Spin-order driven Fermi surface reconstruction 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 YBa(2)Cu(3)O(6.54), 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 YBa(2)Cu(3)O(6+x)--further suggesting that excitations of this phase are important contributors to the unconventional superconducting pairing mechanism.