  We calculate the de Haas-van Alphen (dHvA) effect waveform using the
canonical ensemble for different Fermi surface scenarios applicable to the
underdoped cuprate superconductor YBa2Cu3O6.5, in which quantum oscillations
have recently been observed. The harmonic content of the dHvA waveform of the
principal F ~ 500 T frequency is consistent with the existence of a second
thermodynamically dominant section of Fermi surface that acts primarily as a
charge reservoir. Oscillations in the charge density to and from this reservoir
are shown to potentially contribute to the observed large quantum oscillations
in the Hall resistance.
