Spin susceptibility of underdoped cuprates:: The case of ortho-II YBa2Cu3O6.5

We propose a model to explain the spin susceptibility of underdoped cuprates based on quasiparticle transitions between the arcs of a truncated Fermi surface. Such transitions are resonantly enhanced by scattering to the triplet spin resonance, which we view as the collective mode of a spin liquid. This model is able to explain the presence of magnetic scattering in the pseudogap state and its suppression at low energies on entering the superconducting state. We focus on recent inelastic neutron scattering measurements, which found that the spin susceptibility of detwinned and highly ordered ortho-II YBa2Cu3O6.5 exhibits, in both the normal and superconducting states, one-dimensional incommensurate modulations at low energies and which were interpreted as a signature of dynamic stripes. We show that the anisotropy in the experimental spin response is consistent with this model if the gap at the saddle points is anisotropic.