  We use quantum oscillation measurements to distinguish between spin and
orbital components of the lowest energy quasiparticle excitations in
YBa2Cu3O6.54, each of which couple differently to a magnetic field. Our
measurements reveal the phase of the observed quantum oscillations to remain
uninverted over a wide angular range, indicating that the twofold spin
degeneracy of the Landau levels is virtually unaltered by the magnetic field.
The inferred suppression of the spin degrees of freedom indicates a spin
density-wave is responsible for creation of the small Fermi surface pockets in
underdoped YBa2Cu3O6+x - further suggesting that excitations of this phase are
important contributors to the unconventional superconducting pairing mechanism.
