, found to occupy only 2% of the Brillouin zone in the underdoped
cuprates, exists in isolation against a majority of completely gapped density
of states spanning the antinodes, or whether it is thermodynamically coupled to
a background of ungapped antinodal states. We find that quantum oscillations
associated with the small Fermi surface pocket exhibit a signature sawtooth
waveform characteristic of an isolated two-dimensional Fermi surface pocket.
This finding reveals that the antinodal states are destroyed by a hard gap that
extends over the majority of the Brillouin zone, placing strong constraints on
a drastic underlying origin of quasiparticle disappearance over almost the
entire Brillouin zone in the pseudogap regime.
