  The mystery of the normal state in the underdoped cuprates has deepened with
the use of newer and complementary experimental probes. While photoemission
studies have revealed solely `Fermi arcs' centered on nodal points in the
Brillouin zone at which holes aggregate upon doping, more recent quantum
oscillation experiments have been interpreted in terms of an ambipolar Fermi
surface, that includes sections containing electron carriers located at the
antinodal region. To address the question of whether an ambipolar Fermi surface
truly exists, here we utilize measurements of the second harmonic quantum
oscillations, which reveal that the amplitude of these oscillations arises
mainly from oscillations in the chemical potential, providing crucial
information on the nature of the Fermi surface in underdoped YBa2Cu3O6+x. In
particular, the detailed relationship between the second harmonic amplitude and
the fundamental amplitude of the quantum oscillations leads us to the
conclusion that there exists only a single underlying quasi-two dimensional
Fermi surface pocket giving rise to the multiple frequency components observed
via the effects of warping, bilayer splitting and magnetic breakdown. A range
of studies suggest that the pocket is most likely associated with states near
the nodal region of the Brillouin zone of underdoped YBa2Cu3O6+x at high
magnetic fields.
