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Spin-order driven Fermi surface revealed by quantum oscillations in an underdoped high Tc superconductor

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.